DESCRIPTION

v.category attaches, copies, deletes or reports categories of vector geometry objects. Further on, v.category adds a number given by the cat option to categories of the selected layer. These categories (IDs) are used to assign IDs or to group geometry objects into categories (several different geometry objects share the same category). These categories are also used to link geometry object(s) to attribute records (from an attribute table linked to vector map).

NOTES

Use v.to.db to upload related categories to a linked attribute table.

The type parameter specifies the type of geometry objects to which the category is added; it is similar to an input filter - only the geometry specified in 'type' is processed.

If the type parameter is set to centroid and the option parameter set to add, new categories will be added to existing centroids. Note however, that new centroids cannot be created this way. To do so, they must be added manually using wxGUI vector digitizer or by running v.category with the type parameter set to area.

If categories are copied with option=transfer, a warning is issued if categories already exit in the layer they are copied to. In this case the user must make sure beforehand that copying categories from one layer to another layer does not cause undesired grouping of different geometry objects into the same categories. This can be avoided by specifying only one layer. The module will then find the next free layer number and copy categories to there. The new layer number is reported at the end.

Areas are a special case because it is impossible to attach a cat to an area without a centroid; in this case, the module places new centroids in areas automatically.

The cat parameter is only used with option=add, option=sum and option=del.

Categories can be deleted for the given layer with option=del. If cat is set to -1, all categories for the given layer are deleted. If cat is zero or positive, only this category value will be deleted. By default, cat is set to 1 which means that only categories of value 1 will be deleted.

The ids parameter specifies the list of feature IDs to which the operation is performed; by default, all vector feature ids are processed. The feature ID is an internal (unique) geometry ID that all vector primitives possess, and is separate from any category the feature may also possess. Use

  v.edit map=inputname tool=select

to find out the geometry ids of certain features.

EXAMPLES

Report vector categories

v.category input=testmap option=report

LAYER/TABLE 1/testmap:
type       count        min        max
point          0          0          0
line        1379          1       1379
boundary       0          0          0
centroid       0          0          0
area           0          0          0
all         1379          1       1379

Delete all vector categories in layer 1

v.category input=testmap output=outmap option=del cat=-1

Add vector categories in layer 1 with step=2

v.category input=outmap output=stepmap option=add step=2

# report
v.category input=stepmap option=report
LAYER/TABLE 1/outmap:
type       count        min        max
point          0          0          0
line        1379          1       2757
boundary       0          0          0
centroid       0          0          0
area           0          0          0
all         1379          1       2757

Add categories/centroids to a vector map without categories

v.category input=wkt output=wktnew option=add

Results can be tested using d.what.vect.

Copy categories from layer 1 to layer 2,3,4,5,6,7 and 8

Existing layer will be overwritten, non-existing will be created.

v.category input=observer output=observer_new option=transfer layer=1,2,3,4,5,6,7,8

Print vector categories of given layer

Print vector categories from the first layer, only for feature ids 1-50.

v.category input=roads option=print layer=1 id=1-50

AUTHOR

Radim Blazek, ITC-irst, Trento, Italy
Modified (the id parameter) by Martin Landa, FBK-irst (formerly ITC-irst), Trento, Italy, 2008/02

Last changed: $Date$