[GRASS-dev] Which are the strds 'columns'?

[Sören Gebbert]

[snip]

> (Sorry for bottom-posting, want to break the discussion to a new
> "thread")
>
> It is, and was, clear that the `t.rast.list` module does exactly
> what it is named for. It lists the raster maps of an strds and their
> properties.
>
> On the other hand, the skepsis on t.rast.univar to be able to report
> maps with a where SQL query that accepts mean values as a "column" is of
> interest.

But t.rast.univar is designed to compute the mean values. IMHO, you
can't expect to have a selection option of a value that should be
computed after selection?
The beauty of GRASS GIS is its Unix-like design. Many dedicated
modules that show their strength when combined with other Unix tools.
How about using AWK?

> Univariate statistics are computed and reported. Is it really
> such a computational overhead to keep that "mean", or any other of the
> descriptive statistics, and re-loop over the maps to filter the output?

The temporal database has no "mean" column, so the current where
option can not be used.
However, its really simple to improve that. Feel free to implement the
following steps into GRASS, to avoid extra post-processing by adding
new statistical metadata columns:

1. Patch the raster library to compute and store the mean value on the
fly while writing the raster rows and write tests for validation. It
already stores min and max, so this shouldn't be too difficult. In
addition, you can compute many other statistical values on the fly
like median and so on and add them to the raster map metadata. Be
aware of backward compatibility for raster map layers that do not have
the new metadata.
2. Patch r.info to show the new statistical metadata and write tests
to validate it. Be aware of backward compatibility.
3. Patch the temporal database SQL scripts to add the new statistical
columns to raster layers and space time raster datasets (STRDS) e.g.:
min_mean, max_mean, min_median, max_median, ... .
4. Patch the temporal framework and include the new statistical
columns in the space-time classes for raster layers and STRDS and
write tests for validation. Make sure your improvements are compatible
with older GRASS GIS temporal database versions.
5. Patch the temporal modules to make use the new statistical columns
(where queries, t.info ...) and write tests for validation. Be aware
of backward compatibility.
6. Patch r.univar and t.rast.univar to compute only the remaining
statistical values, write tests and update the documentation.
7. Update the documentation about raster map layers and the temporal
framework using the new statistical metadata for raster maps and STRDS

This is a nice little GRASS development project. :)

> I feel it's worth it to avoid the extra post-processing step, outside of
> GRASS' environment. May I open an enhancement request?

Yes, feel free to open an enhancement request. The workflow howto
implement this is clear i think.

Best regards
Sören

>
> Thank you, Nikos
>