text: how can you determine 0 feet elevation with the national geodetic vertical datum?   i would do a search on the ngs website for a vertical benchmark in your specific area of interest.  find that benchmark and run a level loop from the benchmark to the area of interest and close back on the benchmark.  you should have some surveying experience to do this.  then check that benchmark with the ngs, usgs, and find the corrected elevation.  apply this to your field data.  your question really depends on the required precision and accuracy you are interested in at this point.  to get this information from a map would leave you with a somewhat large error, compared to an actual field survey using specific equipment for the precision you are looking for. good luck