About Grid Locators and Grid Squares
An instrument of the Maidenhead Locator System (named after the town outside London where it was first conceived by a meeting of European VHF managers in 1980), a grid square measures 1° latitude by 2° longitude and measures approximately 70 × 100 miles in the continental US. A grid square is indicated by two letters (the field) and two numbers (the square), as in FN31, the grid square within which W1AW, ARRL's Maxim Memorial Station, resides. Each subsquare is designated by the addition of two letters after the grid square, as FN44ig. These more precise locators are used as part of the exchange in the 10-GHz contest. They measure 2.5 minutes latitude by 5 minutes longitude, roughly corresponding to 3 × 4 miles in the continental US. The Lab Notes column in April 1994 QST covered grid squares in particular and geographical coordinates in general.
How you go about finding your latitude and longitude depends to some extent on whether you want to know your 4-digit grid square or your 6-digit grid square. Finding a 6-digit grid square requires considerably more precision in latitude and longitude. The 4-digit grid squares cover a pretty big area, while the 6-digit grid square is fairly small.
The actual size of a grid square depends on the particular location, as the 1° of longitude represents different distances at different latitudes; longitude lines are closer together at the Earth's poles than at the equator. The first step, then, is finding your latitude and longitude as accurately and precisely as you can.
There are several common methods for doing so:
*Lookup your Lat/Long online at the US Geological Survey GNIS Web site. Enter your town in the "Feature Name" box and select the appropriate state before clicking on the "Send Query" button. It will return a list of features that match the name you entered, along with the lat/long for each one.
*Use a topographic (or topo) map. These maps contain both elevation data, by means of countour lines, and location via coordinate systems. Topo maps of US areas are available from the US Geological Survey (USGS). The most useful series of maps for finding latitude and longitude are the 7.5-minute maps, each of which covers an area of 7.5 minutes of longitude by 7.5 minutes of latitude. Most of these are at a scale of 1:24,000 (1 inch = 24,000 inches, or 2,000 feet).
These maps usually include two coordinate systems to define location. One system uses latitude and longitude-- this is what we want. The other system is the universal transverse mercator (UTM) system that consists of squares 1 kilometer on a side. For finding your grid square, use only the latitude and longitude system.
An online topo map is available at the U.S. Census Bureau Web site. To obtain the 7.5-minute map that covers your location, you need to determine the reference code, map name and state. You'll find this information in the map index for your area. Contact USGS for information about pricing and how to order topo maps.
Many outdoor, sporting-goods and camping stores also sell USGS 7.5-minute maps. Check your local Yellow Pages.
*Use a Global Positioning System (GPS) receiver.
*Call the local library or town hall. They'll often know their own latitude and longitude. If you don't live too far away from the library or town hall, that may be sufficient.
*Use the ARRL Locator. This Web page will let you enter a city and state and will return the location-- including the latitude/longitude and 4-digit grid square). This is sufficiently precise for finding 4-digit grid squares but not for 6-digit grid squares.
*FCC's Degrees, Minutes, Seconds and Decimal Degrees Latitude/Longitude Conversions page. A utility for converting between decimal degrees and degrees, minutes, and seconds. The page also has a link to convert between NAD83 / WGS84 coordinate system and the older NAD27 coordinate system used by the FCC for broadcast authorizations.
You can determine your grid square by entering your geographical coordinates into a computer program.
Several such programs are available for the PC:
- gridloc.zip--GRIDLOC, which runs from gridloc.bas (gridloc.doc is also included in the ZIP file) to calculate Maidenhead grid square and coordinates
- grid.zip--Contains GRIDX, a BASIC program that calculates grid squares and Great Circle headings and grid.exe, a memory-resident pop-up grid locator calculator