QGIS Distance Calculator
I want to find a location that is close to existing industrial areas (red polygons) and away from Sites of Special Scientific Interest (green polygons)
I will do this by using the Proximity (Raster Distance) analysis tool to create distance thematics, then using the Raster calculator to average the distance from each criterion.
Original layers
Convert to Raster
The process only works with input files that are in raster format. As our source is in vector format (it’s a polygon .shp file), we need to convert it to a raster file.
It is good practice to add an attribute column, set its value to 1 so the resulting raster has a value of 1 for all the polygons. This can be done using the Field Calculator in the Attribute table.
QGIS Rasterize tool
Change display properties
The raster initially appears as a grey box. Don’t worry, this is normal. I am going to adjust its display properties so I can see the information.
- Right click on the layer
- Select Properties
- Select Stretch to MinMax under Contrast Enhancement
- Select the style tab
- Tick Invert Colour map. This makes the areas with a value of 1 black and the areas with a value of 0 white
QGIS Layers dialog
The colours look quite stark, so I’m going to apply a transparency:-
- Select the Transparency tab
- In the Transparent Pixel List box, enter 1 under Gray and 50 under % Transparent
QGIS Layers dialog
Its appearance is now less over-powering and I can see other layers too:-
Calculate Proximity/Distance
Now to create a thematic based on the proximity (or distance) between each pixel and the nearest point of a SSSI site:-
- Open the Proximity calculator by selecting Raster menu, Analysis, Proximity (Raster Distance)
- Select the input raster and output raster (I found it works best if the output file is in .tif format)
Proximity (Raster Distance) calculator dialog
- I want to measure the distance to SSSI pixels with a value of 1 (that is what I set to be the value used for areas with SSSI designation)
- Distance units are Geo (geographical) rather than pixels
The output is initially a grey square. I have made the following adjustments:-
- Colour map – Puseudo colour
- Global Transparency – 50%
It is worth exploring the options to invert the colour map and Contrast Enhancement
The SSSI sites are visible as the green polygons, the thematic is red for areas closest to sites and blue for areas furthest away:-
Repeat the process for all the necessary layers:
Proximity Distance Calculator Results
Raster Calculator
The raster calculator is a powerful tool that performs mathematical operations on each cell in a raster. Examples of this can be to calculate elevation, distance or density.
In this case, I am going to use it to identify the areas that are greater than 1km from a SSSI site by applying queries to identify matching pixel values.
- Open the raster calculator
- Enter the following expression SSSI_Distance@1 > 1000
The map units are in metres, so 1,000m = 1km
The resulting raster appears as a grey box. As usual, adjust its display properties (e.g. Contrast Enhancement, Invert Map and Transparency). The areas that are further than 1km from a SSSI site are now highlighted in grey, with the sites visible in green:-
