I-Home improvement

[occams_razor]

You can use a laser level or water level. You can even make a water level with some surgical tubing.

http://www.factsfacts.com/MyHomeRepair/WaterLevel.htm

Put the bucket part at the top of the inclination and bring the tubing part to the bottom and measure the height of the water column. (You'll need a two by four or some other stright piece of scrap wood to get a good height measurement. mark a spot with a pencil on the 2x4 and plum up the 2x4 before you measure the spot's height from the ground) Subtract the height of water in the bucket and that's your dfifference in height. Measure the run by measuring distance of a tape measure stretched from the bucket water line to the top of the column of water in the tube. The two person level is even easier to make and use.

Inverse tangent of the (difference in height)/difference horizontally gives you your angle.

You can also rent a water level or laser level for a few bucks for a day.

For a less accurate measurement if the angle is relatively constant, use a 4 foot carpenter's level. Lie it along the ground and lift one end until the bubble reads level. Use a bevel guage to match the angle on the ground and measure that angle with a protractor.

But make the water level, it's more fun and an extremely useful tool.