[GRASS-user] Export/define region with Map Scale

Hamish hamish_b at yahoo.com
Mon Mar 1 07:25:45 EST 2010

Franz Schiller wrote:
> At GIS Map Display window it's possible to
> define map scale in the bottom bar.

note that it is probably wrong. in order for it to be right
the exact distance across your monitor would have to be known
as Scale is the ratio of distance on map to distance on ground.
If the distance on the screen (in ground units) isn't known,
you can't do that calculation. Typically the system (be it X11
or MS Windows) has a rough idea of the dpi, but it's probably
+/- 30% and not very reliable.

Some software like the GIMP have a calibration tool where you
can make it a bit better (until you change resolution settings
or use a different monitor). but most software just give you
a very rough approximate and hope you don't notice/care.
IMO that scale in the map display window is misleading and
should be disabled. (just because lots of other gis software
pretend to support this doesn't mean we should too; borrowing
the monitor calibration tool idea from GIMP is another option)

> I would like to know
> is:1- Is it possible to export and plot maps
> (rasters/vectorials) with this map scale definition?

yes. the ps.map module knows its output plots will be on paper,
and it knows how big that piece of paper is. therefore it can
correctly calculate and use the scale for PostScript (and so
PDF format too) hardcopies.

> 2- Is it possible to set region using this map scale
> definition in order to have by default this setting?

not directly, but you could write a script wrapping g.region
and d.info which after calibration could keep you somewhat
close to your desired scale. That's probably more trouble than
it's worth though.



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