Garden Level
This is a great inexpensive tool for the gardener or the landscaper.
It works over distances and can be operated by one person.

This is the most accurate, foolproof, trouble-free level I know of. Additionally it's inexpensive and disposable. Since water seeks its own level, the  water in the tube will always migrate until it is at exactly the same level as the water in the jug.
 
Place the surface of the water in the jug at a the position you want things to be level with. Then take the tubing end to the item you want to be level with the water in the jug.
It can be two feet or two hundred feet - it makes no difference except that it may take longer for the liquid in the tubing to stop migrating on longer runs of tubing.

When your car's windshield washer fluid gets 2/3 empty, punch a hole in the top of the handle to accept plastic tubing. Cut the tubing off at an angle so it is unlikely to seal against the side or bottom of the jug. You could also use plain water, or water colored with food coloring to make it easier to see the liquid in the tube.
You can fasten the end of the tubing to a stake or the side of a building . It must be higher than the surface of the water in the jug to keep the water from leaking out the end of the tubing. Then, with the tubeing in the jug submerged, blow into the top of the jug until the liquid comes out the far end of the tube.

It is important that:
( 1 ) - there are NO air bubbles in the tubing.
( 2 ) - the cap is not screwed on tight.
( 3 ) -the hose is submerged in the jug.

Any air in the tubing can cause erratic readings.