[GRASS-dev] r.sum replacement in GRASS 7

[Glynn Clements]

Vaclav Petras wrote:

> what is the right replacement for r.sum? The NewFeatures [1] (1) page says:
> 
>   r.sum: merged into G7:r.statistics, G7:r.statistics2, G7:r.statistics3
> 
> However, it seems to me that r.univar is the replacement. But on the other
> hand, r.univar always computes all statistics, so there is certainly some
> overhead and also reading requires some additional time. This might be
> significant if you use r.univar in loop, for example with temporal data.

r.univar is the replacement for r.sum.

r.statistics and r.stats.zonal (which is the new name for
r.statistics2) create a new map which is a copy of the base map but
with the categories replaced by the aggregate value computed over the
corresponding cells from the cover map.

You could get a similar result to r.univar by using r.stats.zonal with
the -r switch and a base map where all cells contain the same category
value (created using e.g. r.mapcalc "basemap = 1"), then extracting
the value from the reclass table.

But that's inefficient, due to the additional I/O overhead (even for a
1x1 cell map, it will still perform the decoding and rescaling for
each row).

The calculation of additional aggregates by r.univar shouldn't have a
noticeable effect upon performance so long as -e isn't used. The
calculations are trivial compared to the I/O overhead.

-- 
Glynn Clements <glynn at gclements.plus.com>