5.1 Digitally Representing Terrain

Representing the Earth’s terrain is an important role that many maps have played throughout history. Chapter 2 discussed some of the visual techniques used to represent a three-dimensional surface on a two-dimensional page, including the use of isolines, shaded relief, and hypsometric tinting. Maps have used these approaches for a long time, but with computers taking over the bulk of map production, new techniques have been developed to present three-dimensional surfaces, and old maps have been adapted to function in a digital context. This chapter begins with a description of how old map data have been moved into the digital age, and then talks about recent technologies that have improved and enhanced our ability to model the Earth’s surface for use in analysis and mapmaking.

5.2 Digital Raster Graphics

Digital raster graphics, or DRGs, are scanned USGS standard series topological maps, including all information in the map collar (the space “outside” the map). See Fig. 5.1 for an example of a DRG. Since all the information that would be present on a typical USGS topo map is visible in the DRG, it looks visually identical, albeit on a computer screen rather than a sheet of paper. The USGS began a program of scanning topo maps in 1995 as a way of digitizing their paper resources (U.S. Geological Survey, 2013a). Standard topo maps were scanned at 250 dpi and the resulting raster images were georeferenced with the UTM coordinate system.

Georeferencing is a process that takes known ground control points and connects them to the raster in order to apply a coordinate system and projection to the image. Without a projection, GIS software has no way to know where a raster should exist on the globe, and because of that, it cannot analyze it correctly. By giving a coordinate system and projection to the raster, a great number of quantitative GIS tools can be applied to analyze the raster data. Essentially, the georeferencing process takes what was a picture and turns it into geospatial data that can be scientifically analyzed.

In 2001, the standards for DRGs were updated and all scans from then on were captured at 500 dpi (U.S. Geological Survey, 2013b). Under the old standards, colors were limited to 13 different hues to reflect the conventions present in printed topographic maps; the updated standards allow for up to 256 colors to be stored in a DRG to account for more colorful maps added later to the program. Given the long history of the USGS topographic mapping program, these DRGs can be quite valuable as they put historical information into a digital context that can then be included as part of a GIS analysis. DRGs can be downloaded through the USGS EarthExplorer website, and historic topographic maps can be downloaded as GeoPDF files through The National Map’s Historic Topographic Map Collection (U.S. Geological Survey, 2014a, 2016).

As you may have suspected, scanning historical maps is not limited to USGS topologic sources. As humans have been making maps for some time, we have many centuries worth of cartographic sources to work from, and converting them to a digital format has allowed us to apply our GIS analysis abilities to the past. In the subfield of Historical GIS, large-format scanners are often used, or in the case of maps too fragile to be scanned, more traditional photography techniques. Once these maps have been converted to a raster format, the georeferencing process prepares them for GIS analysis. This unlocks an enormous volume of historical data that can be analyzed to study topics as diverse as land cover change, demographics, economies, or any topic that has been previously mapped.

5.3 Digital Line Graphs

Digital line graphs, or DLGs, are also derived from USGS sources of data but are stored in a vector format as opposed to raster. An example of a DLG can be seen in Fig. 5.2. They are generated using both automated and manual techniques, pulling information from both aerial photos and map sources. DLG data are provided at three scales: large, intermediate, and small scale (U.S. Geological Survey, 1996). The large-scale DLGs are derived from USGS 7.5 minute topographic maps (1:20,000, 1:24,000, or 1:25,000 scale depending on the specific map) and primarily use the UTM coordinate system, although some use SPC. Intermediate-scale data exist at the 1:100,000 scale in UTM and is derived primarily from 30 × 60 minute USGS quadrangles. If a USGS quadrangle does not exist, Bureau of Land Management planimetric maps are used as sources. The data in small-scale DLGs come from USGS 1:2,000,000-scale sectional maps from the National Atlas of the United States. Not all layers of data are available in all scales, but DLGs include a variety of thematic content. Table 5.1 shows the available layers with descriptions used by the USGS’s DLG resources website (U.S. Geological Survey, 2012).

