Creating low-cost high-resolution digital elevation models

Ecologists and agronomists are interested in topography because it affects soil, plant, and hydrologic processes. Digital elevation models (DEMs) accurate to several centimeters of vertical elevation are needed but construction is time consuming and expensive when traditional surveying methods are used. Carrier-phase differential global positioning systems can map vertical changes in topography with root mean square errors (RMSE) of 2 to 9 cm, but equipment is expensive ($20,000 to $100,000). Coarse-acquisition code differential global positioning systems (C/A code-DGPS) are much cheaper (<$8,000) and widely available but vertical errors are large with root mean square errors of 100 to 200 cm, which severely limits their usefulness in ecological studies. We combined a coarse-acquisition code differential global positioning system and a laser level (<$1,000) to map topographic change in fields, wetlands, and research plots. Our technique uses the coarse-acquisition code differential global positioning system for longitudinal and latitudinal (X or easting, Y or northing) position while the laser level provides vertical position (elevation) as measured from a ground control point or monument. Measuring elevation across a field scale area is a 2-step procedure. At each sample location the distance from the laser level to the ground is determined and entered as a comment in the differential global positioning systems data logger. In the office, sample locations are differentially corrected and elevation is calculated by subtracting the laser level-to-ground distance from the elevation of the laser. Data is then imported to geographic information system (GIS) software that interpolates between points. The differential global positioning system yields X, Y locations with a root mean square error of between 0.5 and 1.0 m. Elevations measured with our laser level had an accuracy of better than 2 cm across its 230 m working radius. Our technique works best for areas up to approximately 40 ha on open, rolling terrain.