Chapter 2 Equipment.
After reading this chapter, you should be able to:
* List the three categories of equipment.
* Identify the three common types of chains.
* Read a first foot, extended foot, and fully graduated chain.
* Measure distance using a chain.
* Measure distance using an odometer.
* Measure distance using an optical range finder.
* Use a hand level.
* Measure distance by stadia.
* Level a four-legged instrument.
* Identify the parts of an automatic level.
* Identify the parts of a dumpy level.
* Identify the parts of a transit.
* Read vernier scales.
* List the information that should be recorded in a field book.
* Keep notes in a field book.
* Read a rod.
TERMS TO KNOW
first foot graduated tapes
extended foot tape
fully graduated tape
electronic distance measuring
rocking the rod
Surveying is heavily dependent on the use of instruments and equipment. Proper selection, use, and care of this equipment will greatly influence the quality of data collected and the amount of resources that will be required to collect information. The instruments and equipment used may be as simple as two tree branches or as complex as an electronic total station costing several thousand dollars. This chapter will discuss the common equipment used for land measurement and surveying.
One of the most important principles to remember is that surveying instruments are precision instruments, and they are easily damaged by rough use or improper care. Instruments can be easily damaged by being dropped, stored improperly, or having their movements "forced" without being unlocked. A damaged instrument may function correctly, but the data it produces will be wrong, and the operator may not know that the instrument is damaged.
CATEGORIES OF EQUIPMENT
Surveying equipment can be divided into three categories: distance-measuring equipment, instruments for measuring angles and elevations, and accessories.
Distance-Measuring Equipment Common distance-measuring equipment includes:
tapes and chains
Instruments for Measuring Angles and Elevations
Instruments used for measuring angles and elevations include:
Many accessories are used when measuring distances angles and elevations. Some of these include:
rods and targets
Surveying equipment, instruments and accessories are available with different features and capabilities. The following sections will discuss some of these.
EQUIPMENT FOR MEASURING DISTANCE
Distance-measuring equipment ranges in technology from steel tapes to EDM equipment that uses microwaves or lasers. An art in measuring distance is determining the best equipment to use. What is the best equipment for measuring distance is primarily dependent on the topography, the skill of the surveyor, and the use of the data. If a high degree of accuracy is required, then chaining or EDM should be used. If a low level of accuracy is acceptable, then an odometer wheel or even pacing can be used. Regardless of the equipment used, the surveyor must ensure that it is in proper working order and that it is used correctly.
Tapes and Chains
In the United States, the Gunter's Chain was the standard chain for many years. A Gunter's chain is 66 feet long, comprising 100 links, with each link being 7.92 inches. The links were made of heavy wire and connected by rings. The handle was threaded and was used to adjust the length of the chain to compensate for wear (see Figure 2-1).
The Gunter's chain was primarily used for land surveying because of its relationship to a mile, 80 chains = 1 mile. The link size made the chain difficult to use when distances were less than a full chain.
In the United States, the Gunter's chain was replaced by the engineer's chain. Engineer's chains were composed of links also, but they were designed to be 100 feet long and each link was one foot. Engineer's chains were eventually replaced by the modern 100-foot steel tape. Although the modern instrument looks like a steel tape, the term chain is still used by many surveyors. One reason is to distinguish between the steel tapes and cloth tapes. Steel tapes (chains) are more accurate than cloth tapes and should be used when greater accuracy is required. The greatest accuracy is produced by an invar tape. Invar tapes are constructed from a special alloy that reduces the expansion and contraction caused by temperature changes.
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Modern surveying chains may look like a carpenter's tape, but they are designed for measuring long distances. They have two unique characteristics, their construction and the graduations that are used. Graduation refers to the subdivisions of the whole unit. For example, an inch is a graduation of a foot and 1/2 of an inch is a graduation of an inch. Surveying chains are constructed of a steel alloy that has a known coefficient of expansion in response to temperature. This is necessary because steel expands when heated and contracts when cooled. The designed accuracy of a steel surveyor's chain is only true if it is at standard temperature, usually 72[degrees]F, and has a standard tension applied, usually 15 pounds. For surveys with a high level of accuracy, the measured distances must be adjusted for the expansion or contraction of the tape. This is called the temperature correction.
The other difference between a surveyor's chain and a standard tape is the graduations. An individual must be careful when using a surveyor's chain because several different graduations are used. A measurement with carpenter's tape will be in feet, inches, and fractional graduations for each inch. Fractions of 1/2, 1/4, 1/8 and 1/16 of an inch are common. A surveyor's chain uses decimal feet. Standard chains can measure distance to tenths (0.1), hundredths (0.01), and thousandths (0.001) of a foot. In addition, many surveyors' tapes do not have graduations between each foot mark.
Several styles will only have one graduated foot at each end of the tape. This feature defines the three common styles of tapes that are used.
first foot graduated extended foot graduated fully graduated
These three styles of chains are illustrated in Figure 2-2.
In addition to the three styles of chains, manufacturers also place the zero-foot mark at different points on the chain. For some styles of chains the zero mark is at the end of the chain. For others, it will be several inches in from the end of the chain. It is very important for the measuring team to inspect the chain and determine which style of tape they are using before they start recording measurements. One extended foot chain used by the author has an additional six inches between the end of the extended foot and the end of the chain. If a surveyor using this chain assumed that the zero point was at the end of the chain, each 100-foot measurement would have an error of at least 18 inches. The error would be 36 inches if the chain had an extended foot at the 100-foot end.
