Bug report #14508
Updated by Giovanni Manghi almost 7 years ago
In a land cover study I want to derive the total area for each land cover type in a land cover layer (vector data). In order to do this I tried to use Vector/Geometry tools/Single part to Multipart in order to obtain an attribute table with one row per land cover type. By using the field calculator it should then be possible to derive the total area of each multipart land cover feature.
By comparing the obtained areas from the multipart layer with the sum of the single part areas for each land cover type in the original layer it turns out that there are quite significant differences, in the range 5-10 % for most land cover types. A visual comparison of the maultipart and the singlepart layer at a few locations does however not reveal any differences in the geometry that can explain the differences in the areal estimates.
In order to provide the possibility to reproduce the problem I attach an extract from the land cover layer containing individual shrubland polygons.
I have tested the singlepart to multipart conversion using both QGIS 2.8.7 and 2.12.3 and the problem is the same in both versions. I am running QGIS using Windows 8.
By comparing the obtained areas from the multipart layer with the sum of the single part areas for each land cover type in the original layer it turns out that there are quite significant differences, in the range 5-10 % for most land cover types. A visual comparison of the maultipart and the singlepart layer at a few locations does however not reveal any differences in the geometry that can explain the differences in the areal estimates.
In order to provide the possibility to reproduce the problem I attach an extract from the land cover layer containing individual shrubland polygons.
I have tested the singlepart to multipart conversion using both QGIS 2.8.7 and 2.12.3 and the problem is the same in both versions. I am running QGIS using Windows 8.