I have taken a good sample of elevations on a point throughout the day, the vert difference is less than 0.20'. Using VERTCON I converetd the elevations from NAVD 88 to NGVD 29. I believe I am as accurate as FEMA data is, but I did this before and hit a FEMA RM and it was off around 0.5'. All FEMA RM's are gone in this area and the NGS Raduis search turned up nothing local that still existed.
I know the FEMA data in this area is all 1929, but how close to you think our adjsuted field data is relative to the FEMA dataum?
Thanks
sorry to be repetitive as i have posted this before
i spent a few months of my own time (after hours) pulling together all of the historical data in my county. i used all available sources to find the 113 published benchmarks with elevations on both NGVD and NAVD. Why? Because I was finding significant busts in the Vertcon Model based on RTK and Static locations and I wanted to know why. We have as much as a 0.3' difference which can be significant for critical shoreline projects. Even if the flood won't rise to the exact tenth, the insurance adjusters will set the premiums based on the exact tenth or hundredth.
here's a link to the paper
https://www.box.com/s/756712f590caab218a1a
vertcon is a great tool but the conversion does not meet the typical requirements for vertical control.
