[GRASS-user] Query: number of 'areas' reported in v.build output does not necessarily represent different geographical areas

[Markus Metz]

On Thu, May 9, 2013 at 8:05 AM, Nikos Alexandris
<nik at nikosalexandris.net> wrote:
> RichardC wrote:
>
>> Hi,
>>
>> 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,
> ogrinfo reports:
>
>  INFO: Open of `usgs_mangroves2.shp'
>       using driver `ESRI Shapefile' successful.
>
> Layer name: usgs_mangroves2
> Geometry: Polygon
> Feature Count: 1432891
> Extent: (-179.950672, -38.762073) - (179.987788, 32.350173)
> Layer SRS WKT:
> GEOGCS["GCS_WGS_1984",
>     DATUM["WGS_1984",
>         SPHEROID["WGS_84",6378137.0,298.257223563]],
>     PRIMEM["Greenwich",0.0],
>     UNIT["Degree",0.0174532925199433]]
> [cut]
>
>
>> 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.

Try to import with snap=1e-13
then try to import with snap=1e-12
then try to import with snap=1e-11
then try to import with snap=1e-10
etc.

stop when there are no more errors

Markus M

>
>> 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.
>
> Thanks, Nikos
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