Calculating proportion of polygon by value in QGIS?

Question

Using QGIS, I have a single-band raster with with multiple counties. I have reclassified the pixels into 2 categories, using a singleband pseudocolor, based on their value ( > or < 3800), corresponding to non-forest or forest land.

How do I calculate the proportion of pixels that is in each of the 2 categories, in each county?

I have a lot of images over a lot of years so I would prefer to not take the route the I figured out - converting to shp, exporting as csv, sorting csv and calculating percentages in Excel.

it's not entirely clear from your question if your "multiple counties" are available as a polygon layer, can you confirm this? — dmci, Jan 14 '16 at 12:47

score 3 · Answer 1 · edited Apr 13 '17 at 12:34

3

Zonal Statistics https://docs.qgis.org/2.2/en/docs/user_manual/plugins/plugins_zonal_statistics.html

Or just extract the rasters by the polygons Clip raster layer with *raster* mask layer in QGIS

It has been while since I used the zonal statistics but I remember it give a count value.

edited Apr 13 '17 at 12:34

Community

1

answered Jan 13 '16 at 00:43

If you do not know- just GIS

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Yes. I had discovered the Zonal Statistics plugin and had it working nicely - giving me a summary of each county. The problem is that it doesn't give me values per category, ie forested vs non-forested (the ones shown in my TOC). Any way to get it to show me # of pixels/category? – user65148 Jan 13 '16 at 01:15
Thanks makes sense for nominal data, I may be mixing the arc and qgis zonal functions. just make two rasters and give each pixel a value of 1 and then it is the sum in this table or the count. Forested= 1 in one and unforested = 1 in the other. right? – If you do not know- just GIS Jan 13 '16 at 02:04
That doesn't quite make sense to me. How do I give each pixel a 1? – user65148 Jan 13 '16 at 02:27
@user65148 http://gis.stackexchange.com/questions/27988/how-to-reclassify-in-rastercalc – If you do not know- just GIS Jan 13 '16 at 02:31
Im saying if all pixels are 1 or 0 then the sum of the pixels is the count. So <3800 to 1. – If you do not know- just GIS Jan 13 '16 at 02:33
What I don't know is how to add this value to a raster...I know how to do it to a vector, but there isn't an attribute table associated with a raster – user65148 Jan 13 '16 at 02:41
It's a reclass in the earlier link I gave – If you do not know- just GIS Jan 13 '16 at 02:44

score 0 · Answer 2 · answered Aug 26 '19 at 20:53

I know this is an old question, but I wanted to give input as this is something I just had to do. I have a raster with a single band and unique values representing land cover usage and polygons representing historical wildfires. I used the Zonal Histogram tool in QGIS 3.8: to add columns with each value and the count of pixels that contain that value within the wildfire footprint (i.e. i want to get % land use within fire footprint):

Calculating proportion of polygon by value in QGIS?

2 Answers2