One likely possibility is that the map is schematic (not to-scale), like the Washington DC subway map in Figure 1. In that case, MapCruncher cannot transform the map.
Another possibility is that the map is mostly to-scale, but the cartographer has taken a few liberties. MapCruncher had difficulty registering points on the Vashon Island map shown in Figure 2. The note next to the explains why.
Another notorious map with liberal geographic features is the New York MTA Subway map. The remark in the key, as shown in Figure 3, explains why.
Another possibility is that you have selected some dramatically incorrect correspondences.
If you have to fine-tune a lot, you may have a not-to-scale map. Perhaps one or two points are incorrectly matched, and are throwing the rest of the approximation off. Use the error column, and re-check the correspondences with the worst errors. Replace suspect correspondences.
Make sure you are using correspondence points that are widely separated on the source map. If you start with a dense cluster, MapCruncher has to extrapolate for far-away points, and extrapolation produces bigger errors than interpolation. In this Los Angeles Transit map, all of the correspondences are in the city, which leaves features at the edge of the map not tied down, as shown in Figure 4.
Sometimes the road atlas data in Virtual Earth doesn’t match the aerial photography. We recommend registering to features in the Hybrid View, as shown in Figure 5.
Ultimately, you may never get a perfect match. This can happen due to limitations of MapCruncher’s approximate transformation, or due to errors in the source map itself, as shown in Figure 6.
How many correspondence points do I need?
Some source maps, such as aerial photographs or floor plans, are drawn in “conformal projections”, and can be registered with as few as two points, as shown in Figure 7.
Other maps require at least three points. Figure 8 shows the two correspondences are correct, but the source map is obviously locked in an incorrect position: the Northern border of Washington is not East-West, as it appears on the right. That’s because the chosen source map happens to not be a conformal projection.
Adding one additional point tells MapCruncher how to fix the problem, as shown in Figure 9.
MapCruncher rarely needs more than 15 to 20 correspondences, even on detailed maps such as a county-wide map with street-level detail. Beyond that, other sources of error usually limit the fidelity of the results.
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