A Water Level is easy and accurate.

The Water Level is a simple tool that is very accurate. We all know that water always seeks its own level. All you need to do is put the water in a hose, stretch it out and hold up the ends. Once you force out the air bubbles the water will seek it own balance and be the same distance from the top of each end when they are level.

Now to make it easy to see, you attach a piece of clear vinyl hose to each end with a plastic hose coupler. I would buy 8 feet total and cut it in half. So that each end has a 4 foot clear section. The length of the hose in the middle can be any length that is convenient. Now if you have two old yard sticks or pick up some cheap ones that stores like to advertise on, they are perfect to tape to each end so that if you stood them up side by side, resting on the ground, the water level would then read the same number on each yard stick. The numbers are not that important, they will vary depending on how much you fill it. The point is the water will always remain in balance. You can see it is the same height when they are side by side. Now, if you leave one end right where it is and move the other end 50 feet away, the water will still seek its own level. You can be sure that you are level within a quarter of an inch over the 50 foot distance when you position the yard stick to read the same number as it did at the point of beginning.

I also have a inexpensive laser level that I use to confirm the settings. The problem with it is it is hard to balance. A very slight error is magnified the further out you go in distance. That is the beauty of the water level, no matter how far you extend it you are still within your 1/4 inch of accuracy.