FIRST FOOT GRADUATED TAPES
First foot graduated tapes have graduations only within the first foot (between 0 and 1) or first foot and last foot (between 0 and 1, and 99 and 100). When this style of tape is used, it is very easy to make an error of one foot when a partial chain is used. Figure 2-2 illustrates this problem.
When using a first foot graduated tape, if the distance is less than a full chain and less than a whole foot, the head person holds the tape on the nearest foot mark and the rear person takes out the slack and reads the partial foot. An error can occur because the distance being measured is less than the foot mark at the head of the chain. In Figure 2-2, the head person holds the chain on the 97 foot mark and the rear person has the chain on the 0.4 mark. The correct reading is 96.6 feet because it is 0.4 feet less than 97 feet. Because the distance from zero to one foot is used to measure the partial foot, the partial foot must be subtracted from the measurement at the head of the chain.
EXTENDED FOOT TAPE
The extended foot tape is similar to a first foot tape, except that an additional foot has been added to the zero end for the graduated foot. It is also possible that the 100 foot end will have an extended foot. With this style of tape there is less chance for the one-foot error associated with the first foot graduated tape. Using the previous example would result in a measurement of 97.4 feet for an extended foot chain. When using an extended foot chain the partial foot is added to the reading at the head of the chain.
Extended foot chains have their own chance for error. The user must be careful to use the correct zero foot and 100 foot marks. The initial reading must be made using the zero foot mark on the chain not the end of the extended foot. If the end of the extended foot is used as zero feet, the measurement will have an error of 1 foot. For a chain with an extended foot on both ends, the error could be 2 feet for every full chain that is measured.
FULLY GRADUATED TAPE
A fully graduated tape is more similar to a standard carpenter's tape than either the first foot or extended foot tape. The primary differences are that it is constructed from steel and the scale is in decimal feet, instead of feet, inches, and fractional inches.
It is very important for the user to understand the style of tape being used before starting to record measurements. Failure to do so may result in incorrect information.
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USING SURVEYOR'S CHAINS
Surveyors' chains must be used carefully. These chains break easily if a loop is pulled tight. The surveyor's chain should always be wiped clean and oiled after each use. Chains can be thrown and rolled by hand, but a chain holder, as shown in Figure 2-3, is very handy and reduces the chance of breaking the tape.
Until the invention of electronic measuring devices, chaining was the most accurate method of measuring distance. A reasonable attention to detail will produce measurements accurate to 0.01 foot. With the appropriate chain and procedures, accuracy of 0.001 foot is possible.
Chains are still very useful, especially for short distances and where there is a clear path for the route. Electronic distance measuring has replaced chains for many surveying jobs such as measuring long distances, where the route cannot be easily walked or when the slope changes dramatically and frequently.
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As the name implies an odometer wheel is a combination of an odometer and a wheel. They are a very common instrument for measuring distance. An odometer is a mechanical revolution counter. Because it is mechanical, the manufacturer can design the gear ratios so the numbers on the dial represent a distance. Odometer wheels are available with one of several different standard units of measure. For example, the odometer on an automobile is designed to read in miles and tenths of a mile. Odometer wheels used for surveying are manufactured with dials that read in whole feet, feet and inches, decimal feet, and metric units. One consideration when selecting and/or using an odometer wheel is the diameter of the wheel. Odometers with small diameter wheels, less than 18 inches, are designed for hard surfaces. Larger diameter wheels can be used on hard surfaces, but they are designed for soft surfaces and irregular terrain. There is a correlation between the ideal diameter of the wheel and the height of the vegetation and roughness of the terrain. When measuring distances on unimproved surfaces or in tall grass and weeds an odometer with a larger diameter wheel should be used. Odometer wheels are available with wheel diameters up to four feet in diameter (see Figure 2-4).
To measure a distance with an odometer wheel the odometer is set to zero. Then the wheel is placed at the starting point, and the operator "walks the distance." A common error when using odometer wheels is to hold the handle directly above the wheel when setting it to zero and then tilting the handle down to the walking position. This practice will cause an error in the readings.
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A rangefinder is a device that is usually handheld and can be used for measuring distance. Rangefinders can be divided into two categories, optical and electronic.
An optical rangefinder is a useful instrument for estimating distances.
These instruments are commonly used for hunting, but they can also be used for land measurement when a low level of precision is acceptable. Many of these instruments have a precision of 1 yard.
Optical range finders use a splitting mirror and a tilting mirror. When you look through the eyepiece, the view is split into two images. One image is looking straight ahead. The other image is a shadow image produced by the mirror offset to the side.
Using an Optical Range Finder The instrument is used by focusing the instrument on a well-defined object. If the instrument is not set at the correct distance, a shadow image will be visible. The distance is determined by rotating the adjustment wheel until the shadow image is superimposed on the original image. When the images are superimposed, the distance can be read from the scale on the instrument.
