qgisraster
I am trying to find a tool in QGIS that does the same function as Region group in ArcGIS.
What I am trying to do is to identify uniquely each patch of a raster that has a certain value (e.g "Value" =1) using neighbouring cells.
From the region group docs:
For each cell in the output, the identity of the connected region to which that cell belongs is recorded. A unique number is assigned to each region.
You just need to set the two layer (parent : "Map_CorrectionDEF", and child : "crop_main") then set the field that link them, in your case the country field: "unitcode" for both layer.
This way when you click in a cell you will see all entry from the CSV table that share the same unitcode (be aware that relation don't work like join, you wont get the CSV column added to your grid table but you will see a form with, on top the grid attribute and below an extract of the attribute table of related CSV entry. see image below for example from QGIS doc on relation)
If your goal is to get a field in your grid table with a list of all crop for the country you may try the solution from here
For the solution in the referenced question to work you need to define the relationship between your two layer before attempting to use the expression.
Your first steep is to go to Project > Properties... then to select the Relation tab. Here you will add a relation with the "Map_CorrectionDEF" layer as parent layer and the "crop_main" as the child, set the "unitcode" field for both the parent and child layer.
Then you should be able to use the expression :
aggregate( layer:= 'crop_main',
aggregate:='concatenate',
expression:="cropclass",
concatenator:=', ',
filter:=attribute( $currentfeature ,'unitcode')=attribute( @parent ,'unitcode'))
The result would be for each cell of the grid layer a list of all cropclass from the crop_main table that share the same unitcode. This should work with both text and numeric field (it does work with my 3.22.1 QGIS version) and you should not replace the @parent with with the parent layer name (the @parent is in reference to the relation you set as a first-steep).
Take care to properly define the field where the result should go as text with enough length to accommodate all the value you expect to aggregate (including the concatenator sting)
Best Answer
To get a similar tool to Region Group, you need to use r.clump in GRASS, which can be accessed from QGIS if you have installed GRASS using OSGeo4W advanced installer.
The tool can be found in
QGIS -> Processing Toolbox -> GRASS Commands -> Raster -> r.clumpAs a test, I used small portion of land use map with 7 classes, as you can see below:
After running the tool, the land use map was reclassified into 35 groups (35 unique pixel values) with different colors to see the groups clearly, as you can see below: