Need to understand topographical maps

Okay, so these maps aren't so difficult. they never intersect, nor do they have a start or end. I'm this case the 'unit' I should say is the middle being the higher number 'unit' and the outer being smaller. Now not all maps will have it the same. the way you can tell is by looking and the number given at A and B lines in this case and you can see a is 900 and the one in the center is 1500 so you can tell it's getting bigger. Hope this helps