shrug-l: NAD83/90

Mayo, Michele L. Michele.Mayo at
Wed Sep 6 16:48:49 EDT 2006

Many thanks Ronnie.  I thought that the 1990 adjustment was a vertical
correction and I did incorrectly refer to it as a datum.
My apologies to the SHRUG forum for any contribution I made to the confusion,
and thanks for keeping us on our toes!

	-----Original Message----- 
	From: Taylor, Ronnie L. 
	Sent: Wed 9/6/2006 3:56 PM 
	To: Mayo, Michele L.; Eric_Songer at; Thomas, Jim 
	Cc: shrug-l at 
	Subject: RE: shrug-l: NAD83/90


	Let me try to help out here.  It seems everyone has bits and pieces
of the puzzle with no way to put it together. 

	First North American Datum of 1983 (NAD 83), later called NAD 83/86
because of the release date and additional re-adjustments, is the horizontal
datum that is used by the United States, Canada, Mexico, and Central America
and was established in 1983 and released to the public in 1986, thus NAD
83/86, by the National Geodetic Survey (NGS) not USGS.  NGS defines all
datums used by the civilian community and is approved by the Congress of the
U.S. as the only civilian datum accepted by the U.S. Government.   This datum
is based on the Geodetic Reference System 1980 (GRS80) which is defined as
the geocentric center of the earth.

	As for the High Precision Geodetic Network (HPGN), this is what
Florida first called the network (Florida High Precision Geodetic Network)
when it was re-observed in 1988 and 1989 by the National Geodetic Survey
(NGS).  NGS then re-adjusted the stations observed in Florida and called it
NAD 83 Re-Adjustment of 1990 (NAD 83/90).  This has nothing to do with the
Vertical Datum which is completely different.  There was no change in the
definition of the horizontal datum only a name change based on the year of
the re-adjustment.  After Florida was completed several states wanted the
more accurate positions that were being observed by GPS so NGS came up with
the name High Accuracy Reference Network (HARN) which is the term used
nationally.  This name has been dropped, as well as HPGN, and only a year is
assigned to the NAD 83 for each of the states.  Florida has had a couple of
Re-Adjustments with the most notably being NAD 83/90 which different from the
original NAD 83 adjustment by +/- 40 centimeters.  The next notably
re-adjustment is NAD 83/99 which different from the NAD 83/90 by +/- 5

	Without getting deep into the science behind all this you can go to
the NGS website and look under FAQ for some of the answers
to your questions.  

	I hope this helps and I will be happy to answer other questions that
you have. 

	Ronnie L. Taylor 

	NOAA, National Geodetic Survey 
	National Ocean Service Advisor 
	3900 Commonwealth Blvd., MS105 
	Tallahassee, Florida 32399 
	Phone: (850)245-2606 
	Fax: (850)245-2645 
	Email: Ronnie.Taylor at 

	-----Original Message----- 
	From: shrug-l-bounces at
[mailto:shrug-l-bounces at] On Behalf Of Mayo, Michele L.

	Sent: Wednesday, September 06, 2006 11:46 AM 
	To: Eric_Songer at; Thomas, Jim 
	Cc: shrug-l at 
	Subject: RE: shrug-l: NAD83/90 


	The description below is correct as far as the horizontal datums are 
	concerned.  The reason for the change from NAD83 to HARN/HPGN (same
thing) is 
	that the shape of the Earth is actually not a spheroid (a
	regular shape) but rather a geoid (an irregular shape).  The new
	provides a more accurate description of the Earth's true

	The term NAD83/90 refers to a combined horizontal and vertical
	system, with the "90" being NAVD90, or the North American Vertical
Datum as 
	established in 1990. 

	If your data have z values (elevations) then this is the datum to
which those 
	values are referenced. 

	Hope this helps to clarify. 


	Michele Mayo, GISP 
	Florida DEP, Beaches and Coastal Systems 
	Coastal Data Acquisition Section 
	3900 Commonwealth Blvd. MS 300 
	Tallahassee, FL 32399-3000 
	michele.mayo at 

	-----Original Message----- 
	From: shrug-l-bounces at 
	[mailto:shrug-l-bounces at] On Behalf Of 
	Eric_Songer at 
	Sent: Wednesday, September 06, 2006 12:09 PM 
	To: Thomas, Jim 
	Cc: shrug-l at 
	Subject: Re: shrug-l: NAD83/90 


	I'll start this and hopefully somebody else can add to the

	A spheroid is a mathematical formula that describes the shape of the
	(an ellipsoid, like a slightly flattened sphere)  A datum is a piece
	that ellipsoid stretched mathematically to known control points to
	describe a "local" area, because the earth is not all the same at the
surface.  NAD83 is the North American Datum so it's locality is all of 
	North America.  Around 1983 USGS updated the datum with more accurate
points all over N. America.  In 1990 somebody updated it again.  Usually 
	that is a HARN or HPGN update and it can be referred to as that or by
	date just to add confusion. 

	Since there is a lot of variation across N. America, HARN (High
	Reference Network, I think) and HPGN (High Precision Ground Network,
	are done, usually by an individual state, with even more accurate
	These are confusing to me, so maybe somebody else can chime in here,
	basically they are more accurate mathematical descriptions of even
	portions of the earth's surface based on direct measurements. 

	This starts to matter greatly when you collect sub-meter or better
	data.  The HARN adjustments can be a couple of feet away from the
NAD83 in 

	Eric Songer 
	URS Corporation 
	1625 Summit Lake Drive 
	Tallahassee, FL 32317 
	Direct: 850.402.6327 
	Main: 800.842.9671 ext. 327 
	eric_songer at 


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