[Proj] NAD 27 v 83 Distances

[strebe at aol.com]

>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
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and watch the world go round - Rush    | Geospatial Programmer for Rent

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