The principle of optical range finders is based on a trigonometric function. If you know the length of one side and one angle of a right triangle, then you can calculate the length of any remaining sides. The range finder does this using a mechanical linkage and a calibrated wheel.
For the example in Figure 2-5, if the distance between the splitting mirror and the tilting mirror is one foot; thus, the distance from the range finder to the object is:
Tan [theta] = opp/adj
adj = opp/Tan [theta]
= 1.0 ft./Tan 0.19 = 301 ft.
The primary limitation of optical rangefinders is measuring range, accuracy, and precision. For example, the usable range may only be between 20 and 400 yards or 50 and 1,000 yards. The accuracy may be as low is 1/100 and the precision as large as 1 yard. An accuracy of 1/100 for a rangefinder with a precision of 1 yard means that the maximum error for any measurement is 1 yard for every 100 yards measured. Their primary advantage is that they are totally mechanical. They do not require any batteries to operate.
Electronic rangefinders use an invisible signal to measure distance. The devices range from inexpensive hand-held instruments with a precision of 1 yard to tripod-mounted devices with a precision of less than an inch. Simple hand-held rangefinders that have a low level of precision can be purchased for 200 dollars or less. The type used for surveying can cost several thousand dollars and is usually called an electronic distance measuring (EDM) device. Electronic distance measuring is explained more fully in the next section.
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Electronic Distance Measuring
The term electronic distance measuring is used to describe a category of distance-measuring devices that use microwaves or other forms of energy beams to determine the distance between itself and an object, between itself and a reflector, or between a master and a slave instrument. EDM's measure distance by rearranging the equation for velocity. In the standard equation, velocity equals distance divided by time.
V = D/T D = VT
Rearranged, distance equals velocity multiplied by time. EDM devices determine distance by sending out a signal of known velocity and measuring the time elapsed before the signal returns. The instrument must know the number of complete cycles and the point on a partial cycle when the signal returns to the instrument. If a single beam is used, the instrument can determine the point in a cycle when the beam returns, but not the number of complete cycles. Therefore, instruments use multiple beams with different wavelengths. Each wavelength will reach the sending unit at a different point in the cycle. Comparing the return point on the cycle for beams with different frequencies provides the CPU program with the information it needs to determine the number of cycles and the point on the partial cycle that occurred between the time when the signal was sent and when it was received (see Figure 2-6).
One popular example of EDM technology is laser rangefinders.
A laser emits a visible or invisible beam that is very coherent; it diverges at a slow rate. This characteristic makes lasers very useful for surveying. A laser rangefinder determines the time required for a signal to leave the instrument and return. Some lasers use a reflector at the second station to reflect the signal back to the instrument, but others are reflectorless. Reflectorless lasers are point-and-shoot instruments. The object being measured is first centered in the viewfinder, or located with a visible dot of light. Then, either the rangefinder displays the distance or the operator activates the laser to measure the distance. Advantages of using a reflector include increased distance and greater precision.
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Manufacturers provide a variety of additional features on laser rangefinders. One manufacturer includes a compass that is visible in the same screen as the distance. With this instrument a person can stand in one spot and measure the bearing and distance to any object within the range of the instrument. Another feature that is available is a built-in inclinometer. The addition of an inclinometer allows the user to determine vertical angles. With this ability, the rangefinder can be used to determine the height of objects. The combination of an EDM and an electronic transmit results in an instrument called a total station. These instruments are described in the section on surveying instruments.
ADVANTAGES AND DISADVANTAGES OF EDMS
Electronic distance measuring has several advantages:
* The operator doesn't need to walk the distance.
* Distances can be taken across obstacles such as water, trees, and rough terrain. The only requirement is that a signal can reach the second point and return in a straight line.
* Instruments can be purchased that have the capability of downloading the data into a computer, data logger, or electronic field book, thereby eliminating the errors associated with manually recording data.
* Once leveled, EDM instruments can produce a reading every few seconds.
* EDMs require fewer individuals to take measurements.
* EDMs will measure short or long distances. Disadvantages of EDMs include:
* Some models must be calibrated for air density.
* They are electronic instruments and their use may be restricted by environmental conditions.
* They use microprocessors: no electricity = no measurement.
* High-quality instruments are expensive.
One additional method of measuring distance is by stadia. Measuring distance by stadia requires the use of a surveying instrument. This method is covered in Chapter 4.
EQUIPMENT FOR MEASURING ANGLES AND ELEVATIONS
For a given type of instrument, there are many different models available based on options, focal length, and accuracy. The best source of information for a specific instrument is the owners' manual or a manufacturer's catalogue. This section will discuss the common categories of surveying instruments.
As the name implies, a hand level is a level that is held in the operator's hands. Hand levels are more sophisticated than carpenter's levels, but they are the simplest style of level used in surveying. They have a spirit level and a single cross-hair. This style of hand level is used to ensure that chains are level when measuring horizontal distance with plumb bobs, and estimating slope and changes in elevation. The common magnification is from zero to 5X. The more sophisticated hand levels will include stadia hairs for measuring horizontal distance (see Figure 2-7).
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The hand level illustrated uses an external spirit level for controlling the location of the instrument. With this type of level, you must be able to hold the instrument horizontally while looking through the lens. An alternate design includes an internal level. This style uses a split viewing area. When looking through the eyepiece, you can see on one side the bubble for the spirit level and on the other half the cross-hair. It is easier to hold this style of instrument in a horizontal position.
