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Topographic Data Resources

Topographic data are information about the elevation of the surface of the Earth. Two such data types are commonly used with GeoPads. The first are data that represent the information typically found on a topographic quadrangle map, such as contour lines, roads, streams, railroads, towns, etc. For simplicity, this first category of data will be referred to as digital topographic map data. The second are grids of data, for which each cell in the grid represents the elevation at a certain point on the Earth. These data are commonly referred to as Digital Elevation Models or DEMs.

Digital Topographic Map Data

Digital topographic map data exists for the United States in both vector (points, lines and polygons) and raster (image) formats. The vector topographic data are called Digital Line Graphs (DLGs) while the raster topographic data are called Digital Raster Graphics (DRGs). Availability of each are described below.

Digital Line Graphs

Digital Line Graph example image
Example of a large-scale Digital Line Graph. Image courtesy of United States Geological Survey.

Digital line graphs (PDF fact sheet) are files that contain the point, line, and polygon data found on a traditional paper topographic map, including the Public Land Survey System (township and range boundaries), contour lines, rivers, lakes, roads, railroads, towns, land cover, survey control points, etc. The advantage to using DLGs in a GIS setting is that the vector features can be rendered at any scale, such that zooming in and out on the map does not change the quality of the topographic data representation. The disadvantages of the DLG format are that importation of the data to a GIS requires more effort than importing a DRG raster image, and that the DLG data are quite large, sometimes slowing down the screen refresh rate of the GIS software, and that not all data layers are available for all topographic map quadrangles. Data layer projections for DLG data vary from state to state. DLG data is available in three series.

Digital Raster Graphics

Example of a Digital Raster Graphic
Example of a 7.5' Digital Raster Graphic. Image courtesy of United States Geological Survey.

Digital Raster Graphics (DRGs) are scanned images of USGS topographic maps, and are available in the standard USGS topographic map series, including 1:24,000, 1:100,000, and 1:250,000 for the United States (with the exception of 1:63,360 for Alaska instead of 1:24,000), and 1:20,000, 1:25,000, and 1:30,000 for U.S. territories. To display multiple DRGs seamlessly in a GIS, it is best to use DRGs with the map boundary information removed. These DRGs are referred to as 'trimmed', 'clipped', 'collarless', or 'enhanced'. DRGs are available for most of the United States from several on-line sources including:

The states of California, Massachusetts, and parts of Tennessee and the surrounding Tennessee Valley Authority maintain and distribute their own DRGs.

Digital Raster Graphics are also available for a large number of commercial distributors for a fee, and many states choose to distribute DRGs through a state-supported web portal. An Internet search on a state name and the keywords 'geospatial data' or 'gis data' will return many useful resources.

Digital Elevation Model Data

Example of a Digital Elevation Model
Example of a National Elevation Dataset 1 arc-second Digital Elevation Model. Image courtesy United States Geological Survey.

Digital Elevation Models (DEMs) are gridded representations of the surface of the Earth where each pixel in the grid contains an elevation value. Resolution of DEMs is often reported in radial measurements, such as arc-seconds.

Global Digital Elevation Models

Global topographic data including topography and bathymetry is available with a resolution of 1 arc-second (~2 km) as part of the ETOPO1 data set (more info) . The highest resolution DEM data set for the entire world (excluding bathymetry) is the 30 arc-second GTOPO30 DEM, which has a nominal ground resolution of ~1 km. The recent Shuttle Radar Topography Mission (SRTM) returned 3 arc-second (~90 m) resolution data for most of the continents between 60N and 60S.

National Digital Elevation Models

For much of the United States, the National Elevation Dataset contains 1 arc-second and 1/3 arc-second (~30 m and 10 m, respectively) resolution data. SRTM 1 arc-second DEM data are also available for the United States.

Regional Digital Elevation Models

High-resolution digital elevation models are available for small regions within the United States. The National Elevation Dataset contains some 1/9 arc-second (~3m) resolution DEM data, mostly for urban areas. Both the US Department of Agriculture and the US Coast and Geodetic Data have some airborne Interferometric Synthetic Aperture Radar (IfSAR) DEMs with resolution of 1-5 meters for small regions. Several consortiums in the US also offer sub-meter resolution DEMs generated from Light Detection and Ranging (LiDAR) airborne scanners. GEON hosts an on-line tool to process LiDAR point clouds into GIS-compatible DEMs.

Comparison of Data Sets

Topographic data created by airborne or space-based techniques (e.g. LiDAR or IfSAR, including the SRTM DEM data) have the advantage of being generally smooth and continuous data sets. The National Elevation Dataset is derived from digitization of USGS topographic maps, and the gridded data derived from this technique can be prone to a number of data artifacts. On the other hand, Synthetic Aperture Radar is absorbed by water and snow and generally does not return elevation data from these surfaces. Airborne and space-based techniques may also have difficulty in steep, rugged topography. A comparison of datasets may be necessary to find the most appropriate type for a particular project.

Digital Elevation Model Data Resources


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