Q: Why don't the Latitude/Longitude coordinates produced by my GPS receiver agree with the coordinates shown in LandView III?
A: There are several possible reasons for this.
First, the US Department of Defense for national security reasons, intentionally adds a varying degree of error to the GPS system for civilian users. The technical term for this is Selective Availability (SA). The errors can be as great as 100 meters, and the amount of error varies over time. There are a number of Web sites on the topic of GPS (use one of the Net search engines to find them) that describe the issues and techniques of reducing the error.
Second, your GPS receiver may be using a different datum (basically the starting reference for the latitude and longitude) than we are using for the TIGER/Line 1995 data layer which is NAD-83. The difference between these datums can be as much as 100-200 meters within the continental U.S. Most GPS units can be programmed to display lat/long results in NAD-83, or any one of a number of other datums.
Third, the LandView product presents spatial data from several agencies and each agency's data has specific limitations on positional accuracy. You should refer to the LandView III help which discusses the source and limitations of the data..
For example, some of the addresses in the various EPA databases were coded to ZIP code centroids if they could not satisfactorily geocode the address. One can usually tell such situations since several sites appear to be stacking up over a point. There is a disclaimer on page 72 of the LandView III Guided Tour document that says "Much of the data included in the five EPA data bases (sic) have been self-reported by facilities. EPA has not verified the accuracy of all the latitude and longitude coordinates contained in these databases. The validation of these data is an ongoing project."
Those using LandView to calculate a population estimate within a radius should be aware of the limitations of the TIGER/Line 1995 data . Our TIGER/Line 1995 map data comes from a variety of sources, mainly the US Geological Survey's 1:100,000 topographic maps. The specific source materials used in developing the Bureau's internal TIGER data base (from which the public TIGER/Line 1995 file was prepared) are principally the following: 1:100,000 US Geological Survey (USGS) topographic maps that were scanned by USGS for the Census Bureau. The USGS scanned the roads, railroads, hydrographic features, and miscellaneous transportation features layers. We then vertically integrated these layers into a topologically consistent file (not stored as separate layers). For the 2% of the land area covered by the major urban areas we substituted the GBF/DIME files that we had created in the '70s (with updates in '81 to '85). The resulting TIGER data base file was in turn updated by maps obtained from local officials and some field work done by Bureau staff. The Bureau also tagged all the features with the geographic codes needed to identify the areas for which it collects and tabulates data (essentially all administrative and statistical area boundaries).
The Census Bureau's mission to count and profile the Nation's people and institutions does not require very high levels of positional accuracy in its geographic products. Its files and maps are designed to show only the relative positions of elements. Coordinates in the TIGER/Line files have six implied decimal places. The positional accuracy of these coordinates is not as great as the six decimal places suggest. The positional accuracy varies with the source materials used, but at best meets the established National Map Accuracy standards (approximately +/ 167 feet) where 1:100,000-scale maps from the USGS are the source. The Census Bureau cannot specify the accuracy of feature updates added by its field staff or of features derived from the GBF/DIME-Files or other map or digital sources. Thus, the level of positional accuracy in the TIGER/Line files is not suitable for high-precision measurement applications such as engineering problems, property transfers, or other uses that might require highly accurate measurements of the earth's surface. Despite the fact that TIGER/Line data positional accuracy is not as high as the coordinate values imply, the six-decimal place precision is useful when producing maps. This precision allows you to place features that are next to each other on the ground in the correct position, relative to each other on the map, without overlap.
Therefore we suggest that users attempting to calculate population estimates using LandView disregard the GPS reading and rather use the TIGER/Line features to place their coordinate.
