Hi, yes I agree - can you tell me how I find out the datum which Mapblast uses here in Europe?? On the other hand - as I stated before: to what accuracy do we aficionados need the precision differences ?? Jörg Chris Olsson schrieb: > > Bruno Tilger wrote: > >I am surprised at the large difference (between Posns 1&3) in longitude of > >11.174". > > The longitudinal difference between WGS72 and WGS84 at Greenwich is 0.554", > Bruno. > > My typo in which I inadvertently typed "E" instead of W in Pos#3 is to blame > and I apologise to the List if any members were genuinely confused by my typing > error. > > Of course, both the WGS72 and WGS84 positions were in the same hemisphere. > WGS84's zero degree meridian (ignoring a little bit of tectonic plate movement > since 1989) lies 102.478 metres East of the Prime Meridian at Greenwich. > > The difference between WGS72 and WGS84, although quite comprehensive in view of > the fact that the size and shape of the two spheroids are rather different and > the fact that the two datums have a rotational difference, as well as markedly > different geoids, is a mere handful of metres in most places around the work. > The greatest 3-D difference is little more than half a dozen metres or so. > Converting between WGS 72&84 is very easy, so there is little scope for > confusion > > The big geodetic difference is when co-ordinates are misunderstood as to their > geodetic basis. For example, if a Cospar site in Scotland lists its position > to a precision of a tenth of an arc-second of Lat/Long and another Cospar site > in Sweden lists its position to four places of decimal degrees, then there is > an easy trap for the unwary who might make the error of presuming that both > co-ordinates are on some kind of common ground, such as WGS84 or WGS72. > > Note that neither the Scottish nor the Swedish position is wrong in any way. > In fact they are quite accurate and the basis from which they were derived is > fundamentally sound. The potential error is in the presumption that they have > a common basis. In fact: the Scottish point is listed with reference to the > Airy spheroid and the OSGB36 + OD(Newlyn) datums; and the Swedish position is > listed -- equally legitimately -- with reference to its national mapping datum > of Sveriges Lantmäteriverket RT90 datum på Bessel's 1841 spheroid in the > horizontal axis and the local RH70 datum in the vertical axis. > > Each has a sound provenance and each is capable of being used with suitable > precision in 3D calcs upon either WGS72 or WGS84, but the important thing to > recognise is which geodetic basis is associated with any stated position. > > An analogy might be the European price of oil. If I simply state that the > current (epoch: close of business on Friday evening) Rotterdam market spot > price of Brent Crude oil is 25.12345, then my stated price is capable of > misinterpretation unless I declare whether I am referring to Dollars or Euros > and whether or not I confirm that I'm referring to a barrel of crude or a tonne > or a ton or a tun or a firkin or whatever. > > Note too that the stated Edinburgh and Malma locations are not in any way wrong > and that they are just as legitimate as a position which is expressed in one of > the international standards such as ED50 or WGS84. > > It is reasonable to presume that the Swede is using Degrees, not Gon, as his > angular measure, just as it is reasonable to presume that the Scotsman is using > arc-seconds rather than Groats or Drams as his. > > It might be reasonable to presume that both of those well-qualified observation > locations are using a common spatial datum. Such a presumption would be quite > wrong. > > My thesis is simple. I am suggesting that we either adopt a single common > standard or else -- perhaps more practically -- declare what geodetic standard > we are referring to when we list apparently precise geographical (or geodetic) > co-ordinates. The differences between the actual locations of stated positions > may be small or great. We can only know what those discrepancies (they are > *not* errors) are if we have some indication as to what they refer to. > > It is sufficient to simply say that a particular position is with reference to > WGS84 or a particular national mapping datum or whatever the basis of the > stated position is. > > By stating the geodetic basis of stated co-ordinates, we give a positional > indication which is almost as important as stating the epoch of an elset or a > SatObs. It gives a pre- and post- calculable peg upon which other spatial > calcs can be attached and assessed. > > Cheers, Chris Olsson > (In the Northern and Western hemispheres, well above sea level, wrt WGS84!) > > ----------------------------------------------------------------- > Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' > in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org > http://www2.satellite.eu.org/seesat/seesatindex.html -- ===================================================================== Dr. Jörg Kampmann - IBK-Consult for Real-Time and Embedded Systems D-31228 Peine - Tel.:+49-177-276-3140 - Fax: +49-5171-13385 http://www.ibk-consult.de ===== QNX is the better Choice for Real-Time: http://www.qnx.com ==== ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Sat Aug 18 2001 - 23:44:31 PDT