I've done very little work with a digital level but I am doing some volunteer work for an organization. I vaguely remember sometime in the past that I was told not to level across a body of water. If that is true, can anyone give me an explanation of why. Is there some kind of refraction difference or what???
We know there are issues with using the water level itself to transfer accurate elevations, but I'll be interested to see what people say about sighting across water.
How high above the surface would your line of sight be? That could be an important factor if refraction is involved.
NGS published a document on transferring elevations across bodies of water (or other impassable features) in 2014, I believe:
https://www.ngs.noaa.gov/PUBS_LIB/SupplementToChapter4OfNOAAManualNOSNGS3.pdf
And if refraction is a problem, measuring both directions in a short time period may be the solution, as the average should mostly cancel it out.
?ÿ
That is for using zenith angles. Much simpler if the layout and height difference lets you do it with a digital level.
I realize that, but sometimes leveling is not an option, but the work still has to get done.
My point was that the OP said digital level.
And if refraction is a problem, measuring both directions in a short time period may be the solution, as the average should mostly cancel it out.
Agreed, that's what I remember from the texts I have. Reciprocal levelling is the way to go, ideally with two levels at the same time.
Two turning points/benchmarks set on either side of the water, instruments set up so that they have as close to the same backsight length (to the BM on same side as instrument), and same foresight length (to the BM across the river). Like a rectangle with the instruments at one set of cross-corners and the BMs at the other set.
Take the two height differences between BMs as observed by both instruments and average them.
Instrument Height would be about 5 feet above water surface
I'm not sure about large bodies or water, as far as I know you're really only getting a couple hundred feet out of a digital level reading.
You might be concerned with the dew point - if the air around you were colder than the body of water - that might factor in.
There are detailed procedures for river crossings in NGS manuals.?ÿ
I can't see leveling across water being any worse than leveling parallel to the ground as far as refraction effects.?ÿ
I was thinking along the same lines. The length of shots with a digital level are rather short so there shouldn't any significant error introduced.
Do it at night, trig level both directions.?ÿ
If you aren't going for geodetic accuracy, but more like flood certificate accuracy, and conditions are relatively stable, it may be good enough to use one point on each side and one instrument, ferried quickly across between measurements.
