[GRASS-user] new GRASS virtual raster in trunk

[Nikos Alexandris]

* Rich Shepard <rshepard at appl-ecosys.com> [2018-06-04 05:56:16 -0700]:

>On Mon, 4 Jun 2018, Markus Metz wrote:
>
>>A new GRASS virtual raster format has been added to trunk in r72761-4,
>>together with a new module to build a GRASS VRT: r.buildvrt.
>
>Markus,
>
>  For folks like me who have not followed this thread please explain the
>problem for which a raster VRT is the solution.
>
>Best regards,
>
>Rich

Dear Rich,

(sorry Markus for my intervention -- just so excited about this new
creation of yours!)

if I may, virtual raster maps allow to treat an arbitrary number of
raster maps as one map. Hence, it is not required to patch hundreds or
thousands or more (?) raster map tiles, to build a single map.

Why build a single map?  Because the one or the other module or workflow
need it, in order to be able to process the entire computational extent
of interest.


In my case, I treat 177 tiles that cover European territories,
for each of which there exists a raster map (of 10000 x 10000 pixels).
Having one raster map, for the whole of Europe, will make it easier to
extract zonal statistics, for example.


So, in the general case of many raster maps, that form say, a "canonical"
tile grid, zones of interest (say vector boundaries), for which is asked
to extract statistics, are possibly split over two to up to four raster
map tiles. It takes some respectable amount of time of handcrafting, to
get stats for each tile, assembly then results for zones that don't
entirely fall inside one tile.

I am sure there are better explanations or even more complicated
problems for which a Virtual raster is the answer. In short, however,
and the way I understand things, it is a "work-around" to avoid dealing
with huge raster data sets. Huge as in file size. And, while the total
processing time, might not necessarily drop down, depending on the
work-flow, the time to script a solution decreases dramatically.


Nikos, happy to be a GRASS GIS user.
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