Hand levels are primarily used for estimating slope. Slope is the rate of change in elevation. Measuring slope with hand levels is accomplished by standing at the bottom of the slope, and while holding the level in a horizontal position, making note of a landmark where the line of sight strikes the ground. Using the distance to this point and the user's eye height above the ground, the slope can be calculated as follows:
% Slope = Rise/Run x 100
Rise is the eye height of the user, and run is the distance from the observation point to where the line of sight strikes the ground (see Figure 2-8).
Abney levels are hand-held levels, but they are more sophisticated than hand levels. They will usually have a direct reading scale for vertical angles and slope, stadia hairs, and better magnification and optics. Stadia cross-hairs are visible when you look through the eyepiece. They are mounted inside the telescope for measuring distance. A stadia cross-hair is located an equal distance above and below the elevation cross-hair, as shown in Figure 2-9. The use of stadia for measuring distance is explained in Chapter 4.
Most Abney levels have adjustments for both focusing and magnification. When used with a rod and target, they provide sufficient accuracy for preliminary surveying (see Figure 2-10).
USING AN ABNEY LEVEL
Abney levels use a split viewing area. When the user looks through the eyepiece, half of the area is used to view the spirit bubble, and the remaining area is for viewing the target.
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To measure vertical angles or slope, the tilting lock is loosened and the instrument is aligned on a target at the same height as the user's eye. While holding the instrument on the target, the level is tilted until the bubble is centered. Once the tilting lock is secured, the angle or slope can be read from the scale.
The accuracy of both hand levels and Abney levels is improved if they are used in conjunction with a stick or rod of known height. For example, if the centerline of the level is held on the 5.0-foot mark on a stick, the instrument height is 5 feet. Any difference in the rod reading at the unknown station from five feet is a difference in elevation. The use of a stick will also make it easier to hold the level steady.
The dumpy level is one of the simplest types of surveying levels mounted on a tripod. The use of a tripod improves the accuracy of the instrument and provides a reference for horizontal angles. A dumpy level consists of a telescope with a spirit level mounted in parallel. The telescope will have at least one horizontal cross-hair, mounted inline with the line of sight, and may have a vertical cross-hair and two stadia cross-hairs (see Figure 2-11).
The telescope and spirit level are mounted on a mechanism (leveling plate) that rotates in a 360-degree horizontal circle. The entire mechanism is mounted on a plate that can be leveled and attached to a tripod.
Accuracy is insufficient for precise surveying, but is acceptable for general work such as leveling forms for concrete and shooting profiles for drainage work. A dumpy level will also contain a horizontal scale for measuring horizontal angles. The precision of the angle scale will vary because different manufacturers use different systems. Many have a precision of 1 degree, but a precision of 10 minutes is possible.
USING THE DUMPY LEVEL
The first step in using the level is to set up the tripod. The legs of the tripod should be spread until the tripod is stable and the head of the tripod is a good height for the operator. The points of the legs should be planted firmly in the ground. During this process, the head of the tripod should be as close to horizontal as possible. This will reduce the amount of time that will be required to level the instrument after it is mounted.
When the tripod is set, the instrument is mounted on the head. All instruments are attached by either a large diameter thread, 3.5 inches and 8 threads per inch, or a smaller diameter threaded bolt. During attachment, the base of the instrument should be loosened so one hand can grasp the instrument at all times as it is threaded onto the head of the tripod. This prevents dropping the instrument.
Once the instrument is mounted on the tripod, the next step is to level the instrument (set the telescope horizontal).The instrument is leveled by turning the four leveling screws in the correct sequence. This process requires several hours of practice before an individual can level an instrument quickly because dumpy levels use a four-screw, four-leg, leveling mechanism.
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LEVELING A FOUR-LEGGED INSTRUMENT
The first step in leveling the instrument is to align the telescope over any two of the adjusting screws. These two screws are turned in opposite directions until the bubble is between the lines on the spirit level vial. The direction the leveling screws should be turned depends on which way the instrument must be tilted to make it level. In Figure 2-12, the right-hand screw is turned clockwise and the left-hand screw is turned counterclockwise because the right side of the telescope needs to be lowered to level the instrument. The opposite rotation of the leveling screws would be used if the opposite movement were desired. Another way to remember which way to turn the leveling screws is to remember the left thumb rule. The bubble will move in the same direction as the left thumb when a leveling screw is turned. During this process, it is important to turn both screws at the same rate. Usually one hand of the operator is dominant, and this hand will tend to turn one screw faster. When this occurs, the adjustment can either become loose, in which case the telescope will teeter-totter on the base, or the adjusting screws will become too tight to turn. Forcing the adjusting screws too tightly can warp the adjusting mechanism and permanently damage the instrument.
The second step is to rotate the telescope 90 degrees and repeat the leveling process. These two steps may need to be completed several times before the instrument remains level as it is rotated. The instrument is level when the bubble stays within the lines on the spirit level as the telescope is rotated 360 degrees.
