NAVD88 to NGVD29 Conversion?

[briancoward]

This is probably a bit odd but I'm working on a project that is requiring me to translate a set of contours from NAVD88 to NGVD29.  Does anyone know if this is possible?  And if so... what's the procedure to make it happen? 

I've been given a straighforward conversion from a hydralic study of NAVD88 = NGVD29 + 2.57.  Does this make any sense?  Should I really just subtract 2.57 from the z values and the conversion to NGVD29 is done?  Any help would be much appreciated. 

Thanks,

Brian 

[tzzzzoddhepworth4253]

The two vertical datums reflect different mathematical models for the Geoid.  So the difference in elevation is related to the horizontal position on the surface of the earth.  If your project is localized, i.e. over a small geographic area, it might be sufficient to just use that point differential of 2.57' and still be within your tolerances.  how large of an area are we talking?  Acres? Square Miles?  whole PLSS Sections/Townships?

[tzzzzoddhepworth4253 · Accepted solution]

I should add that if you really want to get your hands dirty, you can look into an old USGS routine surveyors use called "VertCON" for the vertical differences and "CorpsCON" for horizontal datum comparisons.  Not exactly your answer, but it may help your understanding of what you're looking at.

[briancoward]

The area is pretty large... I think.  It's the levee system for an entire city and covers 6 or 7 miles along those levees.  I do have an older data set of contours that are are in the NGVD29 but... it's older and it only covers half of the city.  I'm thinking I may be able to use the older version to see how bad any shifting is occuring.   

[tzzzzoddhepworth4253]

I'd fall back on selecting 2 points 6 miles apart and seeing what their differences are from Vertcon. When I get some time, I'll delve deeper.

Alternatively, you might cross post in C3D and request help from Sinc - he'll be presenting on coordinate systems at AU this year.

[briancoward]

I found this conversion tool on the National Geodetic Survey site and I think I'm ready to transform my points to NGVD29.  Todd thanks for the help... I haven't had any luck up until now.   

http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl

[tewjoe]

Try Vertcon in google earth.

Its a snap.

http://www.metzgerwillard.us/verge/vergenl.kml

[tewjoe]

A very easy way to check any area is to use google earth and use VERTCON inside it to check a certain point. The difference betwene NAVD88 and NGVD29 is modeled and is not a direct translation.

here is the website for the kml file to use in google earth:

http://www.metzgerwillard.us/verge/verge.html

and here is an example