Background: I'm working on converting NHD to .osm format NHD is an extremely large data set. It's about 25G of zipfiles and all of this converted to .osm would total about 3 TB. This is about 10x-15x times the size of planet.osm.
There are three factors that lead to this large size. The third is what this email is about 1. The NHD covers a massive area. 2. Some ways are very over-noded. The NHD accuracy standard is <12m error 90% of the time. Running a 1m simplify in JOSM reduces the number of nodes to 25%-50% of what it was before. Like everything with the NHD, this varies from region to region. I'm thinking a 2.5m simplification would be best - it's 1/5th of the accuracy standard. Of course, running a simplification on a dataset this large is a challenge in itself. 3. A lot of NHD is very minor streams "only of use to hydrologists." There are streams that you would be hard pressed to locate if you were there in person and in some cases they do not exist anymore. A sensible solution in any NHD translation may be to drop any FCode 46003 (intermittent) streams without a name. It may also be worth dropping FCode 46006 (perennial) streams without a name. I've looked at a couple of regions with this adjustment made and they seem a lot more reasonable. The data is a lot more manageable and of more relevance to a general purpose map like OSM. There are also a lot fewer cases of streams that no longer exist. Does anyone have any thoughts on what should be done in a NHD translation with these streams? _______________________________________________ Talk-us mailing list Talk-us@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-us
