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Anonymous via the Cypherp
science forum beginner

Joined: 20 Jun 2005
Posts: 17

Posted: Tue Jul 18, 2006 7:45 pm    Post subject: Converting co-ordinates from one datum to an other?

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)?
jimirwin
science forum beginner

Joined: 19 Jul 2006
Posts: 1

Posted: Wed Jul 19, 2006 12:40 pm    Post subject: Re: Converting co-ordinates from one datum to an other?

Anonymous via the Cypherpunks Tonga Remailer <nobody@cypherpunks.to> wrote
in news:20060718194512.27749170B6@mail.cypherpunks.to:

 Quote: 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.

--
Jim Irwin
http://www.holoscenes.com
Eino Uikkanen
science forum beginner

Joined: 26 Jun 2006
Posts: 2

Posted: Wed Jul 19, 2006 5:29 pm    Post subject: Re: Converting co-ordinates from one datum to an other?

"Anonymous via the Cypherpunks Tonga Remailer" <nobody@cypherpunks.to>
kirjoitti viestissä:20060718194512.27749170B6@mail.cypherpunks.to...
 Quote: 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)?

See section "Conversions between coordinate systems", table 11 in my article
http://www.kolumbus.fi/eino.uikkanen/geodocsgb/ficoords.htm. The article is
about Finnish coordinate systems, but table 11 shows quite general
conversion flow with references to respective formulas.

If you want a tool for the job, see by freeware GeoConv:
http://www.kolumbus.fi/eino.uikkanen/geoconvgb/index.htm or easier to use
online tools listed in GeoConv's brief description
http://www.kolumbus.fi/eino.uikkanen/geoconvgb/geobriefgb.htm

Regards,
Eino Uikkanen
http://www.kolumbus.fi/eino.uikkanen/gb/index.htm
Anonyma
science forum beginner

Joined: 03 Jun 2006
Posts: 3

Posted: Thu Jul 20, 2006 5:46 pm    Post subject: Re: Converting co-ordinates from one datum to an other?

jimirwin wrote:
 Quote: Anonymous via the Cypherpunks Tonga Remailer

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,
30.0002238973/-100.0003696013/0m, 30.0002238891/-100.0003695878/232m (Dana)
and 30.0002112611/-100.0003501667 (NADCON). So changing the height
doesn't change it much.
So I then tried different longitudes based on a latitude of 40:

latitude longitude
Dana: 39.9999643708 -64.9992930771

40.0000000000 -70.0000000000
39.9999915613 -69.9994472150
40.0001355250 -69.9994378722

40.0000000000 -80.0000000000
40.0000255521 -79.9997669068
40.0000737694 -79.9997744055

40.0000000000 -90.0000000000
40.0000315820 -90.0000936811
40.0000484805 -90.0001139222

40.0000000000 -100.0000000000
40.0000094676 -100.0004176089
40.0000058944 -100.0004058361

40.0000000000 -110.0000000000
39.9999598811 -110.0007288478
39.9999636277 -110.0007048388

40.0000000000 -120.0000000000
39.9998843289 -120.0010179411
39.9999048055 -120.0010352250

40.0000000000 -130.0000000000
39.9997851067 -130.0012761048
39.9998735527 -130.0013791138

I understand these use different methods and NADCON uses actual recorded
data values in its calculation, so it is probably more accurate.
But Dsna's formula is probably simpler.
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?
David L. Wilson
science forum beginner

Joined: 31 May 2005
Posts: 37

Posted: Fri Jul 21, 2006 12:16 am    Post subject: Re: Converting co-ordinates from one datum to an other?

"Anonyma" <anon-bounces@deuxpi.ca> wrote in message
news:313fbc1b6c8289c8f8941960ea2faca8@deuxpi.ca...
 Quote: jimirwin wrote: Anonymous via the Cypherpunks Tonga Remailer

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

 Quote: 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
David L. Wilson
science forum beginner

Joined: 31 May 2005
Posts: 37

 Posted: Fri Jul 21, 2006 12:55 am    Post subject: Re: Converting co-ordinates from one datum to an other? I meant to also include in my post: If you want to experiment with the difference between WGS84 and NAD83, download and also get HTDP from http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.html But also be certain to see Section 7.1 of http://earth-info.nga.mil/GandG/publications/tr8350.2/wgs84fin.pdf

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