[MetaCRS] NAD83 and WGS84

[Norm Olsen]

Hello Frank . . .

Currently in CS-MAP, there are functions (which do nothing) which convert between NAD83 and WGS84.   Thus, at a code level that can be accomplished.

Better yet, in a couple of days, CS-MAP RFC-2 will be complete and you can define one or more NAD83 to WGS84 conversions in the new Geodetic Transformation dictionary file and determine how and when they are activated using the new Geodetic Path dictionary file.  Also please note that the issue of NAD83 and WGS84 has very close brethren all over the world:

	GDA94   <--> WGS84
	ETRF89  <--> WGS84
	JGDF2K  <--> WGS84
		   .
		   .
		   .


Also note that there are (if memory serves) three current realizations of WGS84 and a fourth probably due soon.  These are not simple improvements in the accuracy of the system, but represent how geography has moved over the time intervals involved.  Thus, for example, there is ETRF89 which pretty much locates things where they were in 1989.  A new "realization" will indicate where they are now.  All continents move, some more than others.  Continental drift is on the order of a couple of centimeters a year.  2010 - 1984 == 26 years == 52 centimeters == half a meter.  This issue will be of increasing importance over the years.

There are some seven parameter transformations for NAD83 to WGS84 floating around.  Implementing these will become important.  It used to be that the difference between these frames of reference was smaller than the accuracy at which we knew where things were.  When I start in this business, I used to routinely make the comment "We don't know where anything is +/- 50 feet anyway, so why concern ourselves with a couple of inches?"  The premise that we don't precisely know where stuff is is not valid anymore.  Thus, there are applications where inches start to become important.

In making a definition it will be important to know which realization is involved.  Also, there is such a thing known as the 14 parameter transformation.  This transformation includes velocity components and represents an attempt to predict where something will be at a future point in time.

Norm 

-----Original Message-----
From: metacrs-bounces at lists.osgeo.org [mailto:metacrs-bounces at lists.osgeo.org] On Behalf Of Frank Warmerdam (External)
Sent: Friday, October 01, 2010 10:52 AM
To: metacrs at lists.osgeo.org
Subject: [MetaCRS] NAD83 and WGS84

Folks,

I have generally treated NAD83 and WGS84 as equivelent for the purposes of
PROJ.4 and this seems to be the general practice in the GIS industry.  I
have a potential client asking about what it would take to add NAD83 to
WGS84 conversions so now I feel obligated to ask if there are established
transformations between them that are meaningful in any sense?

I presume they would be tied to particular realizations of NAD83 or WGS84.
I assume distinguishing realizations is something we are going to have to get
better at over time.  Is that a reasonable statement?

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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