[GRASS-user] displaying points based on attributes

Moritz Lennert mlennert at club.worldonline.be
Wed Oct 11 04:12:11 EDT 2006

Hamish wrote:
> Seb wrote:
>> I have imported a shape file containing simple points, with an
>> associated attribute table.  I can see the attribute table in the
>> 'dbf' directory of the mapset.  When working from the display manager,
>> however, I cannot find how to vary symbols/colors for the points,
>> based on an attribute of choice.  Can somebody please point out how
>> this is done?  Thanks in advance.
> by category:
> add the vector map multiple times, one for each symbol/color
> combination, and then restrict with "use SQL query" box.
> e.g. "ATTRIBUTE > 6" or "ATTRIBUTE = 'large'"  See the SQL help
> pages for syntax.
> graduated by value:
> d.vect.thematic  (Add thematic map layer button next to add vector
> button)

You can also use a grassrgb column in your dbf file, in which you give 
an rgb color value (in the form 154:265:247) in each line. You can do 
this with v.db.addcol and v.db.update (accessible in GIS Manager via 
Databases->Manage Databases). This color then serves as _fillcolor_ (not 
outline !), so you have to chose a point icon which can be filled (e.g. 
circle, box, etc):

v.db.addcol map=YourMap columns="grassrgb varchar(11)" # create grassrgb 

v.db.update map=YourMap column=grassrgb value="'255:0:0'" 
where="attribute=X" # update grassrgb with color red where attribute is 
equal to X (watch out for quoting of color value: single quotes in 
double quotes on command line, single quotes in GUI
v.db.update map=YourMap column=grassrgb value="'255:0:0'" 
where="attribute>X and attribute <= Y" # update grassrgb with color red 
where attribute is between X and Y

Then to display your points:

d.vect -a map=YourMap etc...

A while ago, Martin Landa also said that he might try to implement 
random colors for any attribute column (instead of only category value): 
CC'ing him to see if he ever got around to it. This would allow you to 
use d.vect -c

The biggest problem I see with all this at this stage in GRASS, is that 
with the exception of d.vect.thematic, you cannot easily get a legend 
for the colors used...


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