Digital Map Data (GPS)

Although other countries use the same or similar data formats as the U.S. government for producing digital maps, trying to obtain detailed international map data can be difficult. In some parts of the world, maps and map data are tightly controlled by the government because they’re are considered a key part of a country’s national security.


The U.S. Census Bureau produces Topologically Integrated Geographic Encoding and Referencing (TIGER) data for compiling maps with demographic information. This vector data is a primary source for creating digital road maps of the United States. Figure 2-11 shows a map of downtown San Francisco created from Census Bureau TIGER data.

TIGER data viewed from the Census Bureau's Web-based map viewer.

Figure 2-11:

TIGER data viewed from the Census Bureau’s Web-based map viewer.

TIGER data is free (check out the Census Bureau Web site at geo/www/tiger/index.html) but in some areas isn’t very accurate; roads don’t appear on the map and addresses aren’t in the right locations. The government is improving the accuracy of the dataset, and plans to provide better data in the future.

Mapmaker, mapmaker, make me a map

Data for digital maps comes from government sources and commercial mapmaking companies, which license their maps to other companies. These entities get data from satellites, aerial photographs, existing maps, or sometimes from folks driving and walking around with a GPS receiver, using a laptop to record data. As an example, here are the main providers of digital map data for the United States. (Most countries will have a similar collection of government agencies and commercial sources that produce map data.)

USGS:The United States Geological Survey (USGS) has been making United States land maps since 1879.

Census Bureau: The Census Bureau is in the map-making business because the data it collects is directly tied to location. Its Web site has information on its TIGER data.

NOAA: The National Oceanic and Atmospheric Administration creates and maintains nautical charts and other essential data for marine use.

FAA: The Federal Aviation Administration National Aeronautical Charting Office (NACO) provides aviation charts and data for the United States.

NGA: The National Geospatial-Intelligence Agency (formerly the National Imagery and Mapping Agency) is the Department of Defense agency for producing maps and charts of areas outside the United States.

States, counties, and cities: States, counties, and larger cities have Geographic Information System (GIS) divisions that produce maps. This data is often available to the public for free or a small cost. To find whether digital map data is available for your area, search Google for your state, county, or city and GIS.

NAVTECH: Navigation Technologies is one of the largest commercial suppliers of street and road data. UPS, General Motors, Ford, DaimlerChrysler, and AAA use NAVTECH.

GDT: Geographic Data Technology is a commercial supplier of digital map data. GDT licenses its maps to ESRI, MapInfo, Map-Quest, and Expedia. index.cfm

TeleAtlas: TeleAtlas is a European mapmaking company with a United States presence. TeleAtlas licenses its maps to Lowrance, Microsoft, OnStar, and Rand McNally.

Digital Line Graph (DLG)

Digital Line Graph (DLG) data from the USGS is used to create vector maps. The data includes transportation networks, hydrography (measuring surface waters), boundaries, elevation contours, and man-made features. The format is similar to TIGER data but generally has more accurate roads and other features.

Raster and vector maps

These two terms describe the underlying format of a digital map:

Raster maps are composed of a series of pixels (picture elements) aligned in a grid. Each pixel (or bit) contains information about the color to be displayed or printed. Because bitmap images can be very large, graphics formats such as GIF (Graphic Interchange Format) and TIFF (Tag Image File Format) use compression to make images smaller. An example of a raster map is a paper map that’s been scanned and saved as graphic file.

Raster maps tend to show more detail than vector maps and don’t look as computer-generated. Figure 2-1 is a raster map.

Vector maps are composed of many individual objects. The objects are mathematically based. They can be points, lines, and shapes. Each object has properties that define its appearance and attributes, such as color, thickness, and style. If you’ve used such graphics software as Adobe Illustrator or CorelDRAW (called object-oriented drawing programs), you’ve used a vector graphics program. A map can contain thousands of these objects, but vector maps tend to be smaller than raster maps because it’s more efficient to mathematically describe an object than draw it as a bitmap. Vector maps are scalable, which means you can resize a vector map without distorting the map’s information. Vector maps appear to be created with lines and shapes and are linear looking. Figure 2-2 is a vector map.

Figure 2-12 shows a map of the Atlanta Olympics Savannah Yachting Venue, created with USGS DLG data.

DLG data has basic information such as road types, bridges, and highway route numbers, but only GIS software can easily access these attributes.

A map created with DLG data.

Figure 2-12:

A map created with DLG data.

