Topography

Flat is good. I don't know if I'd note "high". Swap terrain for topography: flat terrain works. I don't think high terrain tells me much but mountainous terrain or mountainous topography would.

I don't find that topography should have additive description such as flat or high. Topography can contain information that describes the area as flat or high but describing "topography" itself seems a misnomer.

The word itself means a description of a place, so to write that an area is highly or flatly described seems odd. The features contained within a topography may be high or flat but the act of describing remains the same.

In statements like these about "topography", the writers are just trying to give the impression of being "chrome domes" and using polysyllabic (four syllables) scientific words instead of the common and simple words that everyone understands. AnWulf made the excellent sugestion of "terrain" (two syllables).

[I do think that millions of people are NOT widely-read, but rather they just pick out polysyllabic words from word lists and dictionaries. They just ignore short words like "terrain" and "about", and they want to use polysyllabic words like "topography", "topology", and "approximately". They do it, too. None other than Sir Winston Churchill spoke out against this.]

"Flat terrain" is a fine and meaningful phrase.
"High terrain" is useless in this context because it has a really different meaning.

To describe opposites of near opposites of flat terrain, we already have these, in alphabetical order:
hilly terrain, mountainous terrain, rolling terrain, and rugged terrain
such as in "the rugged terrain of the Ardennes" and "the rolling terrain of Missouri".

High terrain is found in places that are a long way above sea level, but are not necessarily mountainous (e.g. plateaus). Much high terrain is found in Tibet, Peru, Bolivia, Bavaria, Montana, Wyoming, Alberta, and in central Antarctica, where the South Pole is located at about 3,000 meters above sea level, but the land around it is quite flat..
Be careful! When I mentioned Bavaria, Bolivia, Montana, etc., I did not mean 100 percent of those places, but rather most of their areas.
D.A.W.