GRASS Basics - Desktop GIS: Mapping the Planet with Open Source Tools - page 298

Joe Asks. . .
What Is Topology?
You probably noticed the word topology has cropped up a
couple of times now. In simple terms, topology is the relation-
ship between spatial features. For example, a polygon bound-
ary consists of lines. Adjacent polygons share common bound-
aries. Data formats that are topological maintain this relation-
ship when creating and editing data. The common boundaries
are stored only once.
In a nontopological format, the common boundaries are dupli-
cated, one for each polygon.
Apart from the storage difference, a topological GIS such as
GRASS maintains the spatial relationships as you edit data. If
you move a line, the polygon boundary or boundaries are
adjusted accordingly. This provides consistency in your data
and is essential when performing many geoprocessing tasks.
GRASS 6.2.2 (world_lat_lon):~ > v.in.ogr dsn=/home/gsherman/desktop_gis_data \
output=world_borders layer=world_borders cnames=dog
A datum name wgs84 (WGS_1984) was specified without transformation parameters.
Note that the GRASS default for wgs84 is towgs84=0.000,0.000,0.000.
Projection of input dataset and current location appear to match.
Proceeding with import...
Layer: world_borders
Importing map 3784 features...
-----------------------------------------------------
Building topology ...
If the import's successful, you should be returned to the GRASS prompt
without any error messages. You can quickly confirm that we now have
a world_borders layer using the following:
GRASS 6.2.2 (world_lat_lon):~ > g.list vect
----------------------------------------------
vector files available in mapset gsherman:
world_borders
----------------------------------------------
Now let's bring in a raster using the r.in.gdal command. This command
uses the GDAL library and therefore can import a wide range of formats.
For this example, we will import the NASA world mosaic into GRASS.