Digital Mapping Techniques '00 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 00-325

The U.S. Geological Survey's Revision Program for 7.5-Minute Topographic Maps

By Larry Moore

U.S. Geological Survey
Mid-Continent Mapping Center
1400 Independence Road, MS 509
Rolla, MO 65401
Telephone: (573) 308-3661
Fax: (573) 308-3652
e-mail: lmoore@usgs.gov

ABSTRACT

Various processes are used to revise these maps. Some revisions use traditional analog processes, some use digital processes; some work is done by USGS employees, some by contractors. There are four main categories of map revision: minor, basic, complete, and single edition. Minor revision is done on maps that have few changes since the last revision; it includes boundary updates and corrections of previously reported errors. Basic revision updates features from digital orthophoto quadrangles (DOQ) and aerial photographs. Contour update is an optional part of basic revision and is not often done because of the high cost. Complete revision of all layers is seldom performed because of the high cost. Single-edition revisions are done by the U.S. Department of Agriculture Forest Service using procedures similar to basic revision.

The current revision program was not designed to do replacement mapping. Most map revision is done from remote and secondary data sources, including the following:

• Geometry is controlled and some feature content interpreted from DOQ's.
• Most feature content is interpreted by using stereophotographs from the National Aerial Photography Program.
• Boundary and name information is collected from Federal databases, other maps, and State and local agencies.
• Some content may be field checked by Earth Science Corps volunteers (private citizens who donate time to do field verification work) or by State agencies participating in cooperative mapping projects.

INTRODUCTION

The USGS produces printed maps and digital map data for all States, possessions, and territories of the United States, and Antarctica. This paper discusses only the 1:24,000- and 1:25,000-scale topographic maps in the 48 continental United States. There are 54,890 standard 7.5-minute and 7.5- by 15-minute cells in this domain. Because the two cell sizes overlap, the number of map sheets has varied with time. At present, there are 53,336 map sheets that cover the continental United States. Both cell sizes and scales are referred to in this paper as "7.5-minute maps" or "7.5-minute quadrangles."

The 7.5-minute maps are more detailed versions of other quadrangle series that date back to the formation of the USGS in 1879 (Schwartz, 1980, p. 311). Although 7.5-minute maps were produced by the USGS as early as 1908, the effort to cover the country at this scale was a product of World War II technological advances and 1939 legislation creating a National Mapping Program (Bohme, 1989, p. 167). Initial coverage of 7.5-minute maps in the continental United States is summarized in figure 1. The program grew rapidly from 1945 through 1955, then more slowly, and peaked in 1973. In the early 1980's, it became evident that production rates were not sufficient to finish the series before the year 2000. Beginning in 1982, manuscript maps without final cartographic finishing were published (Bohme, 1989, p. 167). These were designated "provisional maps" (P-maps). A significant production increase in the mid-1980's resulted from the lower cost of provisional mapping (fig. 1). Most of the work on the 7.5-minute maps was finished by 1990, and the series was officially declared complete in 1992.

Figure 1. Original production of 7.5-minute quadrangles. Each data point is the number of quadrangles published in a particular year. Each cell is shown only once, the first time a map for the cell was made. The median date of printing is 1972. The smooth gray curve is a polynomial trendline. The data for this and the other figures in this report are from National Mapping Division databases, including the map catalog (MAPCAT) and the assignment management system (AMS).

MAP REVISION PROGRAMS AND METHODS

Figure 2. Currentness of revised maps compared to original maps. The solid line is identical to that in Figure 1, except it is shifted 5 years to the left to show average date of content rather than date of printing. The dashed line shows the date of content for the most recent revision of each cell. For example, 1981 is the source photography date for the most recent revision of about 3,000 quadrangles. The median currency date for original mapping is 1967; the median for latest revisions is 1979. The data include minor revisions but not maps reprinted łas is˛ to replace low shelf stock.

With the completion of the 7.5-minute mapping program in 1992, the USGS began formulating a graphic revision plan to keep primary series maps current. Decisions about revising 7.5-minute quadrangles are based on user requirements, available resources, and the preferences of funding cooperators. Accuracy assessments, evaluations of existing quadrangle materials, and error reports are also considered. Two primary drivers of the NMD revision program are listed below.

• Cooperative funding from other agencies. The USGS will divide revision costs equally with other State or Federal agencies.
• A list of 5,000 "high seller" maps. These maps are judged to be most in demand and are given priority for revision work. A percentage of these maps are revised each year with or without cooperative funding.

Revision decisions are also constrained by other factors. The most important of these is the availability of recent aerial photography and digital orthophoto quadrangles (DOQ) for the quadrangle under consideration.