ADJUSTING FOR PARALLAX
Before using any instrument, with a telescope, it must be adjusted for parallax. Parallax is a condition that occurs when the line of sight of the eye is not aligned with the line of sight of the telescope. This will cause errors when reading a rod. Parallax is removed by adjusting the eyepiece until the cross-hairs are their darkest. Note: On some instruments, the eyepiece is adjusted by rotating the eyepiece and on others, it is adjusted by sliding the lens. If more than one individual is reading the instrument, the eyepiece must be adjusted each time a different person reads the instrument. Once the instrument is leveled and the eyepiece has been adjusted for parallax, the focusing knob is adjusted to bring the rod and/or target into focus.
TIP When using any surveying instrument, never force any movement. If a part is designed to move, but will not, do not try to force it. Instead, check to be certain that the locks have been released. A slight pressure is all that should be required to move any part of the instrument.
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An automatic level has all the features of a dumpy level but is easier to use. The term "automatic" does not mean that the instrument levels itself. Instead, with the combination of three leveling screws, instead of four, and a bull's eye spirit level, instead of a tube level, the user can set the level much more quickly. Once the instrument is nearly level, an internal compensator completes the leveling process and maintains the line of sight in a horizontal position. The compensator also prevents the instrument from being knocked out of adjustment by slight bumps. The effect of the wind is also minimized because the instrument can compensate for slight movement in the tripod (see Figure 2-13).
Automatic levels are available in several different models. Some are more accurate and more precise than dumpy levels, but all are less accurate and precise than transits and total stations.
A laser level is a level that uses a beam of laser light to establish the line-of-sight, reference line. Laser levels are one of the newest types of instruments. The different types can be divided into four categories: (1) single beam invisible, (2) single beam visible, (3) circular beam visible and (4) circular beam invisible. Circular beam lasers can also be categorized as rotating or nonrotating.
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A single-beam laser will produce a single dot or a short line. A circular-beam laser produces a 360-degree beam.
An invisible beam laser requires the use of a detector. The detector is used with a surveyor's rod and locates the laser beam. The pointer on the detector is used to read the height of the beam on the rod. Some use an electronic tone that beeps at a different rate when the detector is above and below the laser beam, and becomes a solid tone when the detector is on-line with the laser beam. Others use flashing lights in the same manner. Some use a display screen with a triangle that indicates the direction the detector should move to be aligned with the laser. Instruments may use a combination of these methods. The detectors for some laser levels also indicate if the laser is out of level. This feature reduces the chance of errors caused by an instrument that is not level.
One distinct advantage of laser levels is that they can be operated by a single person. The laser level is mounted on a tripod and leveled. Once turned on, the laser does not require any supervision. The surveyor can walk around the area and record rod readings anywhere within the range of the beam (see Figure 2-14).
Another advantage of this system is that multiple detectors can be used with a single laser. This allows more than one person to record data simultaneously.
Some lasers have the ability to establish a beam that is not horizontal. The advantage of this ability is being able to establish a reference line or reference plane at a desired angle. This is a handy feature when installing drains, grading land, and working with grades in two different planes.
Transits are the most complicated and precise mechanical surveying instruments. Consequently, they are useful for a wide variety of surveying jobs. Features of transits include a telescope that can be rotated 360 degrees horizontally and vertically, vernier angle scales that can measure in minutes and seconds, higher-power telescopes, and a magnetic compass.
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Transits are capable of measuring both horizontal and vertical angles with a high degree of precision. Even though total stations have replaced transits as the primary surveying instrument, they are still very useful when a high degree of accuracy and precision is required or when a total station is not available.
USING A TRANSIT
The process of setting up a transit is identical to that of a dumpy level. Before any readings can be taken, it must be leveled, using the four leveling screws; the eyepiece must be adjusted for parallax, and the telescope must be focused on the rod.
The primary difference is the higher level of care required to ensure that the transit is level. Transits have more capabilities than laser or dumpy levels, but they also have a greater possibility of error (see Figure 2-15).
One common error is failing to set the telescope at zero vertical degrees when using it for measuring elevations. Another common problem is reading vernier scales correctly.
Transits and other instruments use a vernier on the angle scales to provide a higher level of precision. A vernier is a smaller scale mounted to the side of the main scale.
Verniers are a mechanical means of increasing the physical size of the last unit on the main scale so that an additional level of precision is available. When reading a vernier, the first step is to determine the smallest possible reading, commonly called least count. This is accomplished by determining the smallest reading on the main scale and then determining the precision of the vernier. The lines on vernier scales are very fine, and the scale is usually small. Some type of hand-held magnification is useful.
Figure 2-16 is an example of a double vernier, and the degree-minute-second (DMS) angle scale that may be found on a transit. The angle scales are labeled at every 10 degrees and marked at each degree. Each degree is divided into three parts. The precision of the angle scale is 20 minutes; 1 degree (60 minutes) / 3 = 20 minutes. The vernier has lines numbered from 0 to 20 with no additional graduations. The smallest reading, least count, for this vernier is 1 minute.
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Figure 2-17 is an example of a single-vernier, angle scale that reads in decimal degrees (DD). In this example, each tenth degree is labeled, and each degree is not divided. The vernier scale goes from 0 to 10. The smallest measurement is 1 divided by 10 or 0.1 of a degree.
The steps for reading a vernier are included in Chapter 7.
