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

Blumentrath, Stefan Stefan.Blumentrath at nina.no
Fri Jan 16 04:49:02 PST 2015

Hi Sören,

And thanks for clarification.
I thought that t.vect.observe.strds would do what I am up to, but it is rather time-consuming for >20k maps.
So I thought it`s main purpose must be to generate a new STVDS from an STRDS.
Using r.what is much, much faster (15 min compared to > 12h for t.vect.observe.strds (and here I have not run t.vect.db.select yet because t.vect.observe.strds is still running (since yesterday evening)). When I only want a matrix (text output) as a result, creating a new STVDS seems to be too much overhead... 
This was the main reason for me to use r.what...


-----Original Message-----
From: Sören Gebbert [mailto:soerengebbert at googlemail.com] 
Sent: 16. januar 2015 13:33
To: Blumentrath, Stefan
Cc: GRASS developers list (grass-dev at lists.osgeo.org)
Subject: Re: [GRASS-dev] Sampling STRDS at point location(s)

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 layer.
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 Soeren

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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