Basics of a Topographic Map
One of the most widely used of all maps is the topographic map. The feature that most distinguishes topographic maps from maps of other types is the use of contour lines to portray the shape and elevation of the land.
Producing a Topographic Map
Years ago, topo maps were compiled and sketched entirely by hand in the field. Later the final materials used for printing were laboriously etched or drafted from the field manuscript.
Today, the map manuscript is compiled in the office, primarily from aerial photographs using a complex process based on photogrammetry. For example, overlapping aerial photographs of an area are viewed through a steroscopic plotting instrument to produce a three-dimensional stereo model of the terrain. From this three-dimensional image, an operator constructs a map manuscript delineating contours, vegetation, drainage, manmade and other features. Fieldwork and research is conducted to assist in plotting map features not visible on aerial photographs and to collect and verify place names, political boundaries, public land corners, roads, water features, and building classifications.
Following any necessary editing, the compiled map manuscript becomes the model for the plates that will be used to lithographically (offset) print the topo map. The map manuscript is first photochmically reproduced onto numerous plastic sheets. A series of scribed sheets, each containing the map details of a single color, is then prepared from the multiple plastic sheets. A composite pressplate film for each color is then produced by photographically compositing the corresponding scribed sheets. A proof copy of the topo map is produced from the pressplate films and carefully edited before the map is printed in large numbers.
USGS topo maps are printed using a special set of ink colors, and features are separated by color. For instance, blue is reserved for water, green for forest, and only contour lines are brown. However, boundaries, text, and geodetic control points are all printed in black. Maps are printed using offset (lithographic) process.
Recent aerial photography is the primary source for map revision. Orthophotos, which are digitally scanned aerial photographs but are accurate like a map, have become an important revision tool.
"Map Separates"
Positive or negative film copies or "map separates" of any color, and in some instances map features, can be ordered from the USGS. The USGS topographic maps are printed using up to six colors (black, blue, green, red, brown, and purple). To prepare your own maps or artwork based on maps, you can order separate black-and-white film positives or negatives for any color printed on a USGS topographic map, or for one or more of the groups of related features printed in the same color on the map (such as drainage and drainage names from the blue plate).
After you receive a film separate or composite from the USGS, you can crop, enlarge or reduce, and edit to add or remove details to suit your special needs. Map separates may be used to prepare your own maps, or as a stable base for other overlaid information. They are also valuable source data documents for building data in a GIS. Compared to paper maps, film separates have much high quality details (sharper and denser lines, point symbols and names), and are more stable in changing temperature and humidity. Thus, they make excellent source materials for manual digitizing or electronic scanning.
For more information on map separates or for ordering visit USGS's web site at:
http://store.usgs.gov/
Map Projections
Different types of map projections are used as a foundation for topo maps. Projections convert the three-dimensionally curved surface of the earth into the flat two-dimensional map surface. However, the three-dimensional earth's surface cannot be transformed and portrayed on a flat map without introducing some distortion. Distortion caused by stretching or compressing the surface can affect shape, area, scale (distance), or direction, depending upon the projection used. Many map projections have been devised to minimize particular distortions, but no single projection gives an exact representation of the surface of the earth.
The two most commonly used projections for USGS topo maps are Lambert Conformal Conic and Transverse Mercator. Both of these projections have varying scale but retain the correct shape of the mapped surface. The Transverse Mercator projection is usually used for mapping areas (states) with a longer north-south extent. The Lambert Conformal Conic projection is usually used for mapping areas (states) with longer east-west extent. Topo maps of Wisconsin are generally based on one of three Lambert Conformal Conic projections: north, central, and south zones.
Please visit the Coordinate Systems section of our website for more information.
Map Revisions
To keep up with the ever-changing environment, topo maps must be periodically revised. Because the rate of change varies from region to region, topo maps are not revised at set intervals but rather as the need arises and as funds are available. Photo inspection, which involves comparing the latest published map to recent air photos to decide whether major changes have occurred, is used to prioritize and authorize revision. The cartographic revision, map reproduction, and publication phases follow.
There are four main categories of map revision:
- minor
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
Basic revision uses aerial photographs from the National Aerial Photography Program (NAPP) to update features. Digital Orthophoto quadrangles (DOQ) made from NAPP photographs are the primary data source. Contour update is an optional part of basic revision and is not often done because of the high cost.
- complete
Complete revision updates all standard feature content, including contours. Information is field checked. Ibis is very expensive and is therefore rarely done. Only four USGS quadrangles were completely revised between 1995 and 2000.
- and single edition.
Single edition revisions are done by the USGS and the U.S. Department of Agriculture Forest Service (FS). The content of the maps includes the features normally shown on USGS maps 3 with additional features required for the management of National Forest System land. 7.5- minute quadrangles that contain National Forest land are revised by the FS but are printed and distributed by the USGS.
Topographic Map Series
Since 1886, the USGS has been producing topo maps of Wisconsin. These maps are mostly grouped into series where each sheet is the same scale and covers a standard area. The table below carries the distinguishing characteristics of these series.
Small-Scale Coverage
The USGS mapped the state on a single sheet at 1:1 million-scale, and also at 1:500,000-scale (which is about 4x4 feet). At 1:250,000-scale, the state becomes too large for a single sheet, so it is divided into blocks of 1 x 2 degrees of latitude x longitude. These blocks are sometimes called quadrangles or "quads".
