I have a global land cover raster that I would like to convert into a shapefile. Rather than converting the entire raster I would like to "extract" just the forest value (40 in this case) and then convert that to a polygon. I haven't worked with Rasters much before so haven't been able to work out a solution. I'm using QGIS.
If you have SAGA installed under QGIS, you can use
Victorising grid classes tool located in
Processing Toolbox -> SAGA -> Shapes - Grid -> Victorising grid classes. As you can see in the image below, you need to change the Class Selection from
All Classes to
One single class specified by the class identifier, Then Choose the class pixel value (in your case 40). You can set the polygon class as
single polygon, then save the output file.
Here is a sample land use
Please note for global land use/cover, it may take long time to convert from raster to vector.