Q: When using the .BNA boundary files with TAP I sometimes notice errors in the locations of some of the objects, such as roads. Why is this?
A: Most of the boundary files available for use with TAP are derived from the "1:2,000,000-Scale Digital Line Graph (DLG) Data" files available from the US Geological Survey. As the name implies, the points that define the objects (county boundaries, roads, rivers, etc.) were digitized by the USGS from 1:2,000,000 scale maps. The stated data accuracy for the digitized data is .003 to .005 inch (0.0762 to 0.127mm) as described in Digital Line Graphs from 1:2,000,000-Scale Maps - Data Users Guide, page 20, (USGS, 1990).
At this scale, 0.1mm on the map (within the stated accuracy range of the data) represents approximately 200 meters (about 656 feet, or 0.12 miles). Therefore, even small discrepancies in the digitizing process can result in noticeable differences in the coordinates in the .BNA files. Naturally, the magnitude of the problem will depend on the scale used to plot the files. For example, if you plot the .BNA files along with a transmitter site, coverage information, etc., at a scale of 1:2,000,000 or 1:1,000,000, the potential discrepancies in the location of the boundary information may not be noticeable, since the plotting scale is on the same order of magnitude as the original maps used to digitize the information. The hypothetical 0.1mm digitizing error will still be only 0.1 or 0.2 mm on the finished plot. On the other hand, if you plot at a smaller scale, such as 1:250,000, there may be noticeable differences, (such as a highway or river on the wrong side of a transmitter site). That 0.1mm digitizing error is now represented by 0.8mm, and you begin to see more of the results of the discrepancies.
If a particular object in a .BNA file is in error, the "Enter/Edit Boundary Line File" program in TAP can be used to make corrections, or to enter a completely new version of the object. While this may involve some manual labor in collecting and entering the coordinates that define the object, it only needs to be done once. Then the resulting more accurate boundary file can be used with all future TAP plots.
The boundary line data .BNA files available for use with TAP are useful for providing general information about political boundaries and natural features on, for example, a shadowing study or field strength threshold map. If you need more detailed or more accurate boundary or road information, you can use TAP to produce a transparency of the coverage information at any desired map scale and overlay the transparency on a pre-printed map of the same scale (such as a USGS topo map, county road map, etc.). If you have a pen plotter, you can also put the map itself into the plotter and plot the coverage information directly on the map. TAP allows you to position the pen to set the site location, registration marks, etc., to aid in locating and aligning the coverage plot with the map.
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