shrug-l: NAD83/90

Taylor, Ronnie L. Ronnie.L.Taylor at
Wed Sep 6 15:56:27 EDT 2006


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

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

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 geometrically
regular shape) but rather a geoid (an irregular shape).  The new datum
provides a more accurate description of the Earth's true configuration.

The term NAD83/90 refers to a combined horizontal and vertical coordinate
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 discussion.

A spheroid is a mathematical formula that describes the shape of the earth
(an ellipsoid, like a slightly flattened sphere)  A datum is a piece of
that ellipsoid stretched mathematically to known control points to better
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 the
date just to add confusion.

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

This starts to matter greatly when you collect sub-meter or better GPS
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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