[GRASS-dev] Sampling STRDS at point location(s)

Sören Gebbert soerengebbert at googlemail.com
Fri Jan 16 04:32:31 PST 2015

Hi Stefan,
from my understanding of your requirements and the study of your
script i would say, that t.vect.observe.strds should be the right tool
for you.

This module has two inputs. The first input is a vector map layer with
vector points. The second input is one or several space time raster
datasets that should be sampled over time at the vector point
positions. The space time raster dataset will be sampled over its
whole temporal extent (from start to end). You can adjust the range
using the temporal where condition t_where. You need to specify a
column name for each input space time raster dataset.

The result is a new space time vector dataset that contains a single
new vector map and as many time stamped attribute tables linked to the
new vector map as raster map layer are present in the input space time
raster dataset.
Hence, for each time step in the space time raster dataset a new
attribute table is created. The GRASS GIS Temporal Framework allows to
time stamp attribute tables that can be linked to a single vector map
The module v.what.rast is used for sampling the time stamped raster
map layers. All sampled values of a single time stamped raster map
layer are written into a new time stamped attribute table.

You can use  t.vect.db.select to print attribute values of the space
time vector dataset to stdout.

Vector map layer sample_points contains the vector points at which the
STRDS temperature_daily should be sampled over time. Output is a new
STVDS sampled_stvds and a new vector map sample_vect that links to the
time stamped attribute tables.

t.vect.observe.strds input=sample_points strds=temperature_daily
output=sample_stvds vect=sample_vect columns=temp

The output is generated with t.vect.db.select and should look like this:

t.vect.db.select input=sample_stvds columns=cat,temp

start_time | end_time | cat | temp
2001-01-01 | 2001-01-02 | 1| -6
2001-01-01 | 2001-01-02 | 2| -4
2001-01-01 | 2001-01-02 | 3| -8
2001-01-31 | 2001-02-01 | 1| -3
2001-01-31 | 2001-02-01 | 2| -2
2001-01-31 | 2001-02-01 | 3| -5

I hope this is what you are searching for
Best regards

2015-01-16 10:08 GMT+01:00 Blumentrath, Stefan <Stefan.Blumentrath at nina.no>:
> Dear all,
> On a regularly basis I am asked by colleagues to extract raster based time
> series data (with > 20k maps) for a (set of) point(s).
> After both t.vect.observe.strds, t.sample and t.vect.what.strds seem to have
> a different purposes,  I do this using a little shell script which mainly
> uses r.what (please find it attached).
> My first question is, did I overlook an existing module which does the job?
> The data behind g.gui.tplot would actually be what I was interested in (also
> for more than just one point)...
> I could further-develop the attached shell-script into a new Python-AddOn if
> that would be of interest, but some improvements in r.what would not make it
> necessary to have an new module. The main problem is the 400 maps limit in
> r.what (which is named as a “to do” in the r.what manual). Are there any
> plans on working on this limitation?
> Another nice feature in r.what would be to be able to name an attribute
> column which is written to the site_name column of the r.what output when
> coordinates are taken from a vector map (like it is done when coordinates
> with a site name are used). The latter can be achieved for the “cat” column
> when “v.to.db”-output is piped to r.what. However it would be nice to be
> able to use clear text names here as well (which at the moment – as far as I
> can see – requires to write the coordinates to the attribute table (v.to.db)
> and then fetch them along with the full-text site name (using v.db.select)
> in order to pipe them to r.what).
> As you can see in the attached script I also transpose the output of r.what
> because for time series with thousands of maps my colleagues will likely get
> problems with the number of columns in the software they are using. Therfore
> a switch in r.what between “horizontal” and “vertical” output (like in
> v.db.select) would be useful when dealing with thousands of maps in r.what…
> Any thought? Would you prefer an enhancement request for r.what or a new
> addon or none of both?
> Cheers
> Stefan
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