It is simply a rather badly expressed sentence.
Normally "elevation" just means "height". We might say "The plateau has an elevation of 500m".
We wouldn't say "the general elevation is from the west to the east". Clearly what the author wanted to express was "The land generally slopes down from the west to the east."
This use of elevation may have a particular meaning in Indian English, or it may just be a badly written sentence. It is possible the author intended to write "The general elevation is [between 500 and 1000m and the land slopes down] from the west to the east" but the part in brackets was edited out for some reason.
Similar ambiguity appears later:
The general elevation of the Central
Highlands ranges between 700-1,000 m above
the mean sea level and it slopes towards the
north and northeastern directions.
It is not clear if the land slopes down, or up, toward the north.