shrug-l: NAD83/90

Eric_Songer at Eric_Songer at
Wed Sep 6 12:08:45 EDT 2006


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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