ArcView with the Spatial Analyst and 3D Analyst extensions are ideal tools to display agricultural field topographic data. By using a high-accuracy real-time kinematic (RTK) GPS receiver, accuracy in the range of 1 centimetre vertical is possible. A specially designed field computer was used for the field-based data collection system. The field computer uses the Esri Shape (SHP) and Dbase (DBF) file format for all data recording. These file formats allow for maximal usefulness of the data collected since the data files can be opened directly from ArcView.
The Topographic data layer in a farm GIS may be one of the most important layers for understanding the variability of yield over the field from year to year. The variation over a field is often correlated with elevation because of the strong relationship between soil moisture and plant growth. For instance in drier years, the low spots may yield more and in the wetter years the higher spots may yield more. In relatively flat farmland, the installation of drainage tiles is an important activity that requires accurate topographic data. When fields are irrigated, often they are leveled first, since some forms of irrigation require a level field, or a flat field with a constant slope. Another cause of variation in yield across a field related to elevation is that often soil type is correlated to elevation. Once the Topographic data is available it is important to have analysis tools like ArcView’s Spatial Analyst and 3-D Analyst to view and analyze the data.
A GPS topographic data collection system consists of an RTK (real-time kinematic) GPS receiver, an RTK Base station, a data radio link for the RTK Base station, and a Field computer with appropriate software to collect the topographic data in a useful format. Although it is possible to collect the RTK GPS data for post-processing, most users prefer real-time operation, in order to see that the data they are collecting is good, so they don’t have to go back and re-collect the data if a problem occurred while data collecting. With real-time corrections, any problems can be addressed immediately. Another reason for real-time operation is that the RTK GPS receiver can be used to guide the driver over the field with a lightbar to insure complete mapping.
A RTK GPS receiver is designed to collect and process carrier phase data, and is normally a dual frequency set. For more information how extremely high accuracy is obtained from a GPS set visit the Trimble web site at http://www.trimble.com/gps/diffgps/gpsfram1.htm The 3-D accuracy of an AgGPS 214 RTK receiver while operating with a nearby RTK Base station is on the order of 1 cm. A GPS receiver of this accuracy will collect extremely good topographic data. For comparison purposes, a high performance DGPS receiver that is not operating on carrier phase data will be capable of collecting vertical data with an accuracy of 1 to 2 meters. This DGPS accuracy is seldom good enough for a topographic map.
The field computer used to collect the GPS data can make the driver’s task much easier if it has a map display to show where the driver is on the field, relative to the edges of the field. The field display software can guide the operator across the field to make sure the topographic data is collected across the entire field. A map display is not necessary for the data collection for topographic data, however, it does make the field work a bit easier.
Since the user will be exporting the GPS data to a GIS, the file format chosen for the Trimble AgGPS 170 Field Computer is the shape file, allowing easy transfer to the office computer using compact flash data cards. As soon as the data is transferred to the office computer, it is immediately available for use by ArcView. Using ArcView and the Spatial Analyst and 3-D Analyst extensions allows for easy visualization of the field collected topographic data.
To demonstrate the capability of a GPS topographic field data collection system and the ease of use of the equipment and software, a demonstration project was initiated. In this project topographic data was collected, transferred to the office computer, visualized and plotted. The study site chosen was golf course greens at a local golf course.
In analyzing the surface of a golf green, sometimes the variation in height over the green is very large. In the middle portion of the greens, which is often the area of most interest for golfers, often have some small variations in topography that would be interesting to visualize. This is an easy task with the ArcView tools available. The edges of the green were cut off and the contours re-computed to maximize the display variations in the area of interest. As a practical matter, both the full plot and the expanded vertical scale pictures are useful when lining up a putt.