Topographic Maps

 

by

Dr. James R. Carter

Geography-Geology Department

Illinois State University

 

 

The topographic map is that form of map that uses symbols to show the form of the land surface, what we often call topography.  On most maps the symbol employed is the contour.  The topographic maps in the series published by governments contain a great amount of information in addition to topographic symbols.  This information is collected and presented in a consistent fashion so that maps from across the country are able to be compared.  The fact that the map information is broadly available in standard formats makes these maps useful for many tasks.

The topographic maps are based on rigorous geodetic bases so that everything is in its correct location.  In the 1980's using better technologies we redefined the shape of the Earth over North America.  This led to the adoption of a new North American Datum in 1983.  For more on NAD83, which replaced NAD27, see my brief overview.

Although we say topographic maps are quite precise, cartographic license is applied where a number of features must be shown but there is not room to show them at their correct scale.  A good example of this is where a road, railroad and drainage ditch run parallel to each other.  On the ground they do not occupy much space but to show them on the map with standard symbols requires proportionally more space than they have on the ground.  We say we exercise 'cartographic license' to fit all of these symbols on the map.  We do this by distorting the map slightly, but we accept that distortion so that we have all of the symbols on the map in their relative positions.

You may see a map that has everything the topographic map has, except the contours.  Without the contours this is not a topographic map.  We call these 'planimetric maps' because everything is in its horizontally correct place.  

Every nation publishes topographic maps of their countries.  However, not every nation makes these maps available to the general public.  In the U.S. the topographic maps are considered to be in the public domain, so that anyone can use the maps.  For this reason you will see many people using the U.S. topographic maps as a base for other maps, such as fishing maps.

In North America the federal governments of the U.S. and Canada produce series of topographic maps.  They maintain web pages giving basic information about their maps.  In the U.S. the Geological Survey has the responsibility for making topographic maps at a variety of scales.  Natural Resources Canada is the agency responsibility for these maps for this nation.  Readers should go to these two pages for additional perspectives on topographic maps and mapping.  Look at the many links from these pages.

Topographic maps can be thought of as part of our infrastructure in that they are capital investments that help make us more productive.  These maps provide an inventory of what exists on the land surface of the country.  Without the knowledge provided by these maps, we would be greatly crippled in what we can do and how we think.  I like to think of these maps as part of our National Spatial Data Infrastructure, or NSDI.  

In the U.S. topographic maps are available at a number of scales.  The basic scale is the 1:24,000, or 1 inch represents 2,000 feet.  The U.S. is the only country publishing topographic maps at this scale, but then we use feet and inches.  The rest of the world uses metric scales with numbers like 1:25,000 or 1:50,000.  The 1:24K series is also known as the 7.5 minute series because each quad is 7.5 minutes of latitude and 7.5 minutes of longitude on each side.

Here is an example of a portion of a 1:24,000 topo map.  This covers the southwest corner of the Normal East, IL, quadrangle, showing the area in and around Illinois State University.  The contour interval on this map is 10 feet.

The 1:24K series is complete for the entire 48 states of the conterminous U.S.  Many of these maps are quite dated but still there is complete coverage.  

Below is a ruler scanned at the same scale as the topographic maps.  This ruler is here so you can see how much exaggeration is displayed on your screen.  We have the ability to set the resolution of the display screen.  At a low resolution the maps will be exaggerated relative to their real size.  At very high resolutions the maps may appear smaller than their real size.

For many years USGS produced a series of maps at 1:62,500.  These maps are also known as the 15 minute quads because each side of the map extends over 15 minutes of latitude and longitude.  Here is the Normal area as presented on this scale of map in 1926.  The contour interval on this quad is 10 feet.

There are still many 15 minute quads around but this series has not been updated in recent years.  The 1:100,000 topographic map series has replaced the 15 minute series.  USGS publishes this series in two formats.  The basic series is the quadrangle, which covers 1 degree of longitude (E to W) and 30 minutes of latitude (N to S).  Here is the same area as portrayed on the Fairbury 1:100K map.  The contour interval on this map is 5 meters.

This map series shows considerable detail and covers a much greater area than the 7.5 minute series.  Thus, these maps have become quite popular for travelers because you can cover a lot of area on a single map.  But, these maps are constrained by the boundaries of the quadrangle, as above.  In this case the area of the twin cities of Bloomington-Normal extend over four separate maps.  Because of such situations, USGS has produced many of these maps on a county basis.

Above is the McLean County 1:100,000 topographic map.  Rather than using the rectangular format this map covers the entire county and thus the twin cities of Bloomington-Normal are shown without interruption on the same map. 

