The National Elevation Dataset (NED) is the primary elevation data product of the USGS. It's updated on a two month cycle and is derived from a variety of sources. NED data is available nationally (except for Alaska) at resolutions of 1 arc-second (about 30 meters) and 1/3 arc-second (about 10 meters), and in limited areas at 1/9 arc-second (about 3 meters). [That's the horizontal resolution.] A wealth of information on the National Elevation Dataset is available at the link below.
http://ned.usgs.gov/
There is also a study on the vertical accuracy of the National Elevation Dataset available at
http://ned.usgs.gov/downloads/...nts/NED_Accuracy.pdf
I will need someone to explain the complete study, but it appears in a test of 9,187 unique point pairs, the relative vertical accuracy (RV) was 1.64 meters.
So, it appears if we use the National Elevation Dataset to determine the start and finish elevations, we can be reasonably certain they are accurate within 1.64 meters. Is that accurate enough to calculate the drop for record purposes? Probably.
Thank you. -- Justin