[GRASS-dev] [GRASS GIS] #2101: Different results from m.proj in different locations

[GRASS GIS]

#2101: Different results from m.proj in different locations
---------------------------------+------------------------------------------
 Reporter:  wenzeslaus           |       Owner:  grass-dev@…              
     Type:  defect               |      Status:  new                      
 Priority:  normal               |   Milestone:  7.0.0                    
Component:  Projections/Datums   |     Version:  svn-trunk                
 Keywords:  m.proj, projections  |    Platform:  Unspecified              
      Cpu:  Unspecified          |  
---------------------------------+------------------------------------------
 I wanted to get the latitude and longitude extent for a map, so I applied
 `m.proj` to the map extent. However, I've noticed that I get different
 results for the same map when running `m.proj` in NC sample dataset
 (Lambert Conformal Conic) and in my custom location (WGS 84 Pseudo-
 Mercator).

 This is related with the #2100 ticket but I'm not sure if this is really
 the same. It it something different at least from user point of view.

 I cannot tell about the sample elevation map but for some other data it
 seemed to me that the LL from Merkator is correct and the the other are
 wrong.

 == How to reproduce ==

 To reproduce, run in NC sample dataset:
 {{{
 eval $(r.info map=elevation -g)
 m.proj -o -d input=- separator=space <<EOF
 $east $north
 $west $south
 EOF
 }}}
 I got:
 {{{
 -78.60830318 35.80918894
 -78.77462049 35.68792712
 }}}

 Then create a new location, I've used EPSG:3857 (WGS 84 Pseudo-Mercator).

 In new location run:
 {{{
 eval $(r.proj -g input=elevation location=nc_spm_08_grass7
 mapset=PERMANENT)
 m.proj -o -d input=- separator=space <<EOF
 $e $n
 $w $s
 EOF
 }}}
 I got:
 {{{
 -78.60835872 35.80956421
 -78.77456513 35.68755254
 }}}
 Which is something close but different. Compare the points at OSM (you can
 change the `mlat` and `mlon` in URL):
  *
 http://www.openstreetmap.org/?mlat=35.80918894&mlon=-78.60830318#map=17/35.80850/-78.60932
  *
 http://www.openstreetmap.org/?mlat=35.80956421&mlon=-78.60835872#map=17/35.80850/-78.60932

 == Relation to the r.proj bounding box ticket ==

 When I apply the (first) patch from the possibly related ticket #2100, I
 get
 different results but still does not match with the others:

 {{{
 -78.60830339 35.80960928
 -78.77462031 35.68750748
 }}}

 OSM link:
  *
 http://www.openstreetmap.org/?mlat=35.80960928&mlon=-78.60830339#map=17/35.80850/-78.60932

 == What g.region -l says ==

 During writing of this ticket I also find the `g.region -l` which I can
 actually use instead of `m.proj`, however I still get the different
 results.

 LCC:
 {{{
 north-east corner: long: 78:36:29.891436W lat: 35:48:33.080191N
 south-west corner: long: 78:46:28.633781W lat: 35:41:16.537646N
 }}}

 Mercator:
 {{{
 north-east corner: long: 78:36:30.091395W lat: 35:48:34.431172N
 south-west corner: long: 78:46:28.434454W lat: 35:41:15.189156N
 }}}

 == Creating the new location ==

 I suggest to create the new location using Location wizard instead of
 `g.proj` because I've tried to write a testing code for this but the
 `g.proj` didn't worked for me. The commands:
 {{{
 g.proj epsg=3857 location=test_location_3857
 g.mapset mapset=PERMANENT location=test_location_3857
 }}}
 create and switch the location but than the `r.proj` fails with:
 {{{
 WARNING: pj_transform() failed: failed to load datum shift file
 ...
 }}}
 If I'm using it right this might be another ticket. I've used it recently
 through Python `grass.script.core.create_location()` function and it
 worked well.

 == Warning about the LL format ==

 When trying to do something related to this ticket and with LL either by
 hand or programmatically be warned about different order and formats
 (speaking about GRASS only). I spent some time with this. Maybe this
 inconsistency is topic for different ticket.

-- 
Ticket URL: <https://trac.osgeo.org/grass/ticket/2101>
GRASS GIS <http://grass.osgeo.org>