Dear Badal Pokharel ,

It seems to me that all input parameters of r.topidx must be in the same projected Coordinate Reference System (CRS), i. e., all input units must be in meters.

For example, the CRS should be in WGS84 in the UTM system in the zone and hemisphere corresponding to the study area.

Then, to solve this problem, you should use the Reprojection tool for your data.

Also, it's a good practice to set your QGIS project to the CRS you are working. This allows you to get the correct extension of the map canvas or selection into the tool.

As suggested by leandro all files should have same coordinate system. The tutorials are also downloadable for all SAGA tools and exercises . You should also go through that to understand the process and detect the reason behind this error.

Leandro França Thank you for your suggestions. I tried re-projecting as well. It didn't work.

You're welcome, Badal Pokharel.

I tested the r.topidx tool and it has some problems with the input parameters (same problem you had).

However, I got good results when I set the cell size (resolution) greater than the DEM resolution.

The parameters I've used is in the attached picture.

I used a DEM with the resolution of 10 meters and the CRS is SIRGAS2000/UTM25S.

I also used the DEM (layer) extension to define the region of the algorithm.

I'm attaching a image with the DEM and the output.