Figure 2 shows the overall currentness of the 7.5-minute maps at the end of 1999. The median currency date for the series as a whole is 1979, so the average 7.5-minute map is almost exactly 20 years old. The data in this figure include all photorevisions and minor revisions but not maps reprinted "as is" to replace low shelf stock. The curve falls rapidly toward zero as it approaches the year 2000, but this does not indicate that the revision program is dying. Aerial photographs and other source materials used for map revision are usually 3 to 5 years old by the time the map is published, so most maps printed in 1999 appear in the years 1994 to 1996 in figure 2.

There are currently four official types of map revision: minor revision, basic revision, complete revision, and single-edition revision. The first three are defined by USGS product standards, the fourth by an interagency agreement with the U.S. Department of Agriculture Forest Service (FS). Numbers of each type of revision produced from 1996 to 2000 are shown in figure 3.

Figure 3. Currentness of revised maps compared to original maps.

Minor Revision

Basic Revision

Even though it depends almost entirely on remote sources, basic revision is not cheap. Basic revisions done with USGS Government labor in 1998 and 1999 required an average of 280 hours per quadrangle, or approximately $17,000. Although costs for contractor-produced revisions in 1999 were comparable, they are expected to decrease as contractors gain experience with USGS standards.

Complete Revision

Single Edition

DATA SOURCES

Aerial Photographs and Digital Orthophotos

DOQ's are made from NAPP photographs, and basic revision compilers also use stereopairs of the original photographs to assist with feature interpretation. The current NAPP plan calls for full coverage of the continental United States in 7 years (1997-2003). This schedule is subject to availability of funding, including State cooperative funding (U.S. Geological Survey, 1996).

It is not necessarily the case that a DOQ made from the most recent photography exists. The NAPP, the DOQ program, and the map revision program are not closely coupled; each has its own customer base and its own funding sources. Nonavailability of recent aerial photographs, a recent DOQ, or the control needed to make a DOQ can make it impossible to revise a particular map.

The photographs for the original 7.5-minute program usually had scales that range from 1:15,000 to 1:25,000. The NAPP photographs used for revision have an average scale of approximately 1:40,000. The smaller scale has some effects on the accuracy of the revision, especially on contour updates.

Other Government Agencies

Geographic Names Information System

Earth Science Corps

CONTOUR UPDATES

The USGS map revision programs have always assumed that topography is much more stable than planimetry. A new road or subdivision disturbs the land surface slightly, but rarely is the disturbance enough to warrant major revision of contour lines with 10-, 20-, or 40-foot intervals. The current map revision program is explicitly tied to DOQ's, and contours cannot be revised from these monoscopic images.

Basic revision follows these guidelines for revising contours:

• Contours are revised only as part of joint funding agreements; that is, only when another agency is willing to share the cost. Revising contours can increase the cost of a revision by 50 to 100 percent.
• The contour overlay is not completely recompiled but rather is updated in areas of significant topographic change. The original map base is used for vertical control.
• In areas of insignificant topographic change, "logical contouring" is used to preserve registration with other features. For example, contours are squared across new roads and routed around new ponds without stereorecompilation.

Contours are revised with NAPP stereophotographs, which are usually smaller scale than the photographs used to compile the original contours. Therefore, improving the accuracy of existing contours is usually not possible except in areas of very significant surface disturbance. This is consistent with the overall objectives of the revision program, which are to maintain the horizontal and vertical accuracy of the existing map.

Most basic revisions do not include contour updates (fig. 3), which means that the topography and planimetry on the revised graphic have different currentness dates. In some cases, this leads to glaring visual artifacts, such as contour lines in large water bodies or new islands with no topography.

ACCURACY OF REVISED MAPS

Contours and spot elevations also were originally compiled to meet the NMAS. At present, the USGS has no testing program to systematically evaluate the vertical accuracy of either the original or revised map. When there is some external reason to believe that contours may not meet NMAS, attempts are made to evaluate the data against independent and higher order control. Significantly improving the quality of contour data is extremely difficult because of the nonavailability of large-scale aerial photographs and vertical control that is independent of the original map base.

CONCLUSIONS

RELEVANT WEB SITES

• USGS Topographic Map Information -- http://mcmcweb.er.usgs.gov/topomaps/

• Digital Orthophoto Quads (DOQs) -- http://mcmcweb.er.usgs.gov/status/doq_stat.html

• National Aerial Photography Program (NAPP) -- http://edcwww.cr.usgs.gov/napp/napp_examples.html, http://mcmcweb.er.usgs.gov/status/napp_stat.html

• Geographic Names Information System (GNIS) -- http://mapping.usgs.gov/www/gnis/

• Earth Science Corps -- http://interactive.usgs.gov/Volunteer/EarthScienceCorps/

REFERENCES

National Research Council, 1990, Spatial Data Needs: The Future of the National Mapping Program: National Academy Press, Washington, D.C., 88 p.

Schwartz, S.I. and Ehrenberg, R.E, 1980, The Mapping of America: New York, H.N. Abrams. U.S. Geological Survey, 1996, The National Aerial Photography Program (NAPP): U.S. Geological Survey, 2 p. [fold out brochure].

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