Combining Raster Calculator with elevations

I want to identify which areas of Chetney Marshes would be flooded by a 2m rise in sea level.  I used LIDAR data in contour format as the elevation data:-

Chetney Marshes map

Chetney Marshes map

Create TIN

I am using the TIN method to create an elevation model as the area is relatively small and the data is supplied as contours. TIN’s are more accurate, especially if the source data isn’t in grid format. However they are slower to process, especially over very large areas.

Select Plugins, Interpolation to open the Interpolation dialogue:-

Raster Calc2

I made the following changes to the display properties:-

  • Colour map – Pusedocolour
  • Transparency – I set global transparency to 50%
Chetney Marsh after colour adjustment

Chetney Marsh after colour adjustment

Raster Calculator

I now have an elevation model of the area. I now use the Raster Calculator to identify each pixel with an elevation of less than 2m. The elevation is stored in the pixel value. The raster calculator will identify each pixel with band value of less than 2.

The expression is elevation < 2

Select Raster, Raster Calculator to open the Raster Calculator:-

Raster Calc4

To change the Display Properties for a layer, Right click on the Layer in the layers panel (Table of Contents) and select Properties. I made the following changes to the display properties so the areas that are less than 2m, and susceptible to flooding appear highlighted in blue:-

  • Style tab: Color map – Colormap
  • Transparency: Global transparency 50%
  • Colormap tab: I added 2 entries:-
    • 0.00 white
    • 1.00 blue
Chetney Marsh showing raster calculation results

Chetney Marsh showing raster calculation results

 

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About Luke Nicolaides

I've been working in GIS for over 10 years. I've used MapInfo and ESRI lots at work, and more recently I've started to experiment with QGIS. This blog will bring together some of my tips and tutorials.

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