What is the importance of topographical survey maps? What is meant by the term brackish?

Topographical maps are based on accurate survey of the land. They show natural features of the ground like hills elevation, drainage, trees and vegetation as well as man-made features like towns, villages, buildings, roads, canals, railways, power transmission lines and other cultural features in detail. Therefore, they are of great importance to military and travelers.

The well whose water is unfit for drinking and cultivation as it is salty is said to be brackish.