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<font face="arial" size="2" color="black"><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>
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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" size="3" color="black"><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>
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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>
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Regards,<br>
— daan Strebe<br>
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<div style="font-family: helvetica,arial; font-size: 10pt; color: black;">-----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" size="2" color="#000000">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"> </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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<div class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: 10pt;" lang="DE">Clifford J. Mugnier, C.P., C.M.S.</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>
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<div><font size="2" color="black"><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>
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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 target="_blank" href="http://pobox.com/%7Ewarmerdam">http://pobox.com/~warmerdam</a>
and watch the world go round - Rush | Geospatial Programmer for Rent
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