OK, I'll give it a shot.
Take a bucket near full of water.
Get a 100' roll of 1/4" clear plastic tubing. A couple of dollars at Lowes, Home Depot, etc.
Put the bucket some where so that the water level in the bucket is higher than anywhere you want to measure. Put one end of the tube in the water and secure it to the bucket. I had the one in the picture tied to the handle with string, a light clamp would be more convenient.
Unroll the hose to the first place you want to check and suck lightly on the end for a second. The water will, quicker than you think, syphon to where you are.
Hold the end of the hose up and the water will stop perfectly level with the water level in the bucket. Tape measure or yardstick down to the ground. That gives you the ground or ditch bottom level relative to the bucket.
Then move the end of the tube to the next place you want to check, measure again, and in your head arithmetic will tell you the difference in elevation of the two spots.
Did I get that right?
When I built the spillway for my pond I used this to get the foundation right for the water level to be 8" below the top of a footbridge 150' away though the woods. Thirteen years later I'm still using the same piece of tube.
Wm
New bucket, though.