[talk-au] Victoria - cutting the data polygon

cam_daw at fastmail.fm cam_daw at fastmail.fm
Wed Sep 11 09:22:48 UTC 2013


Hi Brett (and everyone else),

It looks like your first assumption is probably correct: The Victoria
polygon you're using is too big, it comprises of 10147 nodes (and can be
found at
http://downloads.cloudmade.com/oceania/australia_and_new_zealand/australia/victoria/victoria.poly
).
If you don't mind having small parts of NSW and SA in the VIC polygon,
then I recommend using a super-simple custom polygon for this.

JOSM has a plugin called "poly" that allows you to import and export to
poly files, so I just made up a quick 4 node polygon of Victoria that
should be fine to use:

Victoria
1
   140.955887   -33.713994
   150.349971   -36.765387
   150.312801   -39.210773
   140.958166   -39.214549
   140.955887   -33.713994
END
END

The notes on using the splitter (
http://wiki.openstreetmap.org/wiki/Splitter ) under the --polygon-file
part states that, "The name of a file containing a bounding polygon in
osmosis polygon file format. Splitter uses this file when calculating
the areas. It first calculates a grid using the given --resolution. The
input file is read and for each node, a counter is increased for the
related grid area. If the input file contains a bounding box, this is
applied to the grid so that nodes outside of the bounding box are
ignored. Next, if specified, the bounding polygon is used to zero those
grid elements outside of the bounding polygon area. If the polygon
area(s) describe(s) a rectilinear area with no more than 40 vertices,
splitter will try to create output files that fit exactly into the area,
else it will approximate the polygon area with rectangles."
- Which just means that the splitter likes rectangles that fit lat / lon
lines, and hence doesn't like anything with diagonal lines and lots of
complexity.
The --resolution parameter sounds like it enables you to specify how big
it can make the split off squares when approximating diagonal parts of
the polygon. The bigger the squares, the less it fits with the polygon,
but it'll be much faster to compute. So it looks like a smaller
--resolution number makes for bigger squares, by default it uses
--resolution=13

Regardless, the Victoria.poly file I've provided here should work fine
for you. I've used the poly file from
http://download.geofabrik.de/australia-oceania/australia.html when
creating mkgmap files for use with splitter, and also when using
phyghtmap to get contours australia-wide (at 10m resolution I would
expect about 1GB of contours in pbf format Australia-wide). So the
Australia-wide polygon on the geofabrik.de website has 13 nodes with
diagonal parts and it all works fine, so my 4 node polygon of Victoria
should also work fine, hopefully without any tweaks to its geometry.

Let us know if you have any further issues, and if you can, provide a
link to any largish files in future rather than having them in the email
body or attachment.

Cheers :-)

-- 
http://www.fastmail.fm - mmm... Fastmail...

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