[Talk-us] What to do with unnamed NHD streams

Michal Migurski mike at teczno.com
Mon Oct 29 05:35:53 GMT 2012

On Oct 28, 2012, at 10:29 PM, Paul Norman wrote:

>> From: Michal Migurski [mailto:mike at teczno.com]
>> Sent: Sunday, October 28, 2012 8:58 PM
>> To: OpenStreetMap US Talk
>> Subject: Re: [Talk-us] What to do with unnamed NHD streams
>> Does [NHD] all come in shapefile form? The simplification would be a
>> relatively easy (though time-consuming) task for PostGIS on a server
>> with sufficient storage, outputting new shapefiles for ogr2osm. I can
>> help with this using one of our office servers that we use for such
>> tasks.
> It comes in .mdb or .gdb form. These could be loaded into postgis and I've
> considered doing it. My home server has the storage and cores to do it, but
> I don't think it would help.
> 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.

> 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?


michal migurski- mike at stamen.com

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