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

[Nikos Alexandris]

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.
 
> 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