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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Because it is the <EM>current</EM> legal datum of the United States, and it is based in part on artificial satellite observations. The semi-major axis of the GRS80 ellipsoid has been proven to be correct within a few centimeters based on decades of observing GPS orbits.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>NAD27 is referenced to an ellipsoid computed by hand and published by COL Alexander Ross Clark, R.E. in 1866.</FONT></DIV>
<DIV dir=ltr><FONT size=2></FONT> </DIV>
<DIV dir=ltr><FONT size=2>Since this is a discussion of distances, NAD83 is closest to the truth as a matter of scale <STRONG><EM>and </EM></STRONG>shape.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Best regards,</FONT></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of strebe@aol.com<BR><B>Sent:</B> Fri 01-Oct-10 14:26<BR><B>To:</B> proj@lists.maptools.org<BR><B>Subject:</B> Re: [Proj] NAD 27 v 83 Distances<BR></FONT><BR></DIV>
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<DIV><FONT color=black size=2><PRE style="FONT-SIZE: 9pt"><TT>>The difference of distances should be modest between the two datums, with of
>course the NAD83 distances being the better of the two. </TT></PRE></FONT></DIV>
<DIV style="CLEAR: both"></DIV>
<DIV><FONT face="Georgia, Times New Roman, Times, Serif" size=3><BR>Why would NAD83 be the better of the two?<BR><BR>Regards,<BR>— daan Strebe<BR><BR></FONT></DIV>
<DIV><FONT face="Georgia, Times New Roman, Times, Serif" size=3><BR></FONT></DIV>
<DIV style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: helvetica,arial">-----Original Message-----<BR>From: Frank Warmerdam <warmerdam@pobox.com><BR>To: PROJ.4 and general Projections Discussions <proj@lists.maptools.org><BR>Sent: Fri, Oct 1, 2010 9:28 am<BR>Subject: Re: [Proj] NAD 27 v 83 Distances<BR><BR>
<DIV id=AOLMsgPart_0_9b57683c-3e8a-4cba-9e7c-2f6e471b9f4c style="FONT-SIZE: 12px; MARGIN: 0px; COLOR: rgb(0,0,0); FONT-FAMILY: Tahoma,Verdana,Arial,Sans-Serif; BACKGROUND-COLOR: rgb(255,255,255)"><PRE style="FONT-SIZE: 9pt"><TT>Robert McFaul wrote:
> Hi,
>
> I'm concerned about whether ground surface distances are different between
> NAD27 and NAD83. I'm in California (southern) and thinking about a
> conversion
> of a project to NAD83. I have some cross section distances measured (1000 to
> 3000 feet in length) and suddenly am wondering what sort of differences
> might
> be involved by this change in systems.
Robert,
The difference of distances should be modest between the two datums, with of
course the NAD83 distances being the better of the two. I did a test for a
random point in southern california and the change is datum shift deltas over
one kilometer was about 1cm. You can do similar tests for any area of interest:
cs2cs -f '%.4f' +proj=utm +zone=11 +datum=NAD27 +to +proj=utm +zone=11
+datum=NAD83
440000 3750000
439919.4833 3750196.3019 0.0000
440000 3751000
439919.4784 3751196.3100 0.0000
Best regards,
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, <A href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</A>
light and sound - activate the windows | <A href="http://pobox.com/~warmerdam" target=_blank>http://pobox.com/~warmerdam</A>
and watch the world go round - Rush | Geospatial Programmer for Rent
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