Intermediate-Scale Coverage
A typical Wisconsin county at a scale of 1:100,000 is a convenient sheet size. For this reason, the state worked with USGS to produce a series of 72 such maps. Since the counties vary in size, so do the maps.
The national standard at this size is quadrangles that are 30 x 60 minutes in size. However, those maps often split counties into adjoining sheets. Wisconsin has coverage of both the county and quadrangle series at 1:100,000-scale. Joined into a single statewide coverage, either set occupies about 17 x 17 feet.
The 1:100,000-scale topo map series was primarily compiled from the next more detailed series available nationwide, the 1:24,000-scale series.
A number of Digital Line Graph (DLG) layers have also been developed from this series, most of which are available statewide in 30 x 30 minute units, or further enhanced and restructured in various GIS data layers.
Minor areas of the state have been mapped at other intermediate scales. Around Fort McCoy there are 14 map sheets (each a 15-minute quad) at 1:50,000-scale. An older series at 1:62,500 (also 15-minute quads) was halted before statewide completion.
Large-Scale Coverage
Probably the best known USGS topo map series is published at 1:24,000-scale. Each sheet is a quadrangle covering 7.5 minutes of latitude and longitude.
Although begun several decades earlier, this series was developed primarily in the 1970s and 1980s. Since completion in 1985, onlya small fraction of the sheets have been updated, and as a result, the series overall is aging quite rapidly. In developing urban centers this poses a problem for some useseven though primary physical features such as the terrain and the major water features may remain essentially unchanged.
| Scale | Ratio Scale | Inch Scale | Mile Scale | Approx. Sheet Size | Approx. Image Size | Ground Area (sq. miles) | Contour Interval | # Sheets in State | # Maps Completed | Current Revision Program | Years |
| 7.5' | 1:24,000 | 1"= 2000' | 2.64"=1mi. | 22" x 27" | 15-17" x 23" | 50 - 55* | 10 or 20 ft. | 1154 | 1154 | yes | 1944~95 |
| 15' ** | 1:62,500 | 1"= .99mi. | 1.01"=1mi. | 17" x 21" | 12.8" x 17.5" | 197-282 | 20 ft. | 323 | 230 | no | 1943~80 |
| 15' † | 1:48,000 | 1"= .76mi. | 1.32"=1mi. | 21" x 27" | 16" x 23" | 197-282 | none | 323 | 139 | no | 1904~59 |
| 15' metric | 1:50,000 | 1"= .79mi. | 1.27"=1mi. | 22.5" x 29.4" | 17" x 21.9" | 197-282 | 10 m | 323 | 12 | yes | 1976~86 |
| County | 1:100,000 | 1"=1.58mi. | 0.63"=1mi. | varies | varies | varies | 20, 40 or 80 ft. | 72 | 72 | yes | 1985~86 |
| 30' x 60' | 1:100,000 | 1"=1.58mi. | .63"=1mi. | 29" x 40" | 22" x 32" | 1578-2167 | 10 m | 49 | 49 | yes | 1976~90 |
| 1° x 2° | 1:250,000 | 1"=3.95mi. | .25"=1mi. | 22" x 32" | 18" x 25-27" | 7200-7800 | 50 ft. | 18 | 18 | yes | 1963~82 |
* A few non-standard sheets cover upwards of 60 square miles.
** The 15-minute map series are no longer updated or reprinted; however film separations are still available.
† The 15-minute, 1:48,000-scale maps do not include contours, and thus are not truly topographic maps.
Accuracy and Standards
An inaccurate map is not a reliable map. "X" may mark the spot where the treasure is buried, but unless the seeker can locate "X" in relation to known landmarks, the map is not very useful. Dependability is vital, for example, to engineers, highway officials, and land-use planners who use USGS topographic maps as basic planning tools. All USGS topo maps currently adhere to National Map Accuracy Standards (NMAS), which deal specifically with horizontal and vertical accuracy.
To put horizontal accuracy into perspective, the 1:24,000-scale topo maps are constructed so that at least 90% of features that are "well-defined" in the landscape are positioned within 40 feet (1/30th of an inch at map scale) of their true location. At 1:100,000-scale, this standard equates to 167 feet.
However, this is an absolute standard, based on comparing map position to true latitude and longitude, feature by feature. In practice, relative accuracy based on mapped distances between nearby features is much higher.
Vertical accuracy is described for contour lines, where the actual elevation for at least 90% of test points might not differ by more than 1/2 the contour interval.
Non-spatial accuracy includes issues such as completeness (are any features missing?), classification (are features shown with the correct symbol?), and names. While changes on the land can render a map inaccurate, this is different than errors embedded at publication time. In either case, there are no specific standards for these types of accuracy.
Sometimes a person believes that the name of a map feature (village, lake, etc.) should be changed. There is a process to deal with these requests through the Wisconsin Geographic Names Council.
Accuracy of Revised Maps
The USGS originally compiled topographic maps using procedures to meet the NMAS. Basic revision procedures were originally designed to retain the accuracy of the existing map but not necessarily to improve it. This objective has shifted in the last 2 years, and now the horizontal accuracy goals of basic revision are that the revised map should be at least as accurate as the previous version and that all features should match the DOQ to within at least 73 feet. Both goals are evaluated by statistically comparing the map to the DOQ.
Contours and spot elevations also were originally compiled to meet the NMAS. At present, the USTS 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 be meeting NMAS, attempts are made to evaluate the data against independant 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 independant of the original map base.
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