---------- 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 > ------------------------------------------------------------------------- > This message was transmitted using 100% recycled electrons.... > *************************************************************************