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Home - General / All posts - M8 : Transfer heigth to calculate population sum
Corentin47 post(s)
#22-May-18 16:04

Hi, I am stuck at the same point as this thread : http://www.georeference.org/forum/t113912.17

I want to calculate the population of each village in Senegal :

from here I got the village location : https://data.humdata.org/dataset/senegal-settlements

and from there a raster where each pixel counts the population : https://data.humdata.org/dataset/senegal-settlements

Then I made a drawing with the voronoi area of my villages points, and run a transfer height from the raster to the voronoi area.

Total population according to the raster : 14 Million (counted from a SQL query : selection sum([height (I)]))

Total population counted in the voronoi cells : 16Million.

It seems that the bug reported in this thread is still there http://www.georeference.org/forum/t113912.17 (I have Manifold 8.0.30)

Do you see any other method to allocate the population from the raster to the points drawing?

Corentin47 post(s)
#22-May-18 17:03

I tried to do it manually :

I have copied pasted a sample of my raster data as vector points, then I used spatial overlay to fill the population of the contained point to the voronoi area. The difference between the transfer height result and the spatial overlay result can be large, from 5% to 30%.

I cannot do this method for my entire dataset, it would requires days of computation just for copy/pasting the surface as a drawing of point.

tjhb
8,063 post(s)
#22-May-18 20:30

Yes we can use SQL. See Raster Extensions > HeightSum(s, g).

Requires the Surface Tools extension.

Without Surface Tools you can achieve the same using a join, but the specialist function is much faster.

Corentin47 post(s)
#23-May-18 09:17

I tried with heightsum(s,g) and I got exactly the same wrong result than with transferheight.

Sum of height in the raster : 14.9million hab

Sum of the HeightSum in the drawing : 16.8 Million hab.

Corentin47 post(s)
#23-May-18 10:09

I did it with QGIS 3.0.0 and the zonal statistic tool, much faster, and accurate than M8.

The final sum is 14.93Million hab; compare to 14.96 from the original data.

If there is a method in Manifold I am still intersted.

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