I'm a bit lost with the zonal statistics tool in QGIS. I have a raster layer representing population densities per km2 for Asia. I've clipped the raster to my Vietnam provinces polygon as it is my area of interest. For the analysis, I just need to know how much population is present inside each province. Now I've run the zonal statistics tool, and it gives results as presented below: enter image description here

I just don't understand if the "popsum" field has taken all the pixels and corresponding values and is presenting this sum just for 1km2, or has it actually sum up the entire pixels for the entire area of the polygon? I've set my CRS to UTM Zone 48N corresponding to Vietnam and wider south east asia.

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Popsum should be the sum of all the pixels which fall within the vector polygon. In your case, if each pixel represents population density, it should equal the population. The values in that column very roughly match the population of the provinces according to Vietnamese census data, so it appears to have worked generally properly.

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