:Re: [GRASS-user] Some problem about the grass

Nikos Alexandris nikos.alexandris at felis.uni-freiburg.de
Wed Mar 18 13:51:31 EDT 2009

> Thank you very for your reply!
> In fact I mean the "Categories", and what you said is very helpful for
> my understanding something about the grass though not much.

> I think what I am now confusing about the grass is how it describe a
> map either a raster or a vector (actually the raster is my primacy),
> in other words what can we get from a raster or a vector map?

Maybe all this is known to you, but I am writing it anyway.

These are generic questions. You might want to read about data models in
general (basically raster and vector data). A nice and practical view,
through GRASS, about all this is the GRASS book [*]. You will get an
answer as well about what categories are with respect to raster maps.

You can think rasters as matrices and vectors as points, lines or closed
polygons. Depending on the nature of what you want to represent in a
map, you either choose raster (like for temperature of precipitation
maps) or vector (location of weather stations, roads or burned areas).

Of course there are data that can be represented in both raster and
vector maps. It's always a question of what you have and what you want
to achieve.

> I had read the manual you mentioned "Raster data processing in GRASS
> GIS", it says "The geographic boundaries of the raster map are
> described by the north, south, east, and west fields”, but how?? Is
> there a example?

Absolutely. I (can) set the geographic boundaries for Peloponnese, South
Greece for example like that:

g.region s=36 e=23.6 n=38.5 w=21

Note that the above values are geographic (=degrees) and NOT projected
coordinates. We can do something similar in a projected coordinate
system with values given in metres.

You know that there are many GRASS-related tutorials [1]. And even more,
GFOSS related [2] ?

> 1)     The “r.out.gdal”. When I try to convert a map to a geotiff,
> however the image I got is often a black blank!

That one might NOT be a problem if you just have not set your "region"

It can be a problem sometimes related with color table rendering. You
could try to remove the color table (r.colors -r YourMap) before

There are some tips in the manual about this [3]. And there is a long
discussion in GRASS-trac [4] 

> 2)

Sorry but I have no working experience with GRASS under Operating
Systems (or environments) other than Linux.

> PS:
> I have just register the maillists of the grass and how can I directly
> ask or reply in the web pages  as in a forum?

You can register your self in GRASS' wiki [5] and add/edit almost
anything. The wiki is from and for the community.

Cheers, Nikos

[*] http://www.grassbook.org/

[1] http://grass.osgeo.org/gdp/tutorials.php

[2] http://wiki.osgeo.org/wiki/Educational_Content_Inventory

[3] http://grass.itc.it/grass64/manuals/html64_user/r.out.gdal.html

[4] http://trac.osgeo.org/grass/ticket/73

[5] http://grass.osgeo.org/wiki/Main_Page

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