Manufacturers of surveying equipment often develop instruments based on new technology or for specialized uses. Examples of these are electronic transits, transit levels, theodolites, and total stations.
Transits are distinguished by their ability to measure both horizontal and vertical angles and bearings. Transits also have the reputation of being hard to use because of the difficulty in reading the vernier scales. The electronic transit has the same angle capabilities, but it has been improved to provide the angle readings on a digital readout. This reduces reading errors and speeds up the process of collecting data. Some electronic transits also provide mounts for an EDM.
The terms construction transit and transit level refer to a group of instruments that have characteristics of both transits and levels. They consist of a dumpy level with a telescope that has a few degrees of vertical movement. This increases their capabilities, but they are not considered a transit because the telescope cannot be rotated in a full vertical circle.
A theodolite is very similar to a transit. It differs primarily in the level of precision in measuring angles. A good quality transit will measure angles to the nearest minute. A good-quality theolodite is capable of measuring angles to the nearest second. Theodolites may also be electronic.
Total stations are the instruments of choice for modern professional surveyors. A total station is a combination of an electronic transit and an EDM. They have optical cross-hairs and can still be used visually with a rod like a transit, but they display angle and distance readings electronically. They also use a built-in or detachable EDM. This allows multiple measurements, such as horizontal angle, vertical angle, and distance to be recorded simultaneously. The first EDMs used a reflector, a prism, to return the signal but some later models provide the option of reflectorless technology. The distance a total station can measure in reflectorless mode is dependent on the reflectivity of the object being used as the target. Reflectorless technology allows the convenience of single-person operation, but the distance that can be measured is larger and the precision is smaller if a reflector is used. The use of an onboard CPU enhances the ability of total stations. Many features can be designed into the instrument. Some include storage devices that can be used to record data. They can be programmed to measure in a variety of units, such as feet or meters for distance, and degrees, gon, and mil for angles. Tones can be used to indicate horizontal and vertical position of the telescope or the condition of the EDM, and they can even be designed to automatically compensate for refraction. A timesaving feature on most total stations is the capability to export information to a data logger or small computer. This eliminates the requirement of writing down the readings and the associated errors. The total station and the data-recording devices must be synchronized to ensure that the data is stored in the desired format.
Many different accessories are used to enhance the surveying process. The following sections discuss a few of these.
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Land surveyors have traditionally used special pins when measuring distances with a chain. A surveying pin is usually constructed from heavy gauge wire and painted in alternating white and red stripes. A set consists of 11 pins. When chaining, the individual at the rear of the chain places one pin at the starting point and gives the remaining 10 pins to the person at the head of the chain. The chain is stretched out and the head person places a pin in the ground at the 100-foot mark. The chain is moved 100 feet and the process repeated until the head person reaches the destination. As the chain is moved, the rear person pulls the pins set by the head person. When the head person is out of pins, the survey party has traveled 10 x 100 feet, or 1,000 feet. If the distance to be measured is greater than 1,000 feet, a notation is made in the notes that the pins have been transferred, the 10 pins are transferred to the head person, and the process continues.
During the measurement, the appropriate notations are recorded in the field book. Surveying pins should not be used to mark stations or for other purposes. If the number on the ring is incorrect, it could cause an error the next time they are used for chaining. Flags or stakes should be used to mark stations.
A field book is a notebook especially designed for recording surveying notes. The traditional field book, when opened, has a left half and a right half of a page, not two pages like a normal book. Data in field books must follow the four C's: complete, clear, concise, and correct. It is very important that the information be identified, well organized, complete, and legible. Identification is very important because a perfect set of information is useless if it cannot be identified with the site. Organization is also important. The reader should be able to find the desired information quickly, and easily understand the purpose for each number. Legibility is a necessity. If the notes are not readable, all of the resources expended to collect them were wasted. Legible notes also reduce the chance of errors when someone reads the notes. To meet these requirements, a standard form of organization and locating information on the page has been developed.
TIP A field book must be complete so that a person who was not part of the survey is able to find the same site, complete the same survey, and arrive at the same results.
STANDARD FIELD BOOK FORMAT
The front cover or first page in a field book is used for the owner's identification. The purpose of identification is to provide a means of returning the book in case it is lost or misplaced. A set of survey data can represent many hours of work and thousands of dollars in expenses, all of which would have to be repeated if the book is lost. The next page(s) contain the index. The index should be located in the front of the book and it should include:
Survey title Date of Survey Page Number(s)
If a book is used for several surveys, a page or series of pages is used for each survey (see Figure 2-18).The pages for individual survey should include:
* error checks
* page number
* weather information
* party names
* party jobs
* equipment list
* equipment identification
* sketch of survey
* benchmark description and location
* note keeper's signature
Survey Title Date of Survey Page Number(s)
The title used on the information page must be the same as the title used in the index. The title should be descriptive of the survey. For example, "Profile of drainage ditch." The location description in conjunction with the sketch must provide enough information to allow the reader to find the site. The data is the information that is recorded during the survey. It is important for the data to be well organized and labeled so that it is easy to read and understand. Any information in the data section that is not original should be labeled as such by using footnotes for equations, and so on. For example, when distances are measured by pacing the recorded numbers are in paces, but distances in paces are not usable. It is common practice to convert the paces to feet by using the individual's pace factor. These distances in feet are not original and should be identified with a footnote and the equation that was used for the conversion. The equation should be included at the bottom of the data table. Any mathematical error checks should be included with the data. An example is the note check used with differential and profile surveys. The page number coincides with the number used in the index to aid the reader in finding an individual set of data. Weather information is important for surveys. It may be needed to adjust for errors and may provide information useful for interpreting the data. Temperature data is required to calculate the temperature adjustment for a chain. Temperature and pressure data may be needed to set a total station. Party names and party jobs are included so that if the reader has procedural questions or any other questions about the survey he or she can contact the individual responsible for that part of the data. Some individuals prefer that icons be used for the party jobs. Figure 2-19 illustrates a few of these.
