What is the average vertical accuracy of the NED?
See this document for an RMSE accuracy assessment of values within the United States. The USGS’ discussion on accuracy is here. In practice, most of the values I’ve compared to surveyed benchmarks are generally within 2-3 feet. LatLonToElevation.com makes no warranty concerning the accuracy of any of the elevation data or fitness of the data for any particular use. Please read the documentation on the source elevation data to determine if the elevation data meet your needs.