The opposite of flat is hilly, uneven, slanted, or in more figurative senses of the
word, flavorful, exciting,lively,or sharp.
The antonym of flat is round.
Flat
The word contour does not exactly have an antonym, however, it you use contoured, the antonym would be flat.
flat land
Flat.
Flat, dull, insignificant .
A plain, or flat-land.
As undulate means to rise and fall, become wavy or rippled, an antonym would be to "roughen".
I like chicken wings.
The synonym is character. An antonym could be robotic, wooden, or unfeeling... flat affect in psychological terms.
A synonym for the word "rough" is rugged. An antonym for "rough" is smooth. Please see the related link below.
Synonyms of elevation are flat, demotion, depression, and lowness.