# Creating weighted sum analysis using DEM and population data using QGIS?

I have a shapefile containing population points for Vietnam, and I am aiming to perform a weighted sum analysis using a Digital Elevation Model. Basically, I'd like to combine elevation, population densities and slope to find the best areas for terraced cultivation. I have a full DEM for Vietnam, and population densities.

The ideal locations would be medium slope angles without high population densities. However I'm a bit stuck on what path to follow.

• My question is: is there a way I can perform a slope analysis using the DEM, and combine slope with population data inside a weighted sum analysis to find suitable areas. I need a slope between 10° and 15. My issue is that I don't really see how I can weigh the slope. Mar 21, 2019 at 10:43
• Please use the edit button beneath your question to revise it with any requested clarifications. The question you wish to ask will be obvious when it has a question mark at the end.
– PolyGeo
Mar 21, 2019 at 12:12

Here's an outline of a basic method.

Slope

ii. Use the Raster calculator to create a raster with values of 1 when slope is acceptable (between 10 and 15 degrees), and 0 when the slope is unacceptable.

``````@rasterband * (@rasterband > 10 AND @rasterband < 15)
``````

Population

i. Convert your subjective condition (high/low population density) to an objective range of numerical values. Define what an acceptable range of population density would be.

ii. Create a raster with values of 1 when population density is acceptable , and 0 when the population density is unacceptable.

Elevation

i. Define a range of acceptable elevations.

ii. Use the Raster calculator to create a raster with values of 1 when elevation is acceptable, and 0 when the elevation is unacceptable.

2. Find the area where the filtered layers overlap.

The output is a raster with values between 0 and 3. Any cell with a value of 3 meets all 3 of your criteria.

Note that this method can be modified to create a more nuanced output. Instead of making rasters with only 0 or 1, you can use a range of values between 0 and 1, such that 0 means a cell doesn't meet conditions at all, and 1 means the cell has optimal conditions. It's up to you to decide what you define as "optimal" for each condition.