This tool is found in the Toolbox and allows one to adjust the population distribution raster so that the population found in barrier pixels (as specified in the merged landcover) is redistributed outside of these barriers. The redistribution is done uniformly over all non-barrier pixels found within each of the user-specified sub-administrative units. The correction of the population on barriers is important when you want to ensure that your statistics (e.g. percentage of coverage, number of populations found in each catchment, etc.) accounts
The underlying hypothesis when using this tool is that the methodology used to obtain the population density raster has wrongly distributed some population in places where there should not be any population (e.g. on some rivers, airport, wetlands, etc.).
Please see Appendix 1 for the details of the algorithm implemented in this tool.
Before running the tool, it is assumed that the following three layers are already uploaded into AccessMod and that all of them have the same projection system, extent and resolution than the DEM :
- Population distribution raster;
- The land cover merge raster;
- The shapefile of the Administrative boundaries down to the sub-national level.
The steps that are automatically applied by the tool to correct the population are the following ones:
- The population distribution grid (“total population“) is first masked with the landcover merge grid in order to extract the population outside of the barriers, which gives a “partial population” grid, whose sum of pixels values gives the “partial population”.
- The ratio between total and partial population is computed.
- The “partial population” grid is multiplied by this ratio. This process produces a “corrected population“ grid that has no population in barrier pixels, but whose sum of population is equal to the sum of population of the original population distribution grid.
These three steps are done for each administrative unit found in the input administrative boundaries shapefile. It is therefore recommended to use administrative level 3 or 4 (rather than 1 or 2) so that the population on barriers will be redistributed close to the barriers within a small administrative unit.
The panel for this tool and its functionalities is as follow:
(1) “Select population (raster)”: select the raster format layer containing the spatial distribution of the target population
(2) “Select merged land cover layer (raster)”, select the raster format layer containing the merged land cover resulting from the use of the first tool (see Section 5.5.2) - named "land cover merged"
(3) Under “Select zone name (text)", select the field from the zone boundaries layer
(4) Under “(Add short tags)”, give short tags, we will use “adjusted_population" for the present exercise
(5) You can then click the “Compute" button, in case there are no issues, to launch the calculation