QGIS Raster – Area Calculation for Raster Classes in QGIS

areaqgisrasterraster-calculatorreclassify

I have 3 raster layers each of them is classified into 4 classes and each one has defined weight value. I had to to create FINAL raster by multiplying those 3 rasters with their weight values and adding those 3 togheter, I did that, as shown in the figure below.

Now, here is the problem, I want to calculate the area for each class in the FINAL raster that I created. That raster has also 4 classes. I used raster layer uniqe values report tool for that but I get this report bellow:

As you can see I have range of values, for example class 0 goes from 0,5 to 0,9 and each value has its own area, but I want to see only one class 0 with one area (so sum of these areas shown), one class 1 with its area etc. I dont need those ranges with their areas

Is there a way to do this?

Best Answer

You can reclassify your raster. Use e.g. Reclassify by Table:

This algorithm reclassifies a raster band by assigning new class values based on the ranges specified in a fixed table.

So you get an output raster, reduced to the number of classes you defined.

This is how your reclassify dialog / Reclssification table could look like:

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[GIS] How to find the area of raster values within polygon zones

I will admit to an aversion to the "higher level" functionality in Spatial Analyst, such as the clipping and area tabulation, because too many undocumented operations go on under the hood. In the effort to simplify these operations, the software developers have made them less transparent, whence less reliable, and thereby much less useful. The closer you are to the details of the calculation, the more likely you are to understand it and to get the correct answer.

So, motivated by that, consider using combine to overlay the watershed grid and the permeability class grid. The result of combine is to create one code for every distinct ordered pair of (watershed id, imperviousness class) values occurring within the cells. In addition, two fields are adjoined to the attribute table: one gives the watershed id and the other gives the imperviousness class. As always, there is a [count] field, too: multiplying this by the square of the cellsize (and converting to any area unit you prefer, such as acres or square miles) gives the answers you're looking for. The multiplication is readily done with the Field Calculator or by exporting the attribute table and using other software (like a spreadsheet) or even by multiplying the grid itself (inefficient, but it works), provided the multiplicand is an integer.

(Evidently, tabulate areas attempts to do these two operations at once: combine and multiplication by the squared cellsize. It's not much of a savings in user time, especially when you factor in the additional search for (nonexistent?) documentation about what's really going on...)

[GIS] Calculating areas of different raster classes in R

Even if your raster is lat-long and therefore varying cell area, you can do this in R by adding the areas that correspond to cells with your values using the base tapply function:

First set up some dummy data:

> r = raster()
> r[] = sample(-5:-2, ncell(r), replace=TRUE)

Then do this:

> tapply(area(r), r[], sum)
       -5        -4        -3        -2 
126376977 126301228 125943687 129736879

area(r) creates a raster where each cell has its area as its value. tapply then sums this over areas grouped by the value in r.

Best Answer

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[GIS] Calculating areas of different raster classes in R

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