# Paging Mr. Molodensky... (need WGS84-NAD27 conversion formula)

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I'm writing a software app, and I'm trying to find a straightforward statement of the Molodensky formula with paramters for converting between WGS84 and NAD27.

Since this is probably one of the most common datum conversion ops out there, you'd expect the Internet to be rife with references. But I've spent over an hour looking for a site with the formula, with no luck. I can find tons of recommendations of programs, or even Excel spreadsheets, that perform many datum conversions; but the software isn't open-source and the spreadsheets have their operative code obfuscated. The few statements of Molodensky that I've found are for coversion to weird Australian datums, not NAD27.

This is terrifically frustrating. Help!

David Stein, Esq.

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OK, continued searching (another half an hour... boy, I thought this would be easy!) has led to the following two sites, which provided all the info that I need. I'm supplying the links for posterity (i.e., for those who search these forums for the same data.)

"Untitled Document"

This site provides the Molodensky formula. To do a Molodensky transform, you need five parameters for the datum (in comparison to WGS84): X, Y, Z, A, and F. The site also provides a link to a data table, but, sadly, the data contained therein is only the X, Y, and Z info for each point.

The complete datum list, however, can be found here:

Geodetic Datum List

That should do it. Good luck!

David Stein, Esq.

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try going to this page to see if the fomula is there.

http://www.jeeep.com/details/coord/

No matter where you go, there you are!

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No Australian datum's are weid , just different as all datums are. But yes we can apply single conversion formula to much of Oz as much of the old data are contiguous data sets, unlike the US.

Actually the problem with NAD27 is that it's so disjointed and variable being based on so much data variations over the years that there simply is no one single formula that adequately handles the conversion.

GPSr's do a rather rough converison and some even have a dozen or so "different" NAD27 transformations to cover the US.

Distortion grid model handles this type of conversion far better (and the official way) than any single formula ever will, but I suppose it will depend on just how accurate you want your straightforward conversion to be.

Do a search for NTv2 (National Transformation version 2), which might give you some background into the principle of grid shift systems.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

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Check this in particular and Martindales.

Martindale is a great reference for all kinds of calculations and reference material.

Good luck

Alan

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Oh, I forgot NADCOM which is NOAA's Datum convertor program. Very accurate as it uses thousands of samples from around the USA to calculate the "best" conversion.

Or so they say

Alan

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And the winner is...

quote:
Originally posted by smithdw:

Fairly straightforward, as long as you don't sweat the definition of "second eccentricity squared" and just go with the forumulas provided. I think these should do the trick.

Thanks very much, everyone. Off to do some hardcore, insanity-inducing math coding...

David Stein, Esq.

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quote:
Originally posted by David Stein:

And the winner is...

Thanks very much, everyone. Off to do some hardcore, insanity-inducing math coding...

I hope I'm not too late. Complete with source code:

from libjeeps (open-source) that's ?no long supported?. You can find the whole sha-bang in the GPSBabel source tree.

...

alex

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