Edgar Soldin wrote: >Adrian, > >true, true .. well told story rhat ... thanks alot. > >just to make a point: >if bursa wolf parameters are missing, the parameters for an accurate >ellipsoid shift are missing, so results will be (very?) inaccurate? > >
yes... usually in the dimension of 100m + xxx stefan >kind regards ede >-- > > >>Hey, >> >>In the "for dummies" collection, "geodesy for dummies", from adrian, >>will surely become a best seller :-) >> >>Michael >> >>Adrian Custer a écrit : >> >> >> >> >>>On Mon, 2007-09-17 at 10:46 -0700, Jody Garnett wrote: >>> >>> >>> >>> >>> >>>>Edgar Soldin wrote: >>>> >>>> >>>> >>>> >>>> >>>>>just one question .. what is this bursa wolf parameter option? >>>>> >>>>> >>>>> >>>>> >>>>> >>>... >>> >>> >>> >>> >>> >>> >>>>My impression is that this is scary math I never quite understood. The >>>>javadocs describe it all detail (and have links to papers etc..). >>>> >>>> >>>> >>>> >>>> >>>Well, Bursa was a 9 year old bicyclist from the Alps and...no, no, no, i >>>lie. Actually it's not particularly scary math and quite easy to >>>understand. All you really need to remember is that no one has ever been >>>to the center of the earth. >>> >>>So everyone started surveying (mostly so the repressive central >>>governments could exploit taxes from people and have lots of jolly wars >>>where people could slog through the mud and kill each other so they'd be >>>blood and suffering for all). Each group started from some random place >>>on the surface of the earth. Right away, it becomes obvious to everyone >>>that euclidean rules don't work so well. Some didn't care so much since >>>taxes are basically arbitrary anyway and getting serious about it means >>>you'd have to walk through fields and woods and get lots of mud on your >>>shoes. Others kept at it and resorted to spherical geometry. Once you >>>start doing that precisely and at continental scales you realize that >>>doesn't really work either so you decide to try the next hardest thing, >>>an ellipsoid of rotation. Now how do you know which one to choose? Well >>>you pick one that minimizes your squared errors. All good and nice but >>>(1) you are surveying the ground which is anything but an ellipsoid >>>since it has all those ditches you keep falling into and that keep >>>getting your clothes covered in mud and (2) you are not perfect >>>especially with all that mud on your paper. So you have a bunch of >>>errors. Well everyone that does this comes up with lots of different >>>ellipsoids that work really nice for their data and everyone is sure >>>they clearly have found the 'one true ellipsoid' and they decide to use >>>that for all their work. Then everyone guesses where they actually are >>>on each of their particular ellipsoids which involves lots of going >>>outside at night and looking up from the mud at the stars. But then it's >>>not like the edges of each survey was nice and level on these ellipsoids >>>either --- think of the eastern USA. You can start nice and clean and >>>warm and dry at an inn in Boston on the edge of the sea drinking clam >>>chowder and having a good time but a few months later it will be bitter, >>>bitter cold in that tiny town of Denver because you are somewhere like a >>>mile high up in the air and you're wet and covered in mud from slogging >>>through the plains in a snowstorm. So you've got a pretty good idea that >>>your data is on a major slant but, well, you'll do your best to make up >>>for it but it really doesn't help the effort any, especially what with >>>all that mud that's still itching in your hair. So your errors may be a >>>wee bit big but hey it's all right: it's good enough to wage lots of >>>good wars with lots of mud and blood and to keep collecting lots of >>>taxes so no one cares too much. >>> >>>Fast forward to more recent times where some people want to talk to lots >>>of different governments and work with lots of different data. They take >>>everyone's guess and try to line them up. Well it turns out, when you >>>try to line everything up, that the center points of all the different >>>ellipses aren't really the same points and even the orientation of the >>>three axes are all a bit off because of how everyone guessed where their >>>were on their ellipsoids. So now, to go from one data set to another so >>>they line up "the best," you need estimates of how much to rotate each >>>of the axes and how to shift the center point around; all this beyond >>>even the obvious stuff of changing between the different definition of >>>all those "one true" ellipsoids. >>> >>>When you do this mathematically, you need a bunch of parameters: these >>>now have the names of the wolf and the bursa. Generally, you can only >>>come up with good parameters if you have lots of data to compare and >>>some good software to do the comparing. That's what the EPSG did for >>>everyone. The guys in the pickup trucks that went out looking for oil >>>kept falling into ditches along the way and getting mud on their faces >>>but when they got back to the office they had a good sense of what lined >>>up with what and could say: "yep, that hill there is the same as this >>>squiggle here and there's this big ditch right here that cost us our >>>third flat tire and..." So they collected as much data as they could and >>>compared it and came up with a database of parameters by which you go >>> >>> >>> >>>from one data set to another. So that's it. That's why we use their >> >> >> >>>data; we don't have to fall in any ditches and can avoid getting mud on >>>our clothes. They give us their parameters and we can mostly line up >>>data from one survey against data from another. But you do need some >>>good parameters because the earlier folk had a harder time of the mud >>>and the data they created don't just line up the way we would like them >>>to. >>> >>>Actually doing the math is a bit harder but the concept is pretty >>>straight forward: geographic data all ultimately gets tied into points >>>on the earth surface and that requires estimating where the points >>>really are and how they line up on the estimated ellipsoid being used. >>>That in turn means none of ellipsoids quite line up and we need >>>parameters to move between them. >>> >>>--adrian >>> >>> >>>------------------------------------------------------------------------- >>>This SF.net email is sponsored by: Microsoft >>>Defy all challenges. Microsoft(R) Visual Studio 2005. >>>http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ >>>_______________________________________________ >>>Jump-pilot-devel mailing list >>>Jump-pilot-devel@lists.sourceforge.net >>>https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >>> >>> >>> >>> >>> >>> >>> >>------------------------------------------------------------------------- >>This SF.net email is sponsored by: Microsoft >>Defy all challenges. Microsoft(R) Visual Studio 2005. >>http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ >>_______________________________________________ >>Jump-pilot-devel mailing list >>Jump-pilot-devel@lists.sourceforge.net >>https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >> >> >> > > > > ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. 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