When doing glide testing, it is important to fly over level ground. How can you tell whether your ground is level? There is a simple way used by the Romans to compare surface elevations. It uses a hose filled with water. The water will seek the same level in each end of the hose.
Difficulties with the hose led me to later use a 35′ length of clear 1/4″ vinyl tubing.
We start by filling the hose with water. Hold the end up and run the water until any air is cleared from the hose.
One end of the hose must be held fixed several feet above the water level. You want to be sure you don’t spill water out of the hose. Here I taped it to a post. You could use a chair or anything else that will keep it at a constant elevation.
Hold the other end several feet above the surface and fill it with water.
Find a hard point to use as your bench mark. Slowly lower the hose to your chosen level above the bench mark, for example, two feet. Let the excess water drain out. This establishes your datum plane. Every where you go with this end of the hose, the water level will have the same elevation when the end of the hose is lowered to the water level.
Lay out your tape along the flight path. You will be measuring the water surface elevation at the end of the tape and every five feet along it.
Slowly lower the end of the hose until you can see the water. If you move the hose, the water can slosh back and forth. Wait until the surface stops moving up and down. Be careful to not spill any of the water. If you do, you will have to refill the hose with the end at a previously established location. Place the end of your yardstick on the ground and measure the height to the water surface. Write down the station and elevation. Note that this elevation is the distance of the surface below the water level. It is a negative elevation.
Write the numbers down in your Flight Log.
You can plot a profile to visually judge how level your landing field is. You can look at how the numbers vary from an average.