5.4 Digital Elevation Models

Digital elevation models are a more recent, purely digital source of elevation data. The term “Digital Elevation Model” does not refer to a specific data source or file format, but is rather an umbrella term that describes multiple approaches to collecting and representing elevation data on a computer. Some related terms are digital surface model, which represents the heights of all features including man-made structures, and digital terrain model, which only represents the bare surface of the Earth (See Fig. 5.3). DEMs are stored and displayed in two formats: as raster images where cell value represents elevation, or as a vector-based triangulated irregular network (TIN). Raster-based DEMs have similar advantages to rasters in general in that the mathematics involved in analyses tends to be simpler and faster to calculate than the vector-based TINs. However, TINs are much smaller in file size, and known benchmarks and breaklines (known features such as roads or streams) can be easily added to increase the overall accuracy of the surface. As with any analysis, which format is best depends on the needs of the project, see Fig. 5.4. Raster DEMs do have a distinct advantage in that generally speaking they have more support in GIS software packages.

DEMs are used in a variety of applications, including, but not limited to, viewshed analyses, erosion and flood modeling, precision agriculture, and archaeology. It is also common for aerial or satellite imagery to be draped over DEMs, producing realistic three-dimensional visualizations of the landscape. The source of DEM data most often comes from remotely sensed platforms, including satellite- and plane-based RADAR and aerial LIDAR. An example of LIDAR data representing surface elevation can be seen in Fig. 5.5. These technologies are examples of active remote sensing, as both RADAR and LIDAR send out pulses of energy and measure the amount of time it takes for the energy to return.

Conceptually this is similar to the Microsoft Kinect sensor, the video game peripheral that works with the Microsoft Xbox 360 and Xbox One. In fact, the Kinect device has been used in some research circumstances as a low-cost stand-in for more expensive professional LIDAR setups (Mann, 2011). The Kinect was designed for use in living rooms and only works in small environments, but the process remains similar.

While planes and satellites are commonly used to cover large amounts of ground quickly, the use of unmanned aerial vehicles has grown dramatically in recent years. The availability of these relatively inexpensive platforms has allowed both aerial imagery and elevation data to be collected more easily and affordably than ever before. Small-format aerial platforms such as UAVs, blimps, and kites also typically offer a higher spatial resolution than data collected from satellite sources, making them useful for monitoring specific locations with a high level of precision.

Currently, the most complete global source of DEM data comes from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (aka ASTER) satellite platform. The ASTER Global Digital Elevation Model (GDEM) version two was released in October of 2011, and covers 99% of all land surface on Earth, with data from 83°N to 83°S (ASTER GDEM Validation Team, 2011). This dataset can be downloaded freely for use at the Global Data Explorer or Reverb | ECHO websites (Mitchell, n.d.; U.S. Geological Survey, 2015a). For the United States, a widely used source of DEM data is the National Elevation Dataset (NED) provided by the USGS. The NED dataset is a seamless layer of raster elevation data, although not all data resolutions are available for the entire coverage area. Data within the NED are derived from a variety of sources and can be found in 1/9 arc-second (approximately 3 m), 1/3 arc-second (approximately 10 m), 1 arc-second (approximately 30 m), and 2 arc-second (approximately 60 m) spatial resolutions (U.S. Geological Survey, 2014b). The three-meter resolution data are only available for approximately one-third of the United States at this time. NED data are available through The National Map website (U.S. Geological Survey, 2016). More precise elevation data provided by LIDAR sensors are discussed at greater length in Chapter 7.

5.5 Conclusions

Mapping the terrain of the Earth has long been an important component of our cartographic history. Modern technology has allowed us to unlock the analysis potential of older map resources as well as provided more accurate measurements for a larger land coverage than ever before. Thanks to the Internet, many of these resources are freely available for anyone to use. This access has provided us with more information than ever before, enhancing our ability to study the world and plan for future developments.