[GRASS-dev] Struggling using two layered vector maps with v.what.rast

Sören Gebbert soerengebbert at googlemail.com
Wed Jan 25 07:59:09 EST 2012

Hi Markus,
thanks for your explanation.

2012/1/25 Markus Metz <markus.metz.giswork at googlemail.com>:
> 2012/1/25 Sören Gebbert <soerengebbert at googlemail.com>:
>> Hi Martin,
>> many thanks for your example. It works fine with two layer.
>> But is there a way to add categories to a number of layers larger than
>> two using the same vector map?
> This will add new categories to a new layer, irrespective of how many
> layers are already defined:
> v.category option=add
> Note that a GRASS layer is defined by the existence of at least one
> vector feature with at least one category value in that layer. That
> layer may or may not be attached to an attribute table. For examples,
> see v.in.ogr when importing several layers at once, the LiDAR
> filtering tools, or the vector output of r.stream.extract.

I will have a look.

>> Why is a full vector copy needed in case of category transfer from one
>> to another layer?
> Because category transfer takes place in the vector feature, i.e.
> modifies the coor file of the vector. v.category option=transfer does
> not add an attribute table (AFAIK).

Maybe this function:
off_t Vect_rewrite_line(struct Map_info *Map, int line, int type,
			const struct line_pnts *points, const struct line_cats *cats)

can be used to add new layer using existing categories (from layer 1?)
to an existing vector map?

My problem is that i want to successively add new layers (tables) to
existing vector maps using existing categories. Additionally i need to
timestamp the layer in the temporal database. It would be meaningful
to timestamp layers at file level using v.timestamp too.
An approach which i have in mind is to extent v.timestamp with a layer
option, so that timetsamp text files can be created for each layer in
the vector map directory.

An application in the temporal GIS and for environmental modelling
would be the observation of raster time series at specific vector
point locations. The vector location do not change over time, but the
attribute tables. Hence, for each observed time step a new timestamped
layer and table will be added to the observation vector map and filled
with raster values.

>>  i just started to investigate the
>> new shiny world of multiple layer usage.
> I hope you enjoy it!

 I do. :)

Best regards

> Markus M
>> Best regards
>> Soeren
>> 2012/1/25 Martin Landa <landa.martin at gmail.com>:
>>> Hi,
>>> db.copy from_table=soil to_table=soil_2
>>> -> makes copy of `soil` table
>>> v.db.addtable map=soil table=soil_2 layer=2
>>> -> it would overwrite `soil_2` table, but do nothing in this case, no
>>> feature have category defined in layer 2.
>>> Proposal:
>>> v.random --o -z seed=1 output=soil n=20 zmin=0 zmax=100 column=clay
>>> v.category input=soil out=soil2 option=transfer layer=1,2 --o
>>> db.copy from_table=soil to_table=soil_2
>>> v.db.connect map=soil2 table=soil_2 layer=2
>>> v.db.addcolumn map=soil2 layer=1 columns='sand_frac DOUBLE PRECISION'
>>> v.db.addcolumn map=soil2 layer=2 columns='sand_frac DOUBLE PRECISION'
>>> ...
>>> Martin
>>> --
>>> Martin Landa <landa.martin gmail.com> * http://geo.fsv.cvut.cz/~landa
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