Elevation data

You’re likeliest to run into one of these two main data formats used to represent elevation:

Digital Elevation Model (DEM): The USGS uses this raster data format to record elevation information (based on topographic maps) and create three-dimensional representations of the terrain. Figure 2-13 shows a map of Mt. Bachelor, Oregon, generated from Digital Elevation Model data with the 3DEM mapping program.

National Elevation Dataset (NED): This format shows digital elevation data in shaded relief. It’s designed for seamless coverage of the United States in large raster files.

A map created with DEM data.

Figure 2-13:

A map created with DEM data.

Digital Raster Graphics (DRG)

Digital Raster Graphics (DRG) data is a scanned image of a USGS topographic map. These digital maps are available for free on the Internet or are sold commercially in collections on CDs or DVDs. Figure 2-14 shows a DRG map from the TerraServer-USA Web server. This map is a digital version of a 1:24,000 scale topographic map.

These digital maps are scanned at 250 dpi (dots per inch) and stored in a TIFF file format, using embedded GeoTIFF (geographic information) tags for location data.

A map composed of Digital Raster Graphics data.

Figure 2-14:

A map composed of Digital Raster Graphics data.

You can view the map by itself or both the map and its location data. Use one of the following methods:

View the map by opening the DRG file with any current graphics program that supports large TIFF files.

Use the DRG file with a mapping program that supports GeoTIFF to view the map and access its location data.

For more technical details about USGS digital map data, check out the agency’s product Web site:


Who is Mr. Sid?

This question is more accurately stated as what is MrSID? MrSID is the Multi-Resolution Seamless Image Database. It’s a file format used for distributing large images over networks, originally developed by a company called LizardTech. Graphics in MrSID format are compressed with a lossless compression algorithm (a method of compressing data that guarantees the original data can be restored exactly) designed to produce relatively small, high-resolution images.

The file format is perfect for aerial and satellite images that have large file sizes, and the government is increasingly using it for distributing data. (The Library of Congress is even using it for electronic versions of paper documents.) A number of free viewers support MrSID; use Google to find download sites. (One of my favorites is IrfanView, which is available at

Digital Orthophoto Quadrangle (DOQ)

Digital Orthophoto Quadrangle (DOQ) data consists of a computer-generated image of an aerial photograph. The image is corrected so that camera tilt and terrain relief don’t affect the accuracy. DOQs combine the image characteristics of a photograph with the geometric qualities of a map. Figure 2-15 shows a DOQ map of Mt. Bachelor, Oregon, from the TerraServer-USA Web site.

A Digital Orthophoto Quadrangle map.

Figure 2-15:

A Digital Orthophoto Quadrangle map.

The USGS has DOQs available for the entire United States. Most are grayscale, infrared photos; there are higher-resolution color photos for a few large U.S. metropolitan areas.

A booming business provides high-resolution, color aerial photographs to individuals, government agencies, corporations, and educational and nonprofit organizations. Companies like AirphotoUSA (, Keyhole ( and DeLorme’s TopoBird subsidiary (www. provide imagery with quality and resolution that’s close to what was only available to intelligence agencies. If you want aerial photographs for business or government purposes, check these commercial sources.

Satellite data

Satellites are the most exotic source of data for digital maps. Orbiting several hundred miles above the earth, satellites provide photographs and other sensor data. NASA’s Landsat, the Space Shuttle, and other satellites collect raster data for most of the earth. It’s available both free and commercially.


The highest resolution commercial satellite imagery available is .61 meters, from Digital-Globe’s QuickBird satellite (

By 2006, commercial satellites are expected to have resolution as high as 0.25 meters. That’s pretty close to the resolution of spy satellites,which are thought to have a resolution of 0.1 meters. (The best spy satellites can reputedly discern objects about the size of a baseball. Despite what you see in the movies, license plate numbers are too small to be read by spy satellites, especially considering they don’t face up toward space.)

Resolution defines the smallest object a satellite can distinguish. A satellite with one-meter resolution can distinguish objects down to a meter (a little under 40 inches) in size.

Figure 2-16 is a ten-meter resolution SPOT satellite data of Mt. Bachelor, from DeLorme’s Spot 10 collection of satellite data. (SPOT, which stands for Systeme Pour l’observation de la Terre, is a French commercial satellite program that started in 1986.)

A ten-meter resolution satellite image.

Figure 2-16:

A ten-meter resolution satellite image.

Although most free satellite imagery doesn’t come close to the resolution of some commercial sources, the government is relying increasingly on private companies to acquire data. I expect that government sources eventually will release high-resolution, color imagery to the public.

Next post:

Previous post: