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--- howtos:how_to_calculate_an_area-weighted_average_in_grass [2026-10-23 12:38 am] – created hcho
+++ howtos:how_to_calculate_an_area-weighted_average_in_grass [2026-10-23 01:38 am] (current) – hcho
@@ Line 8: / Line 8: @@
 r.to.vect rast_val out=vect_val type=area
-# overlay the value vector and area vector
+# calculate the areas of the area vector polygons
+v.to.db vect_val op=area col=area_m2
+# overlay the value vector OR the area vector
 v.overlay ainput=vect_val binput=vect_area oper=or output=vect_val_area
-# calculate and validate the area-weighted area value grouped by area vector;
+# add a new column for the area-weighted average
-# this command doesn't update anything
+v.db.addcolumn vect_area col="avg_val double"
-v.db.select vect_val_area col="b_cat,sum(a_value*a_area)/sum(a_area)" group=b_cat
-# add and populate a new column with the area-weighted average
+# populate the new column with the area-weighted average
 v.db.update vect_area col=avg_val \
-  qcol="(select sum(a_value*a_area)/sum(a_area) from vect_val_area \
+  qcol="(select sum(a_value*a_area_m2)/sum(a_area_m2) from vect_val_area \
          where vect_val_area.b_cat=vect_area.cat group by b_cat)"
 </code>