[GRASS-dev] t.rast.univar Unable to get statistics for raster map

[Moritz Lennert]

On 5/12/18 10:47, Margherita Di Leo wrote:
> Hi,
> 
> I have a strds and I try to get statistics for an area within a mask. A 
> couple of images have no data within the masked area, and t.rast.univar 
> gives the warning message:  "Unable to get statistics for raster map". 
> In the resulting csv, the missed dates are skipped, and this is sort of 
> unexpected and creates hurdles when for example I compare it with the 
> behavior of t.rast.what, that keeps the date and associates a no-data to 
> it. Wouldn't it be better if also t.rast.univar would yield a no-data 
> rather than skip the date?

This can probably somehow be solved within t.rast.univar, but IIUC the 
actual issue comes from the underlying r.univar call which has the same 
behaviour, for example when call with the 'zones' parameter and the '-t' 
flag: data is just absent if there are only null values in a given zone. 
I've been struggling with that, for example for addon modules such as 
i.segment.stats. I don't know how easy it would be to change that within 
r.univar, but it would be nice.

IIUC, the issue comes from stats.c [1]:

123	        if (stats[z].n == 0)
124	            continue;

Maybe a flag to fill the stats with NULL values, instead of ignoring 
them, would be appropriate ? But I don't know what an good NULL value 
would be here.

Moritz

[1] 
https://trac.osgeo.org/grass/browser/grass/trunk/raster/r.univar/stats.c#L123