Boundaries¶
Good boundaries are critical to being able to chop the large files into manageble pieces.
Boundary Sources¶
OpenStreetMap¶
It is possible to pull boundaries out of OpenStreetMap, but a warning, many aren't very good. In OSM, some boundaries are a Way (Polygon) some are relations, some are missing sections. Extracting many boundaries is not for the faint of heart... There is a website attempting to do this. If you only need a few boundaries, OSM works fine, but you might have to do a little manual cleanup.
Administrative Sources¶
I use official administrative boundary datasets. While these are all public domain, I have no interest in uploadinmg them to OSM, which would be a whole other project. And I only need them for data chopping anyway. You can get the official boundaries from these two sources.
These datasets are national wise, so need to be split into each forest or park. I wrote a program called tm-splitter that reads in the large file, and then splits each forest and park into a standalone file. This standlone file is a MultiPolygon, as many forests have multiple sections that aren't actually connected. But at this point, you have small enough boundaries to start on making data extracts for conflation.
Boundary Problems¶
Most of the National Park boundaries are ofte a single polygon, as many national parks are smaller than national forests. National Forest boundaries have an interesting set of problems that need to be cleaned up before they're usable.
Small Outholdings¶
National Forests often have multiple small Polygons that are outside of the actual forest boundary. These appear to be administrative buildings. Ranger stations, visitors centers, etc... These are all useles for our goal of improving remote trails and highways.
When the tm-splitter utility parses each MultiPolygon into it's indivugal Polygons, these small outholdings are ignored, leaving only the actual forest boundaries.
Inner Polygons¶
Since we're only interested in using these boundaries for making data extracts, some have interior Polygons that have other ownership or public land designations. Since we're using boundaries to make data extracts, I may drop all inner Polygons, but so far they don't see to cause any problems.