Dear Dr Gaykar

I would put two pieces of information to contribute to you:

First we must consider that the MDEs already represent an average at a given spatial resolution.

Next, you should consider which algorithm and which parameters are being used by them for output raster generation.

The QGIS program is tested and approved by a wide community of users around the world. Their algorithms and their parameters for transformations and modeling are open for study and improvement.

I hope I have contributed. Good job.

Best regards

Dear Dr Gaykar ,

I would suggest to update to latest versions of software. Current QGIS version is 3.8.

Also, I would recommend using SNAP for downloading DEMs and some processing.

Sincerely,

Gregoriy

Dear Avinash,

Differences in the outputs of different software can likely be attributed to different equations or neighborhoods being used.

Regarding slope (and aspect), please consider the R 'terrain' help file for reference:

"When neighbors=4, slope and aspect are computed according to Fleming and Hoffer (1979) and Ritter (1987). When neigbors=8, slope and aspect are computed according to Horn (1981). The Horn algorithm may be best for rough surfaces, and the Fleming and Hoffer algorithm may be better for smoother surfaces (Jones, 1997; Burrough and McDonnell, 1998). If slope = 0, aspect is set to 0.5*pi radians (or 90 degrees if unit='degrees')."

Regarding TRI:

"The terrain indices are according to Wilson et al. (2007), as in gdaldem. TRI (Terrain Ruggedness Index) is the mean of the absolute differences between the value of a cell and the value of its 8 surrounding cells." By the way, TRI is short for Terrain Ruggedness Index. (Surface) Roughness is something else.

You could also comfortably write your own R function to be used in 'focal' if you wanted to use yet another (combination of) equation or (and) neighborhood.

Cheers, Rainer