My first pass at a visitor layer involved manually downloading PDF and TIFF files, erasing areas outside the park boundary, lining up 4-5 points on each map with real world coordinates, and then tiling them. Where parks and forests met with curvy borders, erasing everything outside their respective borders was painstaking. Goof one of the alignment points and I had to delete the existing tiles and then re-tile the map. I got pretty good coverage for California and then gave up. It was simply too much work and too little benefit.
Since then the park and forest services have done a better job releasing maps as GeoTIFFs and GeoPDFs, and I felt like it was time to take another pass at visitor maps. In order to avoid the cropping issues that cost me so much time in the first iteration, I went with two layers, one for the NPS and one for the USFS. There are almost no contiguous national parks, so I could render the park maps as-is without cropping. Forest Service land is a different story, and after investing too many hours tweaking an edge-detection algorithm I said "good enough".
|The original visitor layer. Although close, it's not a perfect fit with the aerial imagery. This is partly the map's fault, and partly my inability to perfectly georeference and warp it.|
|In the new NPS layer, streams in Tuolumne Meadows line up dead-on with aerial imagery.|
|New USFS layer. The automated cropping left a little bit of the left map's border in place.|