[Qgis-user] creating vector layer from slope map with few classes

[Shinoto, PD Dr. Maria]

Hi, 

I have created a slope map from a DTM. 

I have limited the symbology to three meaningful classes and wish to create a vector layer with polygons of three classes from that. The conditions are One-Channel-Pseudocolor, Channel1 (Gray), interpolation discrete, modus "fortlaufend" -- not identical interval or quantile, 3 classes with values <=20, 20 - 40, > 40. 

The classes show up in the layer design panel symbology tab, but the layer browser still (thankfully!) shows the greyscale ramp. 

I wish to create a vector layer with polygons from the 3 classes. 

All solutions I have looked up work under slightly different condition. After all I do not know how to get the data of the DTM into three classes like "Flat-Gentle-Steep"; I think the vectorization should work easily then. 

Can anybody hint me to a solution?


Thanks and best wishes, 
Maria