On Thu, May 10, 2012 at 10:23 AM, Arthur Dent <golgarfrinc...@yahoo.com> wrote: >>Chris Albertson albertson.chris at gmail.com >
> I'm not sure what point you're trying to make but it is a fact, as the OP > pointed > out, that there are differences between the empirical data of 'true > elevation' and > what various GPS receivers will indicate based on whatever model they are > using. Sorry if not clear. My point was that (1) the wgs84 reference his GPS uses is not "wrong". It can't be. It is the definition. It may not be the definition he wants and (2) GPS just is not good at altitude and he'd be better off using a paper map, they are free now so why not. "mean sea level" is not meaningful any more. What shape is the ocean and what if you live in Kanas? How to extrapolate the ocean level to Kanas? The answer is to use a model of some kind Here where I live I can walk down to the beach and pound a stake in the sand and mark the water level. People actually do that (in a more sophisticated way with tide misting stations up and down the coast) But in Kanas you need some kind of model that tell you what the ocean level would be if there were an ocean in Kanas. But BIG PROBLEM. No one knows how to do make such a model. So they simply DEFINE the height of the ocean in Kanas. One definition is wgs84. The trouble with a defining it is that it will not match what you measure with your stick in the sand. So there are any number of local definitions that are closer matches to measured heights The root of the problem is that the earch has a very complex shape. It is "lumpy" in random ways and you can't model this, you have to measure it and then look it up. Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.