Moving SOIL SURVEYING
Into the Digital Age

John Beck, Joey Shaw, and Jim Hairston

Many people are familiar with soil survey reports published by the U.S. Department of Agriculture’s Natural Resources Conservation Service (USDA-NRCS). Land grant universities, such as Auburn, have played a large role in the evolution of the National Cooperative Soil Survey (NCSS), which inventories soils and helps us better manage our land resources. Now, the AAES is facilitating the movement of the NCCS into the digital age.

The NCSS is a nationwide partnership of federal, regional, state, and local agencies and educational institutions. These partners work together to investigate, inventory, document, classify, and interpret soils. This information is disseminated through the publication of soil survey reports. A soil survey report consists of descriptive text, tabular data, interpretation records, and maps depicting soil boundaries.

Increased use of electronic data has prompted Congress to mandate soil survey information be placed into digital format by 2002. To accomplish this mission, NRCS must update (recompile) and digitize existing soil survey information. AAES researchers have developed a partially automated technique to facilitate recompilation and digitization of published soil survey reports. This process has been shown to work well in the Alabama Coastal Plain.

In the past, soil surveys were used independently to assist farmers and land managers in natural resource management. Today, these reports are used in conjunction with other data sets within a Geographic Information System (GIS) to provide information for agriculture and forestry applications, land-use decisions, and urban planning. Geographic Information Systems are computer hardware/software systems designed for interpreting and analyzing geographically referenced data. Geographical Information Systems are often thought of as automated mapping systems, although they are more accurately described as a powerful set of automated tools for collecting, storing, retrieving, analyzing, transforming, and displaying spatial data.

Many soil survey reports have been published at scales different than most national and state data sets. Hydrology, topology, and utility data sets are usually published at a scale of one inch equals 24,000 inches on the ground. In contrast, the maps in many soil survey reports have been published at scales larger (more detailed) than 1:24,000. In addition, most soil survey maps are published on outdated and distorted aerial photographs. These differences can create problems.

New soil survey maps are digitized on 1:24,000-scale georeferenced aerial photographs, commonly known as digital ortho-quadrangles (DOQs). However, many existing soil survey maps need to be updated and digitized to DOQs. Currently, only 15 of 67 Alabama counties have soil survey data available in digital format. An automated method to facilitate the development of 1:24,000-scale digital soil data from pre-existing soil survey reports would greatly accelerate Alabama’s digitizing initiative. To accomplish this task, AAES researchers evaluated the utility of using remote sensing (RS) and GIS software to transfer map units from the distorted 1:20,000-scale aerial photobase to 1:24,000-scale DOQ imagery. The soil survey report from Russell County was used to test this method.

Russell County, located in the southeastern part of Alabama, is approximately 413,940 acres in size. Portions of the Southern Coastal Plain and the Blackland Prairie (Major Land Resource Areas 133A and 135, respectively) are contained in Russell County. Common landscape features include pine forests, floodplains, stream terraces, pastures, and row croplands. To conduct this work, the USDA-NRCS provided the soil data and the DOQ imagery.

Individual soil map sheets (52) were scanned at a resolution of 800 dots per inch (dpi) to capture fine details in the aerial photobase. The digital soil maps were imported into the RS software and its corresponding DOQ was concurrently displayed on a computer screen. Ground control points (GCPs), or features easily identified and located on both images, were placed on each image. These GCPs were well distributed and included road intersections, distinctive water bodies, bridges, buildings, stream junctions, and isolated trees.

Coordinates from the GCPs were used to compute a polynomial transformation equation. A polynomial transformation equation corrects nonlinear distortions in the aerial photos and registers the image with geographic coordinates. In this process, a modified version of a 2nd order polynomial transformation equation was used that applied a series of transformations throughout localized regions of the image to minimize overall distortion. Research showed that this technique corrected the nonlinear distortions and provided a more accurate geo-registered product than a standard 2nd order polynomial transformation equation.

After the pre-existing soil maps were georeferenced, soil lines were digitized and transferred using standard routines combined with on-screen digitizing. Soil lines were verified and updated (recompiled) to match changes in landscape features. Errors were fixed using GIS software and individual vector coverages were appended to create a single coverage for the entire county. Individual polygons (a closed line or set of lines representing a single map delineation) were labeled by placing soil map unit symbols into the appropriate polygons (see the figure).

The methods developed and described by AAES researchers proved to be efficient and accurate for digitizing and recompiling soil survey maps. This technique also has other applications and can be used to digitize and transfer polygon units from any map published on a stable base. By applying these methods, the NRCS in Alabama has accelerated its soil digitization initiative. In fact, the digital database generated for Russell County has been certified and is available on the NRCS website (http://www.ftw.nrcs.usda.gov/ssur_ data.html).

Soil map units for Russell County, AL, labeled by Arcview GIS software.

Soil survey data are the most detailed and accurate natural resource inventories available to agricultural and land managers. In a spatially referenced digital format, these data can provide a wealth of information to Alabama citizens. Soil map unit, acreage, location, and resulting interpretations can now be accessed within a GIS. Soil map units can be linked to ancillary data and interpretations can be queried. Once queried, results can be displayed and disseminated by electronic media. This process will help ensure these data sets are utilized in the digital age.