LOL! Clear as mud! Larry
On 9/17/07, Stefan Steiniger <[EMAIL PROTECTED]> wrote: > we should post this on the wiki :) > love it > > stefan > > Adrian Custer schrieb: > > 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 > -- http://amusingprogrammer.blogspot.com/ ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. 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