[Talk-us] California land cover import

[Lord-Castillo, Brett]

I know we're not that big on commercial software, but the esri smooth/simplify tools will fix that first problem very quickly.
You really cannot do a spline interpolation though, because that requires true curves which are not supported by the OSM format (nor PostGIS). You could do a polyline approximation of a spline interpolation, but after you simplify, interpolation, then approximate off a dataset that is already a generalization, how far away are you from the real world data?
(To make this worse, the California shapefiles are polygon conversions of 5m raster data, hence the points every 5m. The nice part is that you can be reasonable certain that 5/2 * sqrt(2) meters is the correct allowable deviation for a smooth/simplify.
For the last problem, if the ways are overlaying each other rather than overlapping, a conversion to a topological data format will eliminate that problem quickly. If the ways are truly crossing, than an esri topology can go a long ways towards fixing those. Instead though, I would suggest a direct conversion to raster (or get ahold of the original raster data) and then a smoothed raster to polygon conversion to fix that.
Brett Lord-Castillo
Information Systems Designer/GIS Programmer
St. Louis County Police
Office of Emergency Management
14847 Ladue Bluffs Crossing Drive
Chesterfield, MO 63017
Office: 314-628-5400
Fax: 314-628-5508
Direct: 314-628-5407



-----Original Message-----

Date: Tue, 29 Dec 2009 23:43:45 -0800
From: <nmixter at runbox.com>
Subject: [Talk-us] California land cover import
To: <talk-us at openstreetmap.org>
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The state of California has some good landcover shapefiles on the
Department of Forestry and Fire Protection site. They are sorted by
county. The smallest are under a meg while the largest - Fresno - is more
than 400 megs. The average size is around 30 to 40 megs. These would be a
great addition to OSM since they contain valuable metadata. Once the
areas have been added, the state will take on a similar look to states
like Georgia and Massachusetts that have
had statewide imports done. Another good example of what California can
be is the Corine Land Cover (WikiProject Corine Land Cover).

There are several challenges with the data. Here are a couple.
* It is a huge dataset and will need some optimization. The straight
lines have points every five meters, creating jagged edges that
aren't visually attractive. Josm has a plugin but that's probably not
the best way. A spline interpolation would really improve it
dramatically. It will increase data size, but it's worth it. Maybe
there is a batch mode program available to do that. Mapshaper has a
tool to optimize a shapefile by reducing the details in the file. It
may not be working though. Here is a rough idea of what the areas
look like without being simplified.

* Only vegetation data should be used. The
urban/residential/water/unknown should be skipped since the quality
isn't good enough for this purpose and would just create tons of
conflicts with existing data. Also there is/will be better data
available.

* We can use some filters to split the shapefiles into different
features. This is also great to prepare the OSM files for each type
instead having it mixed all together.

* All these polygons have overlapping ways. This should be avoided
because it creates tones of duplicate nodes/ways. Each polygon should
be split in single ways and the area defined with a relation.
According to the description, mapshaper will do that to. There may be
a way to do this with postgis or through a Perl script. Validator
will just merge duplicate nodes, but can't fix duplicate lines. This
is really a tricky thing that many other updates lack. Efficiency
should be considered here. Also editing is easier then. This data
shouldn't need much editing after import.

Please feel free to add your comments and suggestions on the California Land
Cover wiki page. This is also being posted to the imports list.

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