[Proj] NAD 27 v 83 Distances

Fri Oct 1 17:49:34 EST 2010

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.

In regard to "legal," if it's not recognized as a "legal" reference, ... what good is it?

Regards,

Cliff

________________________________

From: proj-bounces at lists.maptools.org on behalf of strebe at aol.com
Sent: Fri 01-Oct-10 15:53
To: proj at lists.maptools.org
Subject: Re: [Proj] NAD 27 v 83 Distances

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.

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. That fact that NAD83 is the "current legal datum" seems irrelevant to the question of accuracy of real distances.

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.

Regards,
- daan Strebe

-----Original Message-----
From: Clifford J Mugnier <cjmce at lsu.edu>
To: PROJ.4 and general Projections Discussions <proj at lists.maptools.org>
Sent: Fri, Oct 1, 2010 12:42 pm
Subject: Re: [Proj] NAD 27 v 83 Distances

Because it is the current 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.

NAD27 is referenced to an ellipsoid computed by hand and published by COL Alexander Ross Clark, R.E. in 1866.

Since this is a discussion of distances, NAD83 is closest to the truth as a matter of scale and shape.

Best regards,

Clifford J. Mugnier, C.P., C.M.S.
Chief of Geodesy,
Center for GeoInformatics
Department of Civil Engineering 
Patrick F. Taylor Hall 3223A
LOUISIANA STATE UNIVERSITY 
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536 [Academic] 
Voice and Facsimile:  (225) 578-4578 [Research] 
Cell: (225) 238-8975 [Academic & Research]
Honorary Life Member of the 
Louisiana Society of Professional Surveyors 
Fellow Emeritus of the ASPRS 
Member of the Americas Petroleum Survey Group

________________________________

From: proj-bounces at lists.maptools.org on behalf of strebe at aol.com
Sent: Fri 01-Oct-10 14:26
To: proj at lists.maptools.org
Subject: Re: [Proj] NAD 27 v 83 Distances

>The difference of distances should be modest between the two datums, with of
>course the NAD83 distances being the better of the two. 

Why would NAD83 be the better of the two?

Regards,
- daan Strebe

-----Original Message-----
From: Frank Warmerdam <warmerdam at pobox.com>
To: PROJ.4 and general Projections Discussions <proj at lists.maptools.org>
Sent: Fri, Oct 1, 2010 9:28 am
Subject: Re: [Proj] NAD 27 v 83 Distances

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, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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