For this month’s Geospatial Frequently Asked Question (G-FAQ), I focus on a set of commonly confused terminology that relates to elevation data: contours, topos, DEMs and 3D. It seems that not a week goes by that I receive a request for elevation data where the client asks for a topo map (or topographic map); but based on their follow up questions and comments, it is clear that what they are asking for is elevation data in an alternate format. It seems that for many in our small industry, these terms are interchangeable when in reality they refer to very distinct forms of elevation data.
To help clear the confusion up, in this edition of G-FAQ, I address this core set of questions:
What do the terms contours, topographic map, digital elevation model (DEM) and 3-dimensional (3D) refer to and how are they created? Why are these terms so commonly confused? How are these terms related? When might you use each of these forms of elevation data?
To start off this discussion, there is a single theme that links all of these terms: they are all ways to represent and visualize the height of our surface’s planet. While this sounds like an easy task, in reality it is quite complex. Consider for example mapping a mountain on the Front Range of the Rocky Mountains. In many places, particularly towards the peak, the ground is too steep to even walk on. If you were able to measure elevation every meter you walked/crawled/climbed along this extremely steep sloping face, you would find places with changes in elevation of a meter or more, even over this short distance. Now, turn around and look at all of the micro-elevation changes that you missed in that meter, such as the large boulder you just climbed over. When you stop and consider the complexity of such a task, it becomes apparent that mapping elevation is not as straight forward as it sounds; and as you will see in this G-FAQ, contours, topographic maps, DEMs and 3D are meant for distinct purposes.
From the research I have done, it appears that contours were one of the earliest ways to represent elevation. A contour is created by connecting points of similar elevation with continuous circles. Contours create a strong visual sense of steepness and direction of slope as lines that are closer together show steeper slopes than lines that are farther apart. The first known contour map was created by a French engineer, Philippe Buache, in 1752 to map the depth of the English Channel. Before the advent of ‘modern’ mapping technology, a contour map was created by collecting spot elevations across a landscape and then manually interpolating the data. Contours were interpolated at a regularly spaced interval, for instance every 100 feet (i.e. the contour interval); and then transcribed to paper maps with labels at key elevations, for instance at 1000 feet. Depending on the scale of the map, an appropriate contour interval was chosen as map scale and accuracy are linked – see the USGS Map Accuracy Standards for more information on this topic.
Today, contours are still an important way to visualize elevation data, however we typically extract them from stereo imagery as DEMs are created. Further, most people have abandoned paper maps in favor of digital vector shapefiles that can be used in Geographic Information Systems (GIS). These digital vectors have elevation attributes embedded in them.
The next advance we find in visualizing elevation is the topographic map. A topographic map has the distinct characteristic of having contours present, but a true ‘topo map’ contains cultural, ecological and geographic information that contours alone would not purvey. The first topo maps created in the United States were the U.S. Coast Guard Survey topographic sheets, or ‘T-Sheets.’ There were 1,887 T-Sheets created from 1834 to 1911. In 1879, the USGS took over the responsibility of mapping our country and they created the best known topo maps that are still popular today, the 1:24,000 or 7.5-minute Quadrangle map series. Completed in 1992, it takes more than 55,000 maps to cover the lower 48 states.
A small section of the 7.5-minute or 1:24,000 USGS Quadrangle topo map over Keystone Mountain, Colorado. (Image Source: USGS)
Most of us are familiar with the 7.5-minute USGS map series with its fine contours lines overlain on pastel colors representing forests, rivers and marshes. These topo maps also feature building footprints, picnic areas, airports, dams and so much more. They were our country’s first attempt at creating a geographic database that included information on the slope and height of the land – its elevation – as well as information on its ecosystems, landforms and human-made structures. In a sense, it was our first attempt at creating ‘Google Earth,’ a tool that many of us use now to understand the landscape of distant and/or foreign lands without ever stepping our foot down in reality.
By combining all elements that comprise a landscape, the USGS topo maps are a powerful visualization tool still found in nearly every engineering office, university geography department and high school across the United States. Perhaps it is this familiarity with the topographic map that explains (at least in part) the use of ‘topo map’ to describe every form of elevation data. It is also the presence of contours on every topo map that would explain the interchangeability of these terms.
In next month’s edition of G-FAQ, I conclude this discussion on the visualization of elevation data with an explanation of DEMs and 3D as well as on the common use cases for each contours, topo maps, DEMs and 3D. Until then, Happy GIS-ing!
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Brock Adam McCarty
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