[GRASS-dev] [GRASS GIS] #4009: 'where' option in v.to.rast can select wrong feature for raster attribute, in areas where features overlap

GRASS GIS trac at osgeo.org
Thu Dec 12 05:18:37 PST 2019

#4009: 'where' option in v.to.rast can select wrong feature for raster attribute,
in areas where features overlap
  Reporter:  florisvdh  |      Owner:  grass-dev@…
      Type:  defect     |     Status:  new
  Priority:  normal     |  Milestone:
 Component:  Vector     |    Version:  svn-releasebranch76
Resolution:             |   Keywords:  v.to.rast sql where
       CPU:  x86-64     |   Platform:  Linux

Comment (by mlennert):

 Replying to [ticket:4009 florisvdh]:
 > I use GRASS 7.6.1 on Linux Mint 18.1, i.e. Ubuntu Xenial (16.04) based.
 ''(Note, this is my first post here, I'm a beginning GRASS user, mostly
 working with R)''
 > I applied something like:
 > {{{
 > v.to.rast input=polygon_layer output=output_x1 where="field1 LIKE 'x1'"
 >          use=attr attribute_column=field2 memory=800 --overwrite
 > }}}
 > Importantly, {{{field1}}} in {{{polygon_layer}}} has several possible
 values such as {{{x1}}}, {{{x2}}}, {{{x3}}} and so on (81 different values
 in my usecase).
 > Moreover, in my usecase several features (polygons) have identical
 geometry, i.e. many sets of 2 or more polygons exist with 100% overlap
 among their own polygons (i.e. identical polygons), while each of these
 polygons has its own specific attributes: different values of {{{field1}}}
 and so on. The problem that I met occurs for those features; possibly the
 same problem will occur for overlapping areas between polygons in general.
 > While the {{{where}}} option in {{{v.to.rast}}} is effective in
 ''localizing the correct areas'', i.e. where {{{field1}}} is {{{x1}}}, the
 **problem** is that the **attribute value** ({{{field2}}}) may come from
 one of the other (overlapping) features at that place, e.g. where
 {{{field1}}} is {{{x2}}}. Which feature of an overlapping set is used for
 the attribute probably depends on the order of those overlapping features.
 > From what I've seen, it appears that {{{v.to.rast}}} will select the
 {{{field2}}} value **from the same feature regardless** of the value for
 {{{field1}}} in the above {{{where}}} option, as long as ''one'' of those
 overlapping features meets the {{{where}}} condition.
 > If this effectively ''is'' a bug in the program, maybe it happens in
 other modules as well, where the {{{where}}} option is available.

 This definitely should not happen. Could you provide the output of:

 v.db.select map=polygon_layer where="field1 LIKE 'x1'" \


Ticket URL: <https://trac.osgeo.org/grass/ticket/4009#comment:1>
GRASS GIS <https://grass.osgeo.org>

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