On Sunday 29 May 2005 17:00, Pierre Abbat wrote:
> On Sunday 29 May 2005 12:03, James R. Frysinger wrote:
> > Surveyor's use the survey foot, which was just the plain ol' American
> > foot before 1959. It equals (1200/3937) m and is about 2 mm/km longer
> > than the "new" foot established here in 1959. Surveyors do not divide
> > this foot into inches but instead measure