[GRASS-user] Query: number of 'areas' reported in v.build output does not necessarily represent different geographical areas
nik at nikosalexandris.net
Wed May 8 23:05:24 PDT 2013
> Snapping gives the maximum distance to another vertex in map units, degrees
> for latlon.
> I don't know the coordinate ref system for the world mangrove shapefile,
Here a link to it: <http://data.unep-wcmc.org/datasets/21>. For the records,
INFO: Open of `usgs_mangroves2.shp'
using driver `ESRI Shapefile' successful.
Layer name: usgs_mangroves2
Feature Count: 1432891
Extent: (-179.950672, -38.762073) - (179.987788, 32.350173)
Layer SRS WKT:
> but if degrees lat/long, a snapping distance of 0.000001 would equate to
> ~0.1 m at the equator, which should be sufficiently small?
My post's target was the (as Markus Metz wrote:) "suggestion of a reasonable
(assuming floating point rounding errors) threshold for snapping".
I'd like to have a confirmation that, given that an error message/suggestion
appears still after using a snapping distance of 1e-14, it means that the
imported vector map couldn't automatically be topologically cleaned and needs
some manual handling.
> Does using smaller values improve topology?
Is this a generic question or about the Global Mangroves vector map? If
generic, I don't think that topology is improved by using smaller values. If I
am not wrong, from things learned here and off-list as well, large(er)
snapping values might cause unwanted damage to the shape of the original data.
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