Digitizing Methodology and Public Land Survey Data
Digitizing Methodology and Public Land Survey Data Quality:
Creation of the 1:24,000 Scale Public Land Survey Database
The Public Land Survey Database was created using software designed specifically to capture projected map data from USGS source maps. The digitizing software was developed by engineers at Platte River Associates, Inc., a leader in developing geological modeling software for the oil and gas industry. Associated processing software was developed by Robert C. White Jr., President and CEO of The WhiteStar Corporation. The use of this software, combined with extensive quality control procedures, has produced a database that not only accurately represents the source maps, but conforms to the rigorous National Map Accuracy Standards for 1:24,000 scale maps, and is in widespread use throughout the oil and gas industry.
Using 7.5-minute U.S.G.S. quadrangles (1:24,000 scale) as a source, section corners are digitized with reference to the actual projection specified on each map. Because the U.S.G.S. has published maps using different projections, digitizing software must account for these differences in order to obtain a consistent, seamless database with the highest levels of accuracy.
For a variety of reasons, off-the-shelf software is unsuited to this task. For example, sections share common boundaries which make them difficult to capture accurately with a CAD system. In addition, adjacent sections often share tiny offsets which are erroneously eliminated by popular Geographical Information Systems as part of their "clean line work" procedure.
With regard to digitizing, each topographic map is registered with eight points on the digitizing table. The operator then keys in the map projection used to generate the paper map. This is a crucial step. If a digitizing program does not take into account the map projection, the positional quality of the data is degraded.
The digitizing software then returns a number representing the degree of fit of the control points. If this number is anything less than 99.9999% of the control points' theoretical location, then the operator must re-digitize the control points and double check the map's projection. Another purpose of using so many control points is to account for and eliminate any paper stretch that routinely occurs on all paper maps due to changing humidity and temperature. Section lines digitized from maps using fewer control points will suffer from the undesirable drift effect as one moves away from the edges of the map.
Once the map is properly registered, each polygon on the map is digitized. This means that each section is digitized four times. The software then computes a spatial average for each corner monument. The software flags any point falling outside an error radius of forty feet, while the map is still on the digitizing table, so that it can be re-digitized immediately.
Following the digitizing process, the maps are assimilated together and inspected graphically for any other possible errors. Proprietary software generates the township boundary, which provides an additional error-checking step. Every effort is made to see that the data is absolutely clean and topologically structured - no gaps or overlaps.
The techniques discussed above are the most advanced being used in the industry today and were used to create the entire database. The systematic use of this technology has produced the most consistent and accurate public land survey database that can be derived from 7.5-minute U.S.G.S. quadrangle maps. The 1:24,000 scale Public Land Survey Database is compatible with all other sources of digital data that conform to the same levels of accuracy described in this document.