I have global
shp files of basins (polygons) and rivers (lines). I wish to calculate a measure of stream length per basin area. It should be comparable across world regions, e.g. I wish to compare basins in China to basins in Brazil.
I wonder if using
$area in a geographic WGS84 reference system to calculate the ratio as described above is comparable between regions?
I have made an attempt to compare to remote regions using the following workflow:
For each region:
A) Use spatial join to get
sum(river.length)in each basin.
ratio = sum(river.length) / $area
C) Re-project river / basin data to corresponding UTM and repeat A - B
I expected to get
ratio == ratio.UTM for each basin, yet the results show non-linear differences between them. See results below (red indicates negative values):
Following this example I became skeptic; now i'm wondering if I can simply use the data referenced to WGS 84 to calculate the ratio, or should I re project it beforehand? Note that projecting the data will be time consuming and complex, since it requires to slice the data according to UTM zones.
I am using QGIS 2.14, and may switch to arcmap 10.3, or grass 7. I have read Measure areas, lenghts in specific regions (Australia, Sout East Asia), but answers given there do not satisfy my query.