DESCRIPTION

d.vect.thematic draws thematic choropleth vector maps based on an attribute column or an expression involving several columns. It takes a list of class breaks (excluding the minimum and maximum values) and a list of colors to apply to the classes (has to be the number of class breaks + 1).

Instead of a list of class breaks, the user can also chose a classification algorithm and a number of classes (nbclasses). See the v.class for more information on these different algorithms.

Important note: The module currently supports only area vector type. For displaying thematic choropleth vector maps of other types (points, lines) can be used d.vect.thematic2 available in AddOns via g.extension.

NOTES

The -l flag instructs the module to print legend information (class min | class max | number of observations in class | color) to standard output for further use in graphical software. When combined with the -e flag, the legend information will be extended with some additional statistical information. If the -n flag is set, the module will only print the legend information without drawing the map. If the user gives a legendfile, the module will write d.graph instructions for painting a legend into that file.

EXAMPLES

Thematic map with classes

d.vect.thematic -l map=communes3 column=pop \
  breaks=111393.250000,222785.500000,334177.750000 \
  colors="255:0:0,0:255:0,0:0:255,0,0,0"

Thematic map with calculated class breaks

The following example uses a calculated attribute (density = pop/area) and the standard deviation algorithm to calculate class breaks for 5 classes: 
d.vect.thematic -l map=communes2 column=pop/area algorithm=std \
  nbclasses=5 colors="0:0:255,50:100:255,255:100:50,255:0:0,156:0:0"
To actually draw the legend for the polygons in the graphical display, the procedure is: 
# first save legend instructions to a file	
d.vect.thematic -n map=communes2 column=pop/area algorithm=std \
  nbclasses=5 colors="0:0:255,50:100:255,255:100:50,255:0:0,156:0:0" \
  legendfile=legend_pop_dens  

# open a monitor
d.mon wx0

# draw the map
d.vect.thematic map=communes2 column=pop/area algorithm=std \
  nbclasses=5 colors="0:0:255,50:100:255,255:100:50,255:0:0,156:0:0"

# draw the legend
d.graph input=legend_pop_dens
In order to further enhance this legend, edit the legend file accordingly and rerun the process.

Thematic map with calculated class breaks

Example for the North Carolina sample dataset, colorizing basin polygons by average elevation: 
# create watersheds from elevation map
r.watershed elevation=elevation threshold=10000 basin=basins_10k

# convert raster to vector
r.to.vect input=basins_10k output=basins_10k type=area column=basin_num

# upload raster statistics to each polygon in vector map
v.rast.stats map=basins_10k raster=elevation column_prefix=elev_

# save legend instructions to a file
d.vect.thematic -n map=basins_10k column=elev_average algorithm=int \
  nclasses=5 colors=228:26:28,55:126:184,77:175:74,152:78:163,255:127:0 \
  legendfile=d_vect_thematic_leg.txt

# open a graphical display
d.mon wx0

# draw outline of polygons
d.vect map=basins_10k type=boundary

# draw color fill
d.vect.thematic map=basins_10k column=elev_average algorithm=int \
  nclasses=5 colors=228:26:28,55:126:184,77:175:74,152:78:163,255:127:0

# set a different font (list fonts with: d.font -l)
export GRASS_FONT="sans"

# and finally draw legend, but edit the legend file first (for example:
# set size to 20, remove feature counts and most decimal digits)
d.graph input=d_vect_thematic_leg.txt
d_vect_thematic example
Thematic map of area sizes

SEE ALSO

v.class, d.vect, d.graph, v.univar

Check also Python module from AddOns: d.vect.thematic2

AUTHOR

Moritz Lennert

Last changed: $Date$