Here again is a ruler scanned at the same scale as the topographic maps.  This ruler is here so you can see how much exaggeration or shrinkage is displayed on your screen. 

The first complete topographic coverage of the U.S. was done with the 1:250,000 map series.  This series was first developed by the Army Map Service and then was taken over by USGS and upgraded to USGS standards.  The maps in this series were widely used for many years, particularly in the western parts of the country where populations are less dense and distances between points of interest are usually fairly far apart.  Below is a portion of the Peoria 1:250K map covering the Normal area.  It employs a contour interval of 50 feet.

The discussion of these four series of maps (1:24,000;  1:62,500;  1:100,000;  and 1:250,000)   gives a good overview of the topographic map series of the USGS.  Of course, there are many exceptions based on special situations.  One variation of note is that many years ago USGS made a few of the topographic quadrangles available as shaded-relief maps.  Here is a portion of the Strasburg, VA, 1:62,500 quadrangle with all of the details of the standard Strasburg quad plus the shading.  This is in the Shenandoah Valley area and as such provides many attractive topographic situations which are enhanced with the shading.  The contour interval is 40 feet.  Not surprisingly, none of the topographic quads in central Illinois have been portrayed with shaded relief.

The U.S. Geological Survey is not the only organization which makes topographic maps.  In recent years DeLorme has published a series of State atlases, built in part on the topographic maps produced by USGS.  These atlases are produced at different scales, depending on the size of the State and the density of features in the State.  In the case of Illinois, the scale of the maps is 1:150,000.  Here is the Normal study area as shown in the DeLorme Illinois Atlas.  The contour interval on this map is 30 feet.

Part of the appeal of the DeLorme atlases is that one can purchase complete topographic coverage for an entire state and have it in a single volume.  With a state-wide index you can find your area and turn to it.  It is much more convenient than sorting through a pile of topographic quadrangles to find the map for the neighboring area.  

Many topographic maps are made for specific purposes.  Below is a portion of a topographic map for Knoxville, TN.  This map was generated from their Geographic Information System, GIS, in 1987.  The map was produced on paper at a scale of 1:2,400, or 1 inch to 200 feet.  The contour interval on this map is 2 feet, far more detailed than shown on any of the maps we have seen.  On this detailed map the shape of  houses is shown in yellow, individual trees are shown as green clusters and forested areas are outlined with a green scalloped line.  The red triangle is a power pole and the exact shape of the road and cul-de-sac are shown.

This can be thought of as a large scale map but there are even larger scale topographic maps.  Below is a portion of a topographic map generated by the Illinois Department of Transportation, IDOT.  This map was created for the design and construction of a piece of highway.  The scale of the map is 1:500, or approximately 1 inch to 42 feet.  Here the contour interval is 0.25 meter.

In this presentation you have seen topographic maps at scales ranging from the large scale IDOT map at 1:500 to the relative small scale USGS map series at 1:250,000.  There are even smaller scale topographic maps showing the landforms of the world with contours or hypsometric tints.  

This discussion of topographic maps refers to the analog maps--those printed on paper.  In the digital age we find many of these maps are stored in digital form or the component parts of the maps are available as layers so that users can pick and choose to construct their own topographic maps.  With the computer maps can be made at any scale, but the quality of the data going into making a map can not be any more precise than the source of the data.  So, if a digital database is derived from the 1:24,000 topographic maps the maps generated from this database can be no better than that series of maps.  REMEMBER THIS !

NAD 27 and NAD 83 -- You will see these terms when referring to geographic coordinates of the U.S.  The Earth is spherical, but in reality the Earth is not a true sphere.  The actual shape of the Earth is a geoid, which locally may be quite complex.  The more general shape of the Earth is an oblate ellipsoid (sometimes called an oblate spheroid).  For mapping, ellipsoids are fit to portions of the Earth's surface and coordinates are fit to the ellipsoids.  

NAD 27 refers to North American Datum of 1927.  At that time North American cartographers used the Clarke 1866 ellipsoid to define the shape of the continent.  All coordinates of latitude and longitude were then established relative to this ellipsoid.  Most of the topographic maps of the U.S. are based on the NAD 27 standard.

When computers and satellites came into existence, we became better able to define the shape of the world.  The World Geodetic System 84 ellipsoid was defined based on better data.  NAD 83 is the new standard that was adopted in 1983.  To see how these two systems are represented on maps, see my example showing a corner of the map in NAD 27 and where that corner would be in the NAD 83 system.

 

 

 

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