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

[Markus Metz]

Maciej Sieczka wrote:
> Markus,
>
> Thanks for your v.in.gshhs GRASS 6 port.
>
> I have a problem: when importing full GSHHS extent, strange horizontal
> lines through the whole longitudal extent are present in the output
> GRASS vector map, which are *visible only in v.digit or QGIS*, but never
> on the regular wxGUI map display or X monitors.
>
> To reproduce please import e.g. gshhs_c.b, zoom to it in wxGUI, then
> open in digitizer and compare. Also please notice that *in QGIS those
> bogus lines are always visible*.
>
> I suppose this is related to the GSSHS data extent 0 - 360 vs. GRASS
> -180 - 180.
Strange. v.in.gshhs converts all lon coordinates to the range of -180 - 
180. The only possible problem is Eurasiafrica (also converted to -180 - 
180), crossing the datum border, all other shorelines are not crossing 
the datum border. I wrote the new v.in.gshhs port so that it displays 
properly, but GRASS latlon handling is not fully consistent, e.g. 
bounding boxes in vector topology ignore the datum border problems 
whereas (most?) lib/gis functions take care of that. My guess is that 
neither the wxGUI vdigit nor QGIS do this on-the-fly wraparound like the 
GRASS display. No idea how to solve this. Glynn once remarked that he 
tried to support -360 - 360 in latlon, and said that this opens a 
problem box of Pandora. I tried myself, and found the same problem box :-(

Best regards,

Markus