It is important to include the equipment list and equipment identification. This information may be useful for assigning equipment costs for the job. It is also important in case problems arise with the data. The instrument can be checked or calibrated to determine if it needs adjustments. The sketch is an important part of the notes. Sketches are not expected to be of draftsman quality, but they must be clear, concise, and easy to read. They are used to provide more information about the location and identification used for all stations, the location of the benchmark, and any other information that is pertinent to the survey. The station identifications used in the sketch must be the same as the station identifications used in the data section. Along with the sketch must be a description of the benchmark. This description must be precise. A benchmark description of "telephone manhole cover at the corner of First and West streets" is not complete.
[FIGURE 2-18 OMITTED]
[FIGURE 2-19 OMITTED]
A description that reads "the benchmark used was an "X" chiseled in the rim of the telephone manhole located 10 feet south and 20 feet west of the corner of First and West Street" is complete. The sketch must include a north-facing arrow. The last thing that should be included in a set of notes is the note keeper's signature. The note keeper should not sign the notes unless he or she is sure that they are complete and correct. Standards of practice for different disciplines may require some differences in the location and methods of recording information.
Modern professional surveyors use electronic instruments that have the capability of recording data with a data logger or small computer. Computer programs organize the information into what is called an electronic field book. Even when data is collected electronically, it is important to collect weather data and party names, and to sketch the area.
A surveying rod is a distance measuring device that is used to determine the distance from the line of sight of the instrument to the ground. They were traditionally constructed of wood, but modern ones can be metallic or constructed from other nonmetallic materials. Standard surveying rods are essentially 13-foot wooden rulers. The primary difference between them is the scale used to record the measurements. Rods can be read to the nearest 1/100th (0.01) of a foot directly and to the nearest 1/1,000th (0.001) when the target is used (see Figure 2-20).
The Philadelphia-style rod can be used in four ways: direct reading, indirect reading, extended rod, and high rod.
DIRECT READING A ROD
Direct reading can occur when the distance between the instrument and the rod allows the rod to be read by the person looking through the instrument. When direct reading the note keeper stays with the instrument and records the rod readings as they are called out by the person on the instrument.
INDIRECT ROD READING
As the distance between the instrument and the rod increases, a point will be reached where the person at the instrument will not be able to read the numbers on the rod. In this situation, the person holding the rod must use a target, a pencil, or some other object to indicate the coincidence of the line of sight and the rod. The person on the instrument has the rod person slide the object up or down until it is aligned with the cross-hairs of the instrument. The rod person reads the rod and calls out the results to the note keeper. Indirect rod reading requires the note keeper to stay with the rod. Indirect reading is also used when the desired level of precision requires the use of the target. The vernier scale on the target provides one additional level of precision. Surveying targets are described in more detail in a next section.
[FIGURE 2-20 OMITTED]
EXTENDING THE ROD
The rod must be extended when the change in elevation is great enough to cause the line of sight to pass over the top of the rod. An extended rod must be read directly from the instrument. If the distance is too great to read the rod directly, a high rod must be used. This is explained in the next section.
A common error in using the extended rod is failure to extend it all of the way and ensure that it locks in place. An extended rod is very difficult to impossible to hold vertical in windy conditions. In windy conditions, it is prudent to shorten the distance by using a turning point until the rod can be retracted.
USING A HIGH ROD
The rod is used in the high condition when the line of sight extends above the rod, and the distance between the instrument and the rod makes it impossible to read the rod directly. The high rod technique requires the use of a target. In the high rod technique, the target is attached at the 7.0-foot mark and locked in place. The instrument person signals the rod person to extend the rod until the target is aligned with the line of sight of the instrument. The rod person locks the rod in place and lowers it for reading. Using the high rod technique requires reading the rod on the backside. On a Philadelphia-style rod the scale runs up the front and continues down the back. A vernier scale is mounted on the backside of the rod. When the rod is retracted, the vernier scale on the backside of the rod will read 7.0 feet. As the rod is extended, distances greater than 7.0 feet are indicated on the backside. The zero point of this scale is used to read the rod to 1/100 of a foot, and the vernier scale can be used to read the rod to 1/1000 of a foot.
READING A ROD
A surveyor's rod is read differently than a carpenter's tape. A carpenter's tape is read by recording the measurement from the closest line on the tape. When reading a surveyor's rod, the number of transitions from black to white must be noted. Each edge of each black line is one unit, as shown in Figure 2-21.
