shrug-l: NAD83/90

Sykes, John John.Sykes at
Wed Sep 6 15:15:27 EDT 2006

Here's Dave Doyle's response.  Please note that the height component in NAD
83 (1990) HPGN/HARN is the ellipsoid (essentially the GPS) height, which is
NOT the same as NAVD 88 vertical datum, which is based on tidal (a.k.a.
sea-level) reference measurements.  Most elevations (maps, surveys, etc.) are
referenced to the NAVD 88 datum.
-- John 


Good questions. Hopefully I can answer them without getting too long winded
(which is very hard for me).

NAD 27 was a purely horizontal datum

NAD 83 in it's original solution of 1986, now referenced as NAD 83 (1986) was
also purely horizontal.

NAD 83 (1990) is both HPGN and HARN. In 1992 the name of the GPS survey
program was called the High Precision GPS Network (HPGN). Sometime in 1992
someone at NGS (not me) had the idea to change it to High Accuracy Reference
Network (HARN). So regardless of the name change the process or concept is
still the same. Now, for the first time a vertical component was added which
did not exist with NAD 27 and NAD 83 (1986), but it's the ellipsoid height
component, which is NOT the NAVD 88. NAVD 88 is strictly a vertical datum and
is not in any way related to NAD 83 other than control points that are the
realization of these two datums may have well determined values.

I hope this makes some sense. If it's not clear please call me

(301-713-3178) and I will be happy to explain this in greater detail.

Regards -- Dave Doyle


From: shrug-l-bounces at
[mailto:shrug-l-bounces at] On Behalf Of Thomas, Jim
Sent: Wednesday, September 06, 2006 1:32 PM
To: shrug-l at
Subject: FW: shrug-l: NAD83/90

I've done more research and the 90 part of NAD83/90 seems to refer to the
1990 adjustment (as John said below).  This is new to me as I've never
referred to NAD83 in another way.  Does anyone know what the latest
adjustment is?  and what ESRI's ArcGIS uses?
I'll have to do more homework on this one.  Thanks for your help.


From: Sykes, John [mailto:John.Sykes at] 
Sent: Wednesday, September 06, 2006 11:59 AM
To: Thomas, Jim
Subject: RE: shrug-l: NAD83/90

My understanding is that the NAD83 is the "standard" replacement for NAD27.
NAD83 HARN and NAD83/90HPGN are, essentially, the same.  Originally it was
called the High Precision Geographical Network, but was subsequently changed
to High Accuracy Regional Network (or something like that).
The difference between NAD83 and NAD27 is several meters (up to around 100
meters in parts of Florida), but NAD83 to NAD83 HARN is usually in the
centimeter range (in most of Florida).
Note:  It is also my understanding that the NAD83/90 is the "standard"
version of NAD83 and, therefore, is redundant.  So usually you would see just
NAD83 or NAD83 HARN, the "90" is assumed (not necessarily the case with HARN,
which I believe is set to a later epoch [2004????]).  HPGN is no longer used.
If you want to confirm the epoch, look up a CORS station in the NGS
datasheets, it will give the epoch information as well as the drift
information (Florida is drifting northwest at about 1 cm/yr).

-- John 



From: shrug-l-bounces at
[mailto:shrug-l-bounces at] On Behalf Of Thomas, Jim
Sent: Wednesday, September 06, 2006 11:27 AM
To: shrug-l at
Subject: shrug-l: NAD83/90

Can someone briefly explain the difference among the following.
Is NAD83 HARN the same as NAD83/90 HPGN?
I'm getting requests to have coordinates reported in NAD83/90 and need to
understand the difference.
Thanks for you help.
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