---------- Forwarded message ----------
Date: Wed, 23 Sep 1998 09:16:39 +0200 (DFT)
From: "A. Hausold" <[log in to unmask]>
To: [log in to unmask]
Subject: details to mean sea level, Alicant
 
Dear members of the list,
 
first of all thanks a lot to all who have already responded to my request
and gave hints.
 
On reading the answer of Darius it seems that I might be on the wrong
track. If you do not mind, I would like to explain want I am planning to
achieve. Maybe you then have even more advise for me?
 
I have airborne scanner data of a testsite in spain and I want to do a
geometric correction. For doing this I need the flight path, derived from
differential GPS measurements/calculations, and some ground control
points. The flight path and the GCPs have to be in the same geodetic
system (ellipsoid, datum) concerning position and height.
 
The DGPS data refer to WGS84 in position and height (so these are
ellipsoidal heights, am I right?).
The map is based on Hayford Ellipsoid, European datum, height reference is
mean sea level in Alicante. So what does this mean for the heights? Maybe
I got this wrong.
I have a program that calculates the geoid undulation for lat/long
positions, and I thought it would be valid for WGS84, as it was determined
for GPS data. On a closer look I saw that it refers to GRS80. So I can not
use it for getting heights refering geoid.
 
So what can I do to get the height informations in the same system?
 
And - I hope not to bother you too much - one more question, maybe I have
not understood this so far:
 
is the "geoid level" the same as "mean sea level". Doubts arose, because
there exist so many "mean sea levels", depending on the country.
 
Maybe these questions are too basic, then I am sorry for that and I would
be grateful for a hint on good literature concerning this.
 
Thank you very much for the time you spend,
 
best regards,
 
Andrea
 
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Andrea Hausold, Cartographer                   [log in to unmask]
DLR German Aerospace Research Establishment
Institute of Optoelectronics                   voice:+49-8153-28-1532
D-82234 Wessling, GERMANY                      fax:  +49-8153-28-1458