Common errors in using rods include incorrect rod reading, transposing numbers, and failing to hold the rod vertical. Practice is the best method of controlling reading errors. A rod can be out of vertical in two directions, parallel with the line of sight and perpendicular to the line of sight. The person on the instrument can determine if a rod is out of vertically perpendicularly because they can compare it to the vertical cross-hairs in the instrument. The person on the instrument can not determine if the rod is not vertical with the line of sight. The rod holder must control this potential error. The rod holder can control this error by using a rod level or by rocking the rod. A rod level is a bull's eye or tube type of level mounted on a short frame that fits against one corner of the rod. It can be permanently mounted, but the rod holder usually holds it in place. When rocking the rod the rod holder slowly leans the rod toward the instrument and then slowly brings it past center and back toward them.
The practice of rocking the rod is based on characteristics of right triangles. As the illustration in Figure 2-22 shows, the shortest rod reading occurs when the rod is vertical.
The person at the instrument watches the rod readings as the rod is rocked. As the rod is moved off vertical, the rod reading increases, and as the rod moves towards vertical, the readings decrease. The correct reading is the minimum reading. It will take some practice for a survey team to determine the best speed at which to rock the rod and the optimum distance to move the top of the rod.
A target is a round or oval device with a rectangular hole in the center and a means to clamp it to the rod. The face of the target is divided into alternating red and white quadrants and will contain a vernier scale. It may also include a fine adjustment lever. Targets are used when the distance between the instrument and the rod makes it impossible for the instrument person to read the number on the rod or when precision to 1/1000's a foot is required. Figure 2-20 (p. 32) illustrates a target attached to a rod.
[FIGURE 2-21 OMITTED]
[FIGURE 2-22 OMITTED]
Range poles (See Figure 2-23) are 5 to 6 foot tubes or rods with a solid, sharp point on one end. They are painted in alternate red and white wide stripes. Range poles are used to provide a visual reference point. They are useful for staying on line when chaining and for marking stations when turning angles with a dumpy or automatic level.
Stakes, flags or other means of establishing stations are necessary accessories for surveying. Stakes are commonly used to establish a turning point or when a semipermanent known elevation is needed. They can also be used to identify the location of stations. Stakes may be 1 1/2 inch to 2 inches square and 18 to 24 inches long, or an oak lathe depending on the desired durability and use. It is a common practice to sharpen one end of stakes to aid in driving them into the ground. Information can be written on stakes, and different colors of paint or ribbons can be used to indicate a specific class of stakes, for example, a centerline or grade line. Surveyors also have the option of using metallic or other nonmetallic markers.
Flags are commonly used to show the route of the survey or stations where rod readings were taken on the ground. Flags of various colors can be used to identify different routes. It is also common practice to use both a stake and a flag at each station. The stake can be used to provide or establish a known elevation or turning point, and a flag or ribbon is used to make the spot more visible.
Surveyors have traditionally used special nails to accurately mark a station when turning horizontal angles and measuring distances accurately by chaining. Surveyor's nails have a cupped head which increases the accuracy when aligning plumb bobs with the nail. The use of nonwooden stakes has eliminated the need for surveyor's nails (see Figure 2-24).
Plumb bobs are slender cone-shaped devices designed so that a string can be attached at the center. They are used to establish a vertical line. Vertical lines are useful when transferring a point vertically from the earth to the chain when chaining horizontally and for setting an instrument over the vertex of an angle. Good-quality plumb bobs are constructed of brass, have replaceable points, and, if cared for properly, should last several lifetimes. The string used to suspend the plumb bob should have braided fibers, not twisted ones. In addition, a sliding lock on the free end of the string eliminates the need for tying knots in the string. Any knots that are used must be removed as soon as possible and should be designed so they do not become too tight to be removed (See Figure 2-25).
[FIGURE 2-23 OMITTED]
[FIGURE 2-24 OMITTED]
[FIGURE 2-25 OMITTED]
Good communication is always a necessity for a survey crew. When technology for measuring distance was limited to chaining and instrument sight distances were limited to less than 400 feet, verbal communication and hand signals were effective. Modern equipment has extended distances beyond what can be spanned by verbal communication and hand signals. Modern equipment is capable of measuring distances exceeding 1,000 feet. The communication method of choice for modern surveyors is the two-way radio. Two-way radios can save many steps and reduce the time required to complete the survey. In addition, they reduce the chance of errors caused by individuals shouting at each other, and they allow the transfer of information that cannot be communicated with hand signals.
In this chapter, the common instruments and equipment used for surveying have been discussed. A good understanding of the names of instruments and equipment, their parts, and how they are used is a requirement of surveying. Attempting to use the wrong piece of equipment for the job and/or using a piece of equipment incorrectly will lead to errors in the data and damage equipment. The following chapters will explain how this equipment is used to complete common surveys.
1. Identify the parts for the dumpy level.
2. Identify the parts of a transit.
3. Determine the reading for the chain in the illustration.
4. Determine the reading for the chain in the illustration.
5. Record the rod reading to 0.01 and 0.0001 feet for the rod and target in the illustration.
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|Author:||Field, Harry L.|
|Date:||Jan 1, 2004|
|Previous Article:||Chapter 1 Principles of land measurement and surveying.|
|Next Article:||Chapter 3 Rectangular system of land identification.|