Coloring points by raster values in QGIS?

Question

While it's simple to color a point shapefile by an attribute field, I can't figure out how to match the colors with those from a raster.

I have a raster file that I'm displaying with a continuous colormap, with values clipped between -5 and 5. Using 5 colors results in colors -5, 2.5 0, 2.5 and 5. I'm forced to use the singleband_pseudocolor symbology so that I can clip the values.

The values in my point shapefile are all negative, between -0.5 and -2.3. I want to match the colors to the 5 colors in the colormap, but the colormaps will only reflect the values in the point shapefile.

I've tried saving/applying the colormap of the raster file, and manually adding extra points to match my limits; the former does nothing and the latter causes the endpoints to mismatch so there is one less color than there should be. I've also tried to rasterize the points but this isn't working at all.

I'm doing this in QGIS and can attach screen shots if necessary.

Answer 1

You can use a data-defined override for the 'Fill color' property of the point layer to achieve this.

First you need to export your singleband raster as 3-band RGB image. Once you apply the colormap, right-click -> Export -> Save As.. In the export raster layer dialog, choose Output mode as 'Rendered Image'. Once exported and loaded, you can use the following expression

Assuming your RGB raster is called 'raster', use the expression as below:

concat(raster_value('raster', 1, $geometry), ',', raster_value('raster', 2, $geometry), ',', raster_value('raster', 3, $geometry))

This samples the value in the R, G and B bands of the raster and creates a string such as 119,20,89 which is used as r,g,b values for the fill color.

Answer 2

The ramp_color(ramp_name,value) allows you to set symbol color based on a color ramp. The value parameter controls the position on the ramp to select the color from (as a real number between 0 and 1).

Your point attribute has numbers between -0.5 and -2.3, but you want to use them as though the range was between -5 and 5. To convert the point attribute into the value parameter, we take the following steps:

• add 5 to convert range from (-5,5) to (0,10)
• divide by 10 to convert range (0,10) to (0,1)

Converted into Expression Builder syntax, that's ("point_attribute"+5)/10, which we'll use in the next step.

Set point symbol color using data-defined override with this expression:

ramp_color('ramp_name',("point_attribute"+5)/10)

Where the expression says 'ramp_name', substitute the name of the color ramp that you used for your singleband pseudocolor raster style. It looks like you're using the color ramp 'RdBu', but you can check the color ramp names in the style panel:

If you used a custom color ramp, the easiest solution is to save that color ramp with a name, and use the name in the expression. It's also possible to create a custom ramp directly in the expression builder. See the expression builder help for the ramp_color expression for more details.