GPS for Dummies.pdf - Engineering Surveyor

Digital Map DataChapter 2: Dissecting MapsHere I concentrate on digital map data that the U.S. government makes availablefor free. You can make your own maps with this data by using freewareand shareware mapping programs discussed in other chapters of this book.39Although other countries use the same or similar data formats as the U.S. governmentfor producing digital maps, trying to obtain detailed international mapdata can be difficult. In some parts of the world, maps and map data are tightlycontrolled by the government because they’re are considered a key part of acountry’s national security. Chapter 22 lists a number of Internet resourceswhere you can get maps and map data for areas outside the United States.TIGERThe U.S. Census Bureau produces Topologically Integrated GeographicEncoding and Referencing (TIGER) data for compiling maps with demographicinformation. This vector data is a primary source for creating digitalroad maps of the United States. Figure 2-11 shows a map of downtown SanFrancisco created from Census Bureau TIGER data.Figure 2-11:TIGER dataviewed fromthe CensusBureau’sWeb-basedmap viewer.TIGER data is free (check out the Census Bureau Web site at www.census.gov/geo/www/tiger/index.html) but in some areas isn’t very accurate; roadsdon’t appear on the map and addresses aren’t in the right locations. The governmentis improving the accuracy of the dataset, and plans to provide betterdata in the future. You’re better off using some of the free and commercialstreet map programs and Web sites discussed in Chapters 12 and 18.