For this month’s Geospatial Frequently Asked Question (G-FAQ), I continue the discussion started in our May edition on the visualization of elevation data and confusion among various terms associated with this topic. To recap, the core set of questions I address in this two part series are:
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?
Last month, I turned my attention to contours and the topographic map, showing how the progression of the ‘simple’ contour map to a more complex topographic map helps to explain (at least in part) the duality of these two terms. This month, I turn my attention to DEMs and 3D as well as offer examples when each form of visualization is the most appropriate for elevation data.
Before the 1950’s, elevation was visualized using only vectors – or line files. With the advent of modern aerial photogrammetric techniques, raster forms of elevation data came into existence. A raster file is one where the world is divided into a rectangular grid and each cell of the grid is a pixel with a single value tied to it as well as a latitude-longitude. In the case of elevation data, this single value would be the earth’s height in the units of the raster file. This type of raster file is commonly called a digital elevation model or DEM.
Today, DEMs are typically extracted from optical stereo pairs of aerial or satellite imagery using both automated and human production techniques. At the root of either technique is determining spot elevations along a regular interval. These spot elevations populate the pixels of the raster file output, i.e. the DEM, so that this single value becomes the elevation of the entire cell – even if it covers an 8-meter (m) by 8-m section of the earth. As we discussed last month, elevation can certainly change rapidly across a pixel that is 8m x 8m and even across one that is 1m x 1m. For this reason, a DEM should also be understood as an interpolation of elevation similar to contours.
There are some interesting connections between DEMs and contours. First, during the creation of raster DEMs, vector contours can be extracted by connecting spot elevations similar as to what was done by hand in the past. Computers can extract smaller contour intervals than the accuracy of the source data might otherwise support – this is referred to as over-interpolation and it is commonly done by less experienced geospatial professionals. Finally, there is a well known connection between contours and DEMs in that many 10-meter USGS DEMs that are part of the National Elevation Dataset (NED) were interpolated from 7.5-minute topographic maps as is shown in the animation below.
The most recent advent in the visualization of elevation data is the three-dimensional model (or 3D model). This technological advance was made possible by improvements in both computing power and graphics capabilities. A 3D model is created by draping imagery (typically high resolution data in natural color) over a co-registered elevation model (typically a DEM). The elevation model is used to create the 3D framework on which the imagery lays, similar to the technology used to create animated Disney movies and PlayStation video games. Once a 3D model is created, animated fly-throughs can be recorded to give viewers a realistic experience of interacting with remote and/or distant lands. Similar to the relationship contours and a topo map have, a 3D model cannot be created without another form of elevation data (be it a DEM or contours) and imagery of the planet’s surface.
While I certainly cannot explain all of the confusion among the terms contours, topos, DEMs and 3D which is common in our geospatial industry, from the story offered in this G-FAQ series, it should be apparent that (at least in part) these terms are interchangeable as they all speak to the same topic – i.e. the visualization of elevation data. As technology has progressed, so too have the forms of elevation data we utilize and hence so too have the terms we use. Starting in the 1700s, contours were the earliest form of elevation data as they could be represented on a paper map with hand-drawn concentric circles and/or lines. In the 1800s, cartographers added land use polygons to contours on paper maps, creating the first ‘topos.’ In the 1950s, with the advent of aerial photography, raster DEMs grew in popularity. And now, in the 21st Century, with the progress of computer technology and animated movies/video games/etc., many geospatial professionals, map users and casual admirers alike expect 3D fly-throughs to visualize a landscape.
While geospatial professionals commonly misuse these four terms, each is a distinct form of visualization that can be better suited to represent elevation depending on the needs of the project. As such, let’s examine some common uses cases for each contours, topographic maps, DEMs and 3D models:
- Contours – often the most appropriate choice to visualize elevation on paper maps; these are popular with architects, engineers and designers as they give an instant indication of grade and slope direction.
- Topographic maps – while not a distinct form of elevation data, these maps uses contours to display height; a topo map is often the choice for adventure seekers and outdoor enthusiasts as they combine the appeal of contours with the added benefit of understanding expected land use/land cover. This can be important, for instance, when navigating unknown back country areas of large national parks.
- DEMs – digital elevation models are commonly used in geospatial data production (i.e. for orthorectification) and for GIS analysis including water flow, accumulation models and watershed delineation.
- 3D – these models are geared to fly-throughs meant for movies, websites, video games and television; it is perhaps the most stunning way to visualize elevation that we currently employ.
As a final point to this two part series on visualizing elevation data, it is interesting to note that as much as things change they seem to stay the same, and this is true even in the world of geospatial data. Case and points here:
- While working with raster forms of elevation data (i.e. DEMs) was made possible by the advent of modern photogrammetric techniques and computing power, when it comes down to it they are made in a very similar fashion to their predecessor, contours – i.e. the extraction of spot elevations. In the case of contours, they are created by connection similar spot elevations with concentric circles; and in the case of DEMs, they are created by extracting spot elevations at regular spacing along a grid.
- And a similar parallel can be drawn between the topo map and 3D, in that a topo map was the first way to portray both elevation and land use/land cover on a paper map. 3D is the means to convey the display the same thing on a computer monitor, replacing contours with a raster DEM and then hand-drawn land cover polygons with satellite and/or aerial imagery.
Until our next edition of G-FAQ, happy GIS-ing!
Do you have an idea for a future G-FAQ? If so, let me know by email at [email protected].
Find Out More About This Topic Here:
- NOAA – Photo Library Voyage to Inner Space
- Penn State University e-Institute – Digital Elevation Model Lesson
- UN FAO – Elements of Topography
- University of Alabama Historical Map Collection – Coastal Topographic Sheets
- USGS – Topographic Map Standards
- USGS – Topographic Map Symbols
Brock Adam McCarty