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. 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