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

Sören Gebbert soerengebbert at googlemail.com
Wed Jan 25 09:36:43 EST 2012

Hi Markus,

2012/1/25 Markus Metz <markus.metz.giswork at googlemail.com>:
> 2012/1/25 Sören Gebbert <soerengebbert at googlemail.com>:
>> 2012/1/25 Markus Metz <markus.metz.giswork at googlemail.com>:
>>> 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.
> Also the v.net.* modules, which make use of layers to distinguish
> between line and node costs with the help of layers.
>>>> 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?
> Adding a new layer always means adding new categories, also if they
> are identical to categories in a different layer. The difference
> between Vect_rewrite_line and the combination of Vect_delete_line,
> Vect_write_line is only that Vect_rewrite_line is supposed to check if
> the new line fits into the space used by the old line.

If i understand this correctly the vector library provides the
mechanism to implement vector modules to add new layer and copying
existing categories into the new layer using the same vector map?

>> My problem is that i want to successively add new layers (tables) to
>> existing vector maps using existing categories.
> The problem is that the categories are not existing for a new layer
> because of the definition of a new layer. If there would be categories
> existing for that layer, the layer would already exist.
> I guess you must either use v.category option=transfer or have a bunch
> of tables but only one vector with one layer and disconnect/reconnect
> the tables as needed, only one table would then be connected to the
> vector at any given time. I have used that successfully for time
> series points some time ago, it's the fastest solution but needs some
> care in handling the tables. Probably not an option for the temporal
> database unless you want to introduce db.table.timestamp...

This is currently not an option.
But i guess i can modify v.category to support the transfer of
categories to more than a single layer of a new map:

 v.category --o input=soils output=soils1 option=transfer

This will copy the cats of vector map soils from layer 1 into the new
layer 1 - 2000 of vector map soils2.
Can you estimate how hard the implementation will be?

Best regards

* v.timestamp with layer support is in svn
* Vector layer timestamp support in the temporal GIS implementation is
in my local svn copy and   needs more testing -> i need to create
vector maps with multiple categorized layer.

> Markus M
>> 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
>> Soeren
>>> 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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