> From: Michal Migurski [mailto:m...@teczno.com] > Subject: Re: [Talk-us] What to do with unnamed NHD streams > > On Oct 28, 2012, at 10:29 PM, Paul Norman wrote: > > > The problem is you need to convert to .osm and *then* simplify. If you > > do this in the other order you have problems where one object > > intersects another (e.g. because they share a geometry for a portion > > of them). You end up simplifying away the intersection points and your > > resulting ways won't end up correctly sharing nodes. > > There are ways around this, by first de-duping the shared edges or > nodes. Topology preservation is not terribly difficult if you prepare > your data, for example by splitting lines and polygons at intersections > (as in your lake example), simplifying only the parts and then > reconstructing the original geometries.
Do you have a link to an example of the PostGIS magic to do this? It's beyond what I could do. A slight complication I found is that you can't just go for intersections but you also have to go to near intersections - sometimes the NHD data is off by a couple cm. I don't know if this will pose a practical issue for the simplification. > > I would *really* like to be able to simplify prior to ogr2osm as it > > would dramatically decrease the size of the nodes data in-memory and > > decrease conversion time, I just can't see how to do it prior to > processing. > > > > JOSM's simplify ways function works okay, although it doesn't deal > > with the case of two ways sharing nodes very well. > > Do you have any sample NHD extracts that might be usable for a test > drive? All of NHD can be found at on the USGS FTP site, but you need to compile gdal with 3rd-party toolkits to be able to use them. This is quite often a pain. I clipped out a small part of Kansas I was testing with early, converted it to a set of shapefiles and posted it at http://took.paulnorman.ca/imports/NHDH0202_shp.zip I should note that some information is lost in the shapefile conversion (long field names). For space reasons I dropped the WBD_HU layers. The first step of my converting is to remove them anyways. _______________________________________________ Talk-us mailing list Talk-us@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-us
