QGIS Count values between two numbers inside a polygon in another layer

Question

I have 2 layers. LayerA are multiple polygons of the area that i'm evaluating (Red boundarys). LayerB is a MSAVI raster to vector with the data (multicolor greens - blues).

I want to calculate the area of each LayerA polygon that has a range value from -1 to 0 from layerB for example like "how much bare soil", "how much dense vegetation"..

What i have done. Calculated MSAVI form clipped raster from the area. MSAVI traspased to vector with "raster pixels to polygons" because of float data. Tried the aggregate function on field calculator.

I'm correct with the process?

Answer 1

1. Reclassify your raster, e.g. range from -1 to 0 becomes class 1 etc. Use the Reclassify tool or Raster Calculator.

2. Use Zonal Histogram to calculate the number of pixels for each category.

3. Multiply the number of pixels for each category with the pixels size to get the area.

Left: initial raster with polygons; right: reclassified raster with 3 classes:

Output of Zonal Historgram (felds HISTO_0, HISTO_1 and HISTO_2) as well as calculated area, based on pixel size 4 (2m x 2m); see rough measurement of the black area (corresponds to HISTO_0) of the larger polygon - confirming that area calculation is correct:

Answer 2

Hmm, tricky... Here's a possible idea:

1. Use the dissolve tool to dissolve the polygons by their attibute (eg: LANDCOVER).
2. Then, clip those dissolved polygons by the boundaries layer.
3. Finally, join attributes by location between the clipped layer and the boundaries layer.

Hope this helps - I cannot provide screenshots at this time, but please reply if you get stuck.