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

Maciej Sieczka msieczka at sieczka.org
Tue Mar 17 15:38:38 EDT 2009

Markus Metz pisze:
> Maciej Sieczka wrote:

>> 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 :-(

I personally like the way QGIS or MapServer handles it - they just don't
care and don't try to treat lat-long data as connected at the datum

This has the major plus that in those applications there is no problem
with manual browsing/editing world-wide lat-long data. One can pan and
zoom just like in any other coordinate system and the display coordinate
extent is not restricted.

GRASS automatic wrapparound is a feature that brings more troubles than
benefits to developers and users IMHO.


Maciej Sieczka

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