On Sun, 2012-10-28 at 20:51 -0700, Paul Norman wrote: > 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. >
Yes to this. > 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 think that excluding 46003's is generally O.K. They can be useful, but are not really necessary for the import. I do think that not including 46006's without names would exclude many important and obvious waterways. Here in NC, some of these actually do have (local) names and many are significant especially for hiking/biking trails as they represent places where feet can get wet or there is a big dip/rocky area. My vote would be to keep them. James _______________________________________________ Talk-us mailing list Talk-us@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-us
