[hanfordbill]

I've got a huge backyard that I know has low spots, high spots and drainage issues.

What's the best way to determine them?

I've attached a few photos from different views.

 

[Bondo]

Ayuh,.... There's a Few options,....

Simplest is a clear hose, fulla water....

A Sight Level, or better yet, a Lazer Level can be rented at most rental shops,...

Once ya set the corner pins, 'n establish the high point, 'n low point,....
Lotsa string-lines, 'n pins gives ya something to work from....

or a Good operator on a dozer could probably grade it from the seat...

String-lines are the Key though...

 

[redman88]

you can go old school

http://explorable.com/roman-roads

 

[cleveman]

Redman is correct.

You should use a Cuba Libra.