Select point features from delimited text layer if crossing color of raster layer?

Question

I'm using QGIS 3 to see if point features I have from a delimited text layer can be selected based on if they cross / are overlayed on a specific color from a separate raster layer. I'm using point source emission data to analyze emission levels in cities for a university project. The raster layer (geotiff) was obtained from this source: https://www.arcgis.com/home/item.html?id=fc92d38533d440078f17678ebc20e8e2

I'm trying to see what point emission values interact with the "Built Area" portion of the raster which is a specific color. If there is a way to select and export only the "Built Area" portion of the raster that would also work, but I wasn't sure if this could be done either.

Answer 1

I am using QGIS 3.26.3.

If your raster is categorised and the colour corresponds to a particular value, you can use Select by expression on your points like so:

// select points that fall inside pixels (in band 1) with value 3
raster_value('my raster layer', 1, $geometry) = 3

However, if you are working with an RGB image, you could get the RGB values of your raster and filter/select your points based on ranges of colour.

Calculate a field called R:

// Red band of a raster layer called 'qgis'
raster_value('qgis', 1, $geometry)

Repeat for G and B using band 2 and band 3 in place of 1.

Then select your points based on the RGB values. For example here I choose the most red and least blue points using Select by expression:

"R" > 150 and "B" < 100

You could also use an array to store the RGB values, or indeed use the raster_value function directly in select by expression. I used 3 separate fields here for clarity.

The raster_value function was introduced in a fairly recent version of QGIS but I don't remember which off the top of my head.