Indiana is in the midst of a data collection program that will yield high-resolution elevation data for the entire state. There are no teams of surveyors using transits to sight in locations on the ground or calculating elevations with barometers; now geographic information specialists use remote sensing data collected by aircraft. The technology is known as LiDAR — Light Detection And Ranging. LiDAR sensors operate on the same principle as sonar or radar, but instead of aiming sound or radio waves at an object, light (laser) beams are used. It is possible to accurately measure small objects at a great distance and, in this instance, the objects are features on the Earth.
Historically, Indiana's statewide elevation coverage was provided primarily through the U.S. Geological Survey, which, for nearly a century, has produced its popular topographic 7.5-minute quadrangle map series in a convenient printed format at a scale of 1:24,000 (1 inch on the map representing 2,000 feet on the ground). On Indiana maps, contour intervals typically are 10 feet. The new high-resolution elevation LiDAR data set for Indiana will permit much greater positional accuracy and contour intervals of 2 feet.
LiDAR technology research began in the 1970s with efforts by NASA to measure atmospheric and oceanic phenomena from satellites. Only recently, however, has the convergence of global positioning systems (GPS) navigation, laser technology, and high-capacity and inexpensive computing allowed for airborne LiDAR instruments to be widely available for topographic mapping.
Statewide coverage of this nature is unusual, and few states in the nation have it. In Indiana, the data are being collected over a 3-year period from 2011 through 2013. For this coverage to be possible and affordable, the elevation data are being collected sequentially in successive years in three north-to-south swaths, beginning first with the central part of the state (2011), the eastern part (2012), and finally, the western part (2013). In addition to digital elevation data, a separate aircraft is gathering high-resolution color photographs with a resolution of 12 inches; the option is available for counties and cities to purchase 6-inch or even 3-inch resolution imagery. For the aerial photographs to be most useful, they are collected after winter snow has melted and before the leaves emerge on trees. Consequently, the state is being "flown in a leaf-off mode" in March or April.
Complete aerial photographic and LiDAR coverage for the entire state is understandably expensive. Moreover, ensuring that the coverage is total and of sufficiently high quality is a time-consuming process. Nevertheless, through a partnership involving many state and federal agencies and some private sponsors, the 3-year $4.6 million project is coming to fruition. The coordination of the resources available for the funding, the data collection, quality control, and delivery of the data were provided by the State Geographic Information Office and the Indiana Geographic Information Council, a nonprofit partnership composed of state, municipal, local, federal, business, and private members interested in the use of geospatial data. When the final photographic imagery and elevation data are delivered in December 2013, coverage of Indiana's 36,000 square miles will be freely available over the Internet to everyone.
The physics behind LiDAR technology is easy to understand. Pulses of laser light (light constrained to a particular frequency) are aimed at an object, and the time it takes for the reflected light beam to return to a sensor is measured. The time is then converted into distance. Because laser pulses travel at the speed of light, any slight difference in two successive pulse returns is almost imperceptible, yet measurable. That difference represents a change in distance or, in the case of the Earth, a change in elevation. For a large area to be rapidly assessed, an airplane flying at about 5,000 feet is used, with a laser shooting up to 150,000 pulses per second “sweeping” the ground. Some of the light pulses are scattered in different directions by irregular reflecting surfaces, but the time it takes those light signals that do return to the aircraft's sensors, is measured along with intensity. Surfaces above the ground, which are also “seen” by the laser pulses, may also be detected, such as trees, other vegetation, rooftops, and even power lines.
All these data require sophisticated instrumentation to accurately note the precise location of the aircraft, the moment of laser pulse firing, and the time and intensity of reflected light beams. The multitude of reflecting surfaces define a “point cloud” of data. Nevertheless, each point of data in the cloud can be defined by its location (x and y on the Earth), its elevation (z), and its intensity (i). Collectively, and when they are suitably processed, this point cloud can then be sorted and displayed. Those signals which return to the aircraft sensor's first are closest to the aircraft and are called “first returns.” They usually represent the closest objects to the airplane, such as treetops, rooftops, or tower tops. Sometimes even birds in flight are recorded. Successive returns closer to the ground are reflections from branches and leaves and are called second and third returns. The last reflection is called the “last return,” and it usually represents the ground surface.
When processing the multiple returns, it is possible to filter out parts of the point cloud that are of little or no interest. For example, if one were interested only in the elevation of the Earth's surface features, the early returns can be subtracted to render a bare Earth image. The elevation of the points showing the bare Earth can be used to derive a digital elevation model (DEM). What is remarkable about the LiDAR data for Indiana is the elevation resolution: It is about 7 inches (18.5 cm). Previous DEMs for Indiana, which were produced using best available technology at the time, were 100-foot (30-meter) and 30-foot (10-meter) DEMs. Hence the resolution in the accuracy of elevations continues to markedly improve with each technological advance.
With height (elevation) resolution of this accuracy, the applications are many, some of which include, in alphabetical order:
- Climatology (for example, ice surface mapping)
- Energy industry assessment and monitoring
- Geomorphology (terrain mapping and landform analysis)
- Hazards planning (for example, flood-plain mapping)
- Land-use planning
- Mining and quarrying
- Resource assessment and management
- Transportation planning
At the Indiana Geological Survey, LiDAR data promise to provide investigators with an entirely new look at the Earth, very much like putting on a strong pair of glasses and seeing surface details sharply for the first time. Bare-earth images, DEMs, will allow mappers to trace faults where they were once obscured by vegetation. Similarly, areas of past (and possibly future) landslides will be revealed through geomorphic analysis of steep slopes. Likewise, subtle variations in elevations along river and stream channels will show where water flowed and sediments were deposited in the recent past. Most importantly, LiDAR DEMs will display the geology of the state in a whole new format: Differences in rock strata may be detected through the integration of LiDAR intensity and elevation data. This means that the geologic mappers of Indiana will have a much firmer impression of the geology in the lab before they put boots on the ground to verify their analyses in the field.
Remotely sensed LiDAR data are providing Hoosiers with an exciting new and affordable technology with a multitude of research and practical applications, which we are only now beginning to explore.