[GRASS-dev] Re: v.in.gshhs - bogus horizontal lines

Markus Metz markus.metz.giswork at googlemail.com
Fri Mar 20 05:54:36 EDT 2009

Maciej Sieczka wrote:
> Markus Metz pisze:
>> I thought this is true for GRASS. Generally, if you zoom/pan beyond 
>> the extend of the feature (raster or vector), nothing is displayed. 
>> This should not happen in latlon,
> Why not?
Because it does not happen in GRASS:-) and because latlon is not a true 
projection and because IMHO this is a nice feature and because other 
GRASS developers have thought long about how to handle latlon in lib/gis 
and I see no reason to question the current solution.
> What about other world-wide CRSs, like those based on sinusoidal,
> Goodge, Mercator projections - does GRASS wraps world edges for them as
> well?
No idea, you can check.
> Is there anything that could be done for v.in.gshs in GRASS 6.5 so that
> it's output (when the input is a full GSHHS extent) would be free of the
> horizontal longitudal extent-wide artifacts in case of:
> 1. editing in v.digit
> 2. exporting with v.out.ogr (that's another issue I haven't mentioned 
> yet)
> 3. displaying and editing in QGIS
Yes for 1, can't say for 2, No for 3. AFAICT, v.out.ogr itself does not 
produce these horizontal lines, they are produced by the application 
rendering the vector. Different applications have different, often 
incompatible policies, therefore it is not possible to accommodate every 
application. Either the GRASS display is 100% ok or digitizing is 100% 
ok, both is not possible with the full dataset. Actually, the mere 
display is always ok, but zooming to the selected map may or may not 
work. It does not work when the longitude map extends are > 360, but 
then the digitizer works... QGIS insists on straight lines also without 
wrap around in v.in.gshhs. It seems that QGIS wraps coordinates beyond 
-180 and 180 around into the -180, 180 range, then may draw these 
horizontal lines, and restricts the display area to -180, 180.
I want v.in.gshhs to produce output that is compatible with GRASS, and 
there I stop because it is not possible to accommodate every application 
(see also r.out.gdal discussion).
FWIW, I have submitted changes to make the digitizer work, no more 
horizontal lines there, tested with grass-6.5.


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