A few months ago, I tried to get started on trying to resume the NHD import in my area - and some of the places where I hike. I'm trying to check results with both P2 and JOSM, and tripping over a lot of things, which made me put the project back on hold for a while. (I had some other things to be about.)
But now I'm starting to reconsider again, after hearing that there are others out there thinking the NHD import is desirable for an "Open Trail Map." I mentioned this in a message earlier today. So, let me review some of the things that have me scratching my head. (1) The mapping from NHD feature codes to OSM tags is incomplete (and not quite consistent). That's fine; for all the FCodes in my area, I've been able to find features that I'm familiar with that I can describe. So I think I have that problem fairly well licked. (I may have to invent a tag or two, like "waterway=rise" and "waterway=sink" to describe streams that go underground and appear again in karst terrain.) (2) One historic complaint I've seen about the NHD import is that it clutters the map, and that the assignment of "river" and "stream" is difficult. What I'd propose is to tag as "river" anything that has more than two nodes of its flowline as "Artificial Path", and "stream" anything that's smaller. In my area (eastern New York), for instance, that would mark out the Schoharie Creek, the Mohawk, the Hudson, and the lower reaches of the Catskill, the Kaaterskill, the Esopus, and maybe a few others. Is this a reasonable approach? (3) Is it necessary to tag shorelines with name=? It would be something of an effort to identify what flowline belongs to a shoreline, and it's somewhat ambiguous near confluences. I'm inclined not to do anything about this issue unless there's an overwhelming consensus that it needs to be addressed. (4) What about cadastral lines (administrative boundaries, and land use, leisure, etc.) that appear to follow flowlines? My inclination is not to touch these at all, even if the flowline is being refined. Oftentimes, when a stream changes course, the cadastre remains unchanged. Only the recorder's office in the jurisdiction in question would actually know the situation. I think I'd rather see slightly inconsistent boundaries than mess up something like that. (5) In the area I have in mind, there are very few ways that actually would need to be conflated (a few major rivers). Is it likely to be called vandalism if I confine myself to copying the OSM-specific tags from the OSM ways onto the NHD ones? I trust a land survey rather more than I do someone tracing a shoreline from Bing imagery: in well-graded streams, the shorelines can be variable and ambiguous, and NHD has pretty sound information about high water marks. (6) When I try a limited import, I get a lot of JOSM warnings about waterways crossing highways. Do people think that all of these have to be fixed before importing? In that case, I'll have to confine myself to the very small area that has highway-crossing-stream that I can visit myself (to try to settle what the boundaries of the bridge, dam or culvert are, and what type of waterwork it is). Is it considered acceptable simply to leave the crossings unmarked? (They still render correctly in Mapnik, for what it's worth.) (7) In the event that I find a node tagged with something other than a waterway (or landuse=reservoir, man_made=dam, etc.) that collides with an NHD node, is the correct approach to conflate the nodes, introduce a duplicate, or offset the new node? Does this question even have a correct answer? I'm trying seriously to "do no harm", and getting paralyzed by the fact that there seem to be no firm guidelines on what is acceptable. I'm trying to start with areas where I have firm local knowledge, although clearly I cannot survey streams on private lands, so I have to depend on NHD there. But the result is not going to be of much use to me unless I can generalize what I learn to do better NHD imports in areas that I know less well. -- 73 de ke9tv/2, Kevin _______________________________________________ Talk-us mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk-us
