---------- Forwarded message ----------
Date: Wed, 23 Sep 1998 15:50:24 -0500
From: "Balentine, Timothy C" <[log in to unmask]>
To: Maps and Air Photo Systems Forum <[log in to unmask]>
Subject: RE: details to mean sea level, Alicante: reply (fwd)
NIMA has a good primer called Geodesy for the Layman at the following URL:
http://www.nima.mil/publications/pub.html
> -----Original Message-----
> From: Johnnie Sutherland [SMTP:[log in to unmask]]
> Sent: Wednesday, September 23, 1998 1:08 PM
> To: [log in to unmask]
> Subject: Re: details to mean sea level, Alicante: reply (fwd)
>
> ---------- Forwarded message ----------
> Date: Wed, 23 Sep 1998 16:35:48 +0100
> From: Darius Bartlett <[log in to unmask]>
> To: Maps and Air Photo Systems Forum <[log in to unmask]>
> Subject: Re: details to mean sea level, Alicante: reply
>
> At 09:36 23/09/98 -0400, Andrea Hausold wrote:
> >---------- 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'll do my best. What follows is quite long: those not particularly
> interested in basic geodesy may hit the delete button now! :-)
>
> >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?).
>
> Yes. But WGS84 is just one of many possible ellipsoids (see below). It
> happens to be the one that has been adopted as "standard" for GPS and
> similar applications.
>
> >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.
>
> The Earth, as we all know, is not a perfect sphere. One approximation is
> that it is an oblate spheroid - an ellipsoid, with a major and a minor
> axis, and a defined centre that is some distance removed from what would
> be the centre if the Earth was a true sphere.
>
> There are many different ways of defining these ellipsoids, depending on
> where you place the origin and where the resulting axes intersect the
> surface
> of the earth. In general terms, conversion from one ellipsoid to another
> is
> normally achieved by using what is known as the Molodensky Datum Transfer
> Proceedure, or Molodensky Constants. These constants comprise three
> values - the Delta-X, Delta-Y and Delta-Z shifts required to convert
> from WGS84 to any thus-defined new datum.
>
> If you have access to the manual for IDRISI for Windows version 1.0 or
> above, Appendix 2 has a comprehensive list of these Constants for a
> very long list of alternative geodetic datums. Unfortunately, though,
> the Hayford Ellipsoid does not appear to be one of these, and I have no
> source of reference for these myself. The writers of the IDRISI manual
> actually cite their own source as being "a data file accompanying the
> MAD-TRAN V.9109.04 datum conversion software (1992) available from
> the US Defense Mapping Agency (DMA Stock No MADTRANIBMPC Edition No. 002)"
> as the source of their data, so it may be possible that the parameters
> you require are in this original document.
>
> >So what can I do to get the height informations in the same system?
>
> In a nutshell, if you can somehow track down the relevant Molodensky
> Constants for the Hayford Ellipsoid, European Datum, this should give
> you the conversion details you need. Then, most GIS software can perform
> the required adjustments - Idrisi and Arc/Info both have facilities
> whereby you feed in the constants and the software does the rest.
>
> >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.
>
> No, the geoid level is very different from mean sea level. As I said
> above,
> the Earth is not a sphere. For most applications, including most
> topographic mapping, it is sufficient to approximate the shape of the
> Earth to one of the ellipsoids mentioned above. The actual choice depends
> on whereabouts on the planet you happen to be. However, there are times
> when a very much more detailed, "precise" definition of the shape of the
> Earth is required: this is the geoid (the name actually means
> "Earth-shaped" :-)
>
> A formal definition of the geoid is that it is a sea-level gravity
> equipotential surface for the planet. Robinson et al (Elements of
> Cartography, 6th Edition, 1995, p44) say of the geoid that it is "the
> three-dimensional shape that would be approximated by mean sea level
> in the oceans and the surface of a hypothetical series of sea-level
> canals crisscrossing the continents". They also add the information
> that the geoid surface can deviate from the ellipsoid by up to 100m
> in certain locations, so it is clearly a factor to be taken seriously.
>
> Also, as you say, there are many "mean sea levels" - depending on the
> length
> of the time series of measurements is used for computing the "mean", on
> the
> actual tidal range (very different in the micro-tidal location of Alicante
> and the more macro-tidal range of the North Sea for example), and
> depending
> on which phases and states of the tide are used as minima and maxima
> (spring
> tides, neap tides, ordinary tides?).
>
> >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.
>
> No, not too basic at all - indeed this is something I think is very
> often overlooked and misunderstood. I would only claim to know the very
> basic principles myself. If you do want to know more, I recommend the
> book by Robinson et al cited above as a good introduction to some of
> the issues. They also cite the following, which I have not seen myself,
> but which looks as if it could be useful:
>
> Langley, R.B. "Basic Geodesy for GPS". GPS World 3 (1992) 38-43
>
> Finally, "Photogrammetric Engineering and Remote Sensing", the journal
> of the American Society for Photogrammetry and Remote Sensing (ASPRS)
> has a very useful regular column on the subject of geodetic datums by
> Clifford Mungier at the University of New Orleans.
>
> Hope this helps!
>
> Darius
> ************************************************************************
> Darius Bartlett Darius Bartlett
> Department of Geography Roinn na Tireolaiochta
> University College Cork Colaste na hOllscoile Corcaigh
> Cork, Ireland Corcaigh, Eire
>
> Phone: (+353) 21 902835 Fax: (+353) 21 271980
> Mobile: (+353) 86 8238043
> E-mail: [log in to unmask] Web URL: http://www.ucc.ie/ucc/depts/geography/djb
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