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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>There were next to no classical triangulation arcs in California as of 1860. Therefore, the "fit" by COL Clark was really only to the East coast and west to the Lower Mississippi Valley, at best. The NAVD88 is off only by a couple of centimeters according to the latest geoid models in Louisiana for NAD83/GRS80. The best geoid computed on the Clarke 1866 was an astrogeodetic geoid model computed by Dr. Irene Fisher of Army Map Service in the middle 1960s. Besides which, the NAD27 was off (internal accuracy) by METERS in California - that's why the National Academy of Science recommended we go to a "new" horizontal datum.</FONT></DIV>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>In regard to "legal," if it's not recognized as a "legal" reference, ... what good is it?</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Regards,</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Cliff</FONT></DIV></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 15:53<BR><B>To:</B> proj@lists.maptools.org<BR><B>Subject:</B> Re: [Proj] NAD 27 v 83 Distances<BR></FONT><BR></DIV></DIV>
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<DIV id=AOLMsgPart_2_6dcd06f7-3fb9-484f-ae85-8f394392260b><FONT face=arial color=black size=2><FONT face="Georgia, Times New Roman, Times, Serif" size=3>Does that follow? Aren’t the local datums, with ellipsoids fitted to the local surface, generally more accurate than a single ellipsoid for the entire earth? I’ve certainly found that vertical coordinates are generally worse on WGS84.<BR><BR>Granted, NAD27 covers a vast region, so perhaps it deviates further from reality than some of the more local datums in use around the world. But without a study computing deviations from the geoid on both, it seems a bit breezy to just claim the newer one is “better” given that its purpose is the best global system rather than the best local system. </FONT></FONT><FONT face="Georgia, Times New Roman, Times, Serif" color=black size=3><FONT color=black>That fact that NAD83 is the “current legal datum” seems irrelevant to the question of accuracy of real distances.</FONT></FONT><FONT face="Georgia, Times New Roman, Times, Serif" size=3><BR></FONT><FONT face=arial color=black size=2><FONT face="Georgia, Times New Roman, Times, Serif" size=3><BR>Obviously not important in the context of the original posting; either surface is a fiction and real distances over real terrain will deviate from either datum far more than distances on the two datums will deviate from each other.<BR><BR>Regards,<BR>— daan Strebe<BR></FONT><FONT face=arial color=black size=2>
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<DIV style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: helvetica,arial">-----Original Message-----<BR>From: Clifford J Mugnier <cjmce@lsu.edu><BR>To: PROJ.4 and general Projections Discussions <proj@lists.maptools.org><BR>Sent: Fri, Oct 1, 2010 12:42 pm<BR>Subject: Re: [Proj] NAD 27 v 83 Distances<BR><BR>
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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>
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<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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<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN lang=DE style="FONT-SIZE: 10pt">Clifford J. Mugnier, C.P., C.M.S.</SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt">Chief of Geodesy,</SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><B><SPAN style="FONT-SIZE: 10pt; FONT-VARIANT: small-caps">Center for GeoInformatics</SPAN></B></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt">Department of Civil Engineering </SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt">Patrick F. Taylor Hall 3223A</SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><B><SPAN style="FONT-SIZE: 10pt">LOUISIANA STATE UNIVERSITY </SPAN></B></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt">Baton Rouge, LA<SPAN> </SPAN>70803</SPAN></DIV>
<DIV class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="FONT-SIZE: 10pt">Voice and Facsimile:<SPAN> </SPAN>(225) 578-8536 [Academic] </SPAN></DIV>
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<FONT face=Tahoma size=2><B>From:</B> <A href="mailto:proj-bounces@lists.maptools.org">proj-bounces@lists.maptools.org</A> on behalf of <A href="mailto:strebe@aol.com">strebe@aol.com</A><BR><B>Sent:</B> Fri 01-Oct-10 14:26<BR><B>To:</B> <A href="mailto:proj@lists.maptools.org">proj@lists.maptools.org</A><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><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>
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<DIV style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: helvetica,arial">-----Original Message-----<BR>From: Frank Warmerdam <<A href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</A>><BR>To: PROJ.4 and general Projections Discussions <<A href="mailto:proj@lists.maptools.org">proj@lists.maptools.org</A>><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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