> The magical
> mythical point on the Mississippi upon which early surveyors
> decided is now known to be 694.10 feet above USGS sea level.
Thanks for clarifying, Phil. Sorry I got the sign flipped before.
> This is not comparable to today's surveyor choosing a random
> point on a single building site.
Ok, that part was intended tongue-in-cheek. (Hence the <G> indicating a
big grin.)
Perhaps I should explain that I worked for many years with satellite
surveying software (first Transit then later GPS) which produced positions
relative to global datums like WGS-72 and WGS-84. Converting to local
datums (not to mention local "flat earth" map coordinates) was a major
headache for us. Hard to claim with a straight face that we provide
centimeter-level accuracy when the datum conversion isn't known to even 10
meters ! And from our point of view, every little locality on the face of
the earth has chosen their own little datum/mapping system, based on little
more than "this looks like a good spot". There are something like 500
different conversions just to cover the California State mapping system
(which is a bunch of overlapping Lambert grids). So in that sense, it does
seem comparable, at least enough to base a joke on.
Sorry you didn't enjoy the joke.
BTW, the USGS has updated their estimate of MSL several times in the past
century, with changes in some inland areas varying by a meter or more. To
really quote .01 foot accuracy, you need to specify *which* USGS map you are
referring to. As I understand it, it's not even done by the USGS any more,
but was transferred to the new National Imagery and Mapping Agency a few
years ago. Fortunately I no longer work with surveying, so I don't care.
Randall
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