Extract raster properties and use them in QGIS graphical model

Question

I would like to generate a tool that reclassifies rasters in 10 classes with equal intervals. Since QGIS reclassifies tools does not allow to automatically create classes with such methods, I was wondering if I could do it myself inside the graphical modeller this way:

• Extract the min and the max of a given raster
• Divide the range of those values in ten breaks
• Use those breaks to generate a reclassification table
• Import that reclassification table inside the reclassify tool.

I know that the last step is possible, but I'm not sure about the other ones... Can we process data and informations like this inside the graphical modeller? I know it's possible with Python but for a teaching purpose, I would like to know if it's possible with the graphical modeller specifically.

Answer 1

I have created a model you can download here from my github repo.

Thats how it looks:

Where:

• r is the input raster
• NClasses is a numerical input greater 0 (the amount of classes you wish to have)

And:

• you need to set a dependency for "Reclassy by table", because "Raster layer statistics" needs to execute first
• you need to insert a rather complex expression to "Reclassify by table", see below

The Expression as explanation (uses different variables than in the actual model above, better to explain this way):

-- CREATE VARIABLES
with_variable('rmin',1,( -- Equals to result of Raster layer statistics MIN result
with_variable('rmax',100,( -- Equals to result of Raster layer statistics MAX result
with_variable('nclasses',10, -- Equals to model input NClasses
-- for copy paste into your own model, only copy below, and change variables

-- NEST ARRAYS
array_to_string(array_foreach( 

-- ADD LOWER CLASS VALUES
array_remove_at(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@nclasses)
,@element

-- ADD UPPER CLASS VALUES
-- concat
||','||
-- array of upper classes
array_get(
array_filter(
array_foreach(
generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses)
,
array_get(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),
array_find(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@element)+1)
),@element is not null)
,
-- get the index of initial array to insert at this position
array_find(
array_remove_at(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@nclasses)
,@element))

-- ADD CATEGORY VALUES
-- concat
||','||
-- array of class values
array_get(generate_series(1,@nclasses),
-- get the index of initial array to insert at this position
array_find(
array_remove_at(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@nclasses)
,@element))

)) -- closing array nesting

-- for copy paste into your own model, only copy above, and change variables
))))) -- closing with variable

*Only tested in QGIS 3.16.1

---

Further explanation on the expression:

The expression basically consists of four parts:

1. Nest the three arrays below together, to get one single array_to_string like: lowerclass1value,upperclass1value,class1reclassvalue,lowerclass2value,upperclass2value,class2reclassvalue,...
2. An array to get the lower borders for each class
3. An array to get the upper borders for each class
4. An array to get the new values for each new class

#1 is done by:

array_to_string(array_foreach(...),THE_THREE_ARRAYS)

just some complicated nesting, I cant really explain...

#2 is done by:

array_remove_at(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@nclasses),@element

Generating a series from raster min value to raster max value in steps by raster range devided by nclasses-1. This will result in n equal classes. Since we only want the lower borders, we remove the last entry (raster max) from the array. The last @element is for nesting.

#3 is done by:

array_filter(
array_foreach(
generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses)
,
array_get(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),
array_find(generate_series(@rmin,@rmax,(@rmax-@rmin)/@nclasses),@element)+1)
),@element is not null)

Generating a series from raster min value to raster max value in steps by raster range devided by nclasses-1. This will result in n equal classes. Since we only want the upper borders here, we remove the first entry (raster min) from the array.

#4 is done by:

generate_series(1,@nclasses)

To generate class values from 1 to the maximum of classes. Step 1.

Cant really explain better.... Maybe best to try out yourself and stitch the parts together.

Proof: