The Colarado Springs Gazette reports that Delaware is the first state to adopt the use the new National Geodetic Vertical Datum of 1988 (NAVD 88) for its paper maps (as opposed to the 1929 Datum – NGVD 29 – used by virtually all maps). Although I have not seen a copy of the maps, I checked to see the impact on Ebright Azimuth and the highpoint loses more than a foot in the conversion!
The reason for the discrepancy is that gravity has an effect on measuring instruments that was not realized in 1929. Consequently denser, higher mountains stand to be higher and lower lying areas next to the coast are actually lower.
Colorado has enthusiastically jumped on the bandwagon – noting the Rockies are between 5 and 7 feet higher (Elbert is 6.5 feet higher). The Mile High line on the state capitol steps was dropped three feet.
The mountains and hills for the most part keep the same relative height differences.
The Gazette article notes the cost for redoing the 56,000 USGS maps would cost billions. The USGS is throwing its initiative to the National Map
However the map is only visible on the web through a slow browser window that does not have detailed street level maps or contours.
Further, details for the impact are not totally available for Hawaii and Alaska.
NGS has an online conversion calculator for the Lower 48 using the Vertcon 2.1 program.
The program is a bit intimidating because he asks you to input a stating elevation. The elevations do not in reality enter into the equation. The calculations are based on longitude and latitude and the elevation question is just to give you a starting point (you could enter “0” and it would give you the same relative height differential.
You can download the entire program (2 megs zipped) at:
The readme.txt instruction manual is also intimidating. But this program is also easy if you download the zipped program, extract it and run it with vertcon.exe. The command line instructions walk you through the steps and gives you instructions on using fixed width batch files.
You can get other NGS files from the database.
http://americasroof.com/wp/archives/2005/01/10/delaware-first-adopt-navd-88-for-state-maps-and-ebright-loses-foot/
Earlier THread:
http://www.network54.com/Forum/message?forumid=3897&messageid=1036788959
RSS Feed For This Forum