Adjust volume measurements for web mercator

plugins/measure/api.py

@@ -1,6 +1,6 @@
 import os
-
 import json
+import math
 from rest_framework import serializers
 from rest_framework import status
 from rest_framework.response import Response
@@ -47,7 +47,13 @@ class TaskVolume(TaskView):
             output = result.get('output', '')
             cols = output.split(':')
             if len(cols) == 7:
-                return Response({'volume': str(abs(float(cols[6])))}, status=status.HTTP_200_OK)
+                # Correct scale measurement for web mercator
+                # https://gis.stackexchange.com/questions/93332/calculating-distance-scale-factor-by-latitude-for-mercator#93335
+                latitude = task.dsm_extent.centroid[1]
+                scale_factor = (math.cos(math.radians(latitude))) ** 2
+
+                volume = abs(float(cols[6]) * scale_factor)
+                return Response({'volume': str(volume)}, status=status.HTTP_200_OK)
             else:
                 raise GrassEngineException(output)
         except GrassEngineException as e:

plugins/measure/calc_volume.grass

@@ -11,7 +11,7 @@ v.buffer -s --overwrite input=polygon_area type=area output=region distance=1 mi
 r.external input=${dsm_file} output=dsm --overwrite
 
 # Set Grass region to vector bbox
-g.region vector=region
+g.region vector=region res=0.01
 
 # Create a mask to speed up computation
 r.mask vect=region
@@ -26,7 +26,7 @@ v.surf.rst --overwrite input=polygon_points zcolumn=height elevation=dsm_below_p
 r.mapcalc expression='pile_height_above_dsm=dsm-dsm_below_pile' --overwrite
 
 # Set region to polygon area to calculate volume
-g.region vect=polygon_area
+g.region vect=polygon_area res=0.01
 
 # Volume output from difference
 r.volume -f input=pile_height_above_dsm