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Posted by David L. Wilson on July 20, 2006, 8:16 pm
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> jimirwin wrote:
>>
>>> Is there a simple formula to convert a lat/long set of
>>> co-ordinates from one geodetic datum to another (such as the
>>> Hayford International to the 80/84 Worldwide Datum)?
>>>
>>>
>>
>> Search for Molodensky Transformation and/or Helmert Transformation.
>
> Thanks, I think Iv'e found a good one:
> http://www.colorado.edu/geography/gcraft/notes/datum/gif/molodens.gif
>
> I also found an online calculator:
> http://www.ngs.noaa.gov/cgi-bin/nadcon.prl
>
> I tried Peter Dana's version of Molodensky's formula and his example and
> it works. I then tried it on NADCON and got slightly different answers,
...
> I understand these use different methods and NADCON uses actual recorded
> data values in its calculation, so it is probably more accurate.
> But Dana's formula is probably simpler.
There is a subtle point here. NADCON is the US legal conversion
standard --even though it is based on certainly some mismeasured points. So
you need to define what is meant when you say accurate. If you think about
it, they needed to do that as one does not want to be arguing old property
line.
But generally the only recommeded accurate way to locate a point in one
system is to measure it in that system. If converted to another system, it
is best to state which transformation was used.
You should also get the following 2 excellent pieces of software
CORPSCON (a windows implementation of NADCON)
http://crunch.tec.army.mil/software/corpscon/corpscon.html
and GEOTRANS.
http://earth-info.nga.mil/GandG/geotrans/index.html
> The next question is is there a formula for finding Deltas X, Y and Z,
> perhaps a way to make them variables, thereby increasing the accuracy at
> the global level?
> If not, is there a table somewheres that gives the different values
> between the different datums?
There certainly are talbles of the deltas. Run, do not walk to the following
websites
http://earth-info.nga.mil/GandG/coordsys/csat_pubs.html
http://earth-info.nga.mil/GandG/coordsys/datums/index.html
But keep in mind that these should be take as approximations. In addition
there is some ambiquity in some of the definitions. Through on top of that
that land masses drift--and by different amounts so things are time
dependent transformations in some cases.
Make sure you read the red at
http://earth-info.nga.mil/GandG/coordsys/datums/notice_8142002.html
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