Digitizing
Methodology and
Public Land Survey Data Quality:
Creation of the 1:24,000 Scale
Public Land Survey Database
Developed
by Platte River Digital Cartography
A division of the WhiteStar Corporation
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 linework" 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 redigitized 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.
© 1999 WhiteStar Corporation
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Lakewood CO 80226
303-781-5182
