[GRASS-user] Searching for technical details on how v.rast.stats calculates means and other statistics

[RichardCooper]

Hi Markus, 

Thanks for your comments.

It still appears that v.rast.stats is giving me different values to
r.univar.

Essentially I'd like to clarify how the values are derived from
v.rast.stats.  I'm guessing that the discrepancy between my r.univar and
v.rast.stats outputs relates to how I'm using g.region?

The following shows my approach for deriving statistics from both r.univar
and v.rast.stats.

Cheers,
Richard


1. Using r.univar and zones to output stats
# set g.region for rasterizing vector 
g.region -p -a vector=BMR_bndry_epsg4326 at prcp05216ijk res=0.22                  
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      14:31:12N
south:      13:12N
west:       99:39:36E
east:       101:12E
nsres:      0:13:12
ewres:      0:13:12
rows:       6
cols:       7
cells:      42

# rasterize vector to be used for zones
v.to.rast --overwrite input=BMR_bndry_epsg4326 at prcp05216ijk type=area
output=BMR_bndry_epsg4326_rasterized use=cat
Reading areas...
Reading features...
Writing raster map...
Converted areas: 6 of 0
v.to.rast complete.

# set g.region for zones with r.univar
g.region -p rast=BMR_bndry_epsg4326_rasterized at prcp05216ijk                     
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      14:31:12N
south:      13:12N
west:       99:39:36E
east:       101:12E
nsres:      0:13:12
ewres:      0:13:12
rows:       6
cols:       7
cells:      42

g.region -p align=cahpa_i0apr_05216_nc_remapped_nc_1 at prcp05216ijk               
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      14:35:24N
south:      13:03N
west:       99:34:12E
east:       101:19:48E
nsres:      0:13:12
ewres:      0:13:12
rows:       7
cols:       8
cells:      56

r.univar -g -t map=cahpa_i0apr_05216_nc_remapped_nc_1 at prcp05216ijk
zones=BMR_bndry_epsg4326_rasterized at prcp05216ijk
zone|label|non_null_cells|null_cells|min|max|range|mean|mean_of_abs|stddev|variance|coeff_var|sum|sum_abs
1|Category
1|4|0|3.58209799742326e-05|7.74326545069925e-05|4.16116745327599e-05|5.64720030524768e-05|5.64720030524768e-05|1.53324774982512e-05|2.35084866234379e-10|27.1505820043313|0.000225888012209907|0.000225888012209907
2|Category
2|2|0|3.53023679053877e-05|6.23035812168382e-05|2.70012133114506e-05|4.88029745611129e-05|4.88029745611129e-05|1.35006066557253e-05|1.82266380072614e-10|27.6634913694027|9.76059491222259e-05|9.76059491222259e-05
3|Category
3|1|0|8.14341983641498e-05|8.14341983641498e-05|0|8.14341983641498e-05|8.14341983641498e-05|0|0|0|8.14341983641498e-05|8.14341983641498e-05
4|Category
4|3|0|5.68314899282996e-05|8.2188387750648e-05|2.53568978223484e-05|6.57776896938837e-05|6.57776896938837e-05|1.16198714234862e-05|1.35021411898352e-10|17.6653687254186|0.000197333069081651|0.000197333069081651
5|Category
5|4|0|3.15697689075023e-05|4.9253627366852e-05|1.76838584593497e-05|3.7798413359269e-05|3.7798413359269e-05|7.09251495570204e-06|5.03037683968571e-11|18.7640546926894|0.000151193653437076|0.000151193653437076
6|Category
6|1|0|1.89598704309901e-05|1.89598704309901e-05|0|1.89598704309901e-05|1.89598704309901e-05|0|0|0|1.89598704309901e-05|1.89598704309901e-05
(Sun Feb 22 13:23:02 2015) Command finished (0 sec)                             


2. Check outputs against v.to.rast

# set g.region for raster (stats to be derived from this layer):
g.region -p -a raster=cahpa_i0apr_05216_nc_remapped_nc_1 at prcp05216ijk
res=0.22  
projection: 3 (Latitude-Longitude)
zone:       0
datum:      wgs84
ellipsoid:  wgs84
north:      35:38:24N
south:      15:10:48S
west:       89:45:36E
east:       140:34:48E
nsres:      0:13:12
ewres:      0:13:12
rows:       231
cols:       231
cells:      53361

v.rast.stats map=BMR_bndry_epsg4326 at prcp05216ijk
raster=cahpa_i0apr_05216_nc_remapped_nc_1 at prcp05216ijk
column_prefix=teststat method=number,minimum,maximum,range,average,sum

Outputs from attribute table:
cat,province,teststat_number,teststat_minimum,teststat_maximum,teststat_range,teststat_average,teststat_sum
1,krung_thep_maha_nakhon_bangkok,2,3.58209799742326e-05,6.23035812168382e-05,2.64826012426056e-05,4.90622805955354e-05,9.81245611910708e-05
2,samut_prakan,1,3.53023679053877e-05,3.53023679053877e-05,0,3.53023679053877e-05,3.53023679053877e-05
3,nonthaburi,2,5.02148177474737e-05,8.14341983641498e-05,3.12193806166761e-05,6.58245080558117e-05,0.000131649016111623
4,pathum_thani,3,4.46328085672576e-05,8.2188387750648e-05,3.75555791833904e-05,6.12175620820684e-05,0.000183652686246205
5,nakhon_pathom,3,3.81464305974077e-05,6.24195599812083e-05,2.42731293838006e-05,4.99398726484893e-05,0.000149819617945468
6,samut_sakhon,2,1.89598704309901e-05,7.74326545069925e-05,5.84727840760024e-05,4.81962624689913e-05,9.63925249379827e-05




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