[GRASS-dev] [GRASS GIS] #3300: v.rast.stats: wrong results for non-first category values in the case of multiple category values per feature

[GRASS GIS]

#3300: v.rast.stats: wrong results for non-first category values in the case of
multiple category values per feature
----------------------------------+-------------------------
 Reporter:  mlennert              |      Owner:  grass-dev@…
     Type:  defect                |     Status:  new
 Priority:  normal                |  Milestone:  7.2.1
Component:  Vector                |    Version:  unspecified
 Keywords:  v.rast.stats overlap  |        CPU:  Unspecified
 Platform:  Unspecified           |
----------------------------------+-------------------------
 From a discussion and test on the users mailing list
 [http://lists.osgeo.org/pipermail/grass-user/2017-February/076024.html
 [1]]:

 v.rast.stats seems to incorrectly take into account the non-first cat
 values when there are multiple cat value per polygon.

 See the attached image for an example of a situation with several polygons
 of which polygon 3 overlaps with respectively 1 and 2.

 I also created a second file with only the 2
 non-overlapping polygons (cat 1 and 2). The two input file were created in
 QGIS so as to be able to have "nice" topologically incorrect data.

 I'll attach a .pack file of the test vector map with the three overlapping
 polygons.

 test_nooverlap can be created from test with

 {{{
 v.extract -d test cat=1,2 out=test_nooverlap
 }}}


 I then imported the two files into GRASS GIS with v.in.ogr and ran


 {{{
 v.rast.stats map=test raster=elevation column_prefix=e
 v.rast.stats map=test_nooverlap raster=elevation column_prefix=e
 }}}

 And here are the results:


 {{{
 > v.db.select test
 cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_median|e_third_quartile|e_percentile_90
 1|1|242621|76.8941955566406|145.712738037109|68.8185424804688|114.397437714797|13.5650727983398|184.01120002426|11.8578467047128|27755220.7358017|104.996|115.337|124.654|131.38
 2|2|131854|77.3364105224609|126.733070373535|49.3966598510742|99.4588066985328|10.6819214497898|114.103445859479|10.7400458585508|13114041.4984283|91.5773|98.7388|106.926|114.645
 3|3|99039|71.4758224487305|139.941131591797|68.4653091430664|107.223648245541|13.7481896908839|189.012719776527|12.8219752972784|10619322.8985901|97.4993|105.891|118.752|126.703

 > v.db.select test_nooverlap
 cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_median|e_third_quartile|e_percentile_90
 1|1|242621|76.8941955566406|145.712738037109|68.8185424804688|114.397437714797|13.5650727983398|184.01120002426|11.8578467047128|27755220.7358017|104.996|115.337|124.654|131.38
 2|2|131854|77.3364105224609|126.733070373535|49.3966598510742|99.4588066985328|10.6819214497898|114.103445859479|10.7400458585508|13114041.4984283|91.5773|98.7388|106.926|114.645
 }}}

 The values for polygons 1 and 2 are strictly
 identical. Overlapping areas are thus correctly taken into account.

 To test even further, I ran


 {{{
 v.extract -d test cat=3 out=test3
 }}}


 to get the entire polygon with cat=3.

 Then


 {{{
 v.rast.stats test3 rast=elevation colprefix=e -c
 > v.db.select test3
 cat|id|e_number|e_minimum|e_maximum|e_range|e_average|e_stddev|e_variance|e_coeff_var|e_sum|e_first_quartile|e_median|e_third_quartile|e_percentile_90
 3|3|224074|71.4758224487305|141.793502807617|70.3176803588867|110.21428988784|13.2281044492473|174.982747320197|12.0021681968|24696156.7923279|100.265|110.004|120.916|127.836
 }}}

 There thus seems to be a bug in v.rast.stats as it does not calculate
 the correct values for the polygon with cat=3 when there area polygons
 with multiple cat values. Running v.to.db with op=area on test3 I get an
 area that corresponds to the number of pixels identified in test3, i.e.
 224074.

 When I run


 {{{
 v.overlay ain=test3 bin=test_nooverlap op=not out=t3_no1_2
 v.to.db -p t3_no1_2 op=area
 }}}

 I get an area that corresponds to the number of pixels identified for
 polygon 3 in test, i.e. 99039. v.rast.stats thus only takes into account
 the non-overlapped part of polygon 3.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/3300>
GRASS GIS <https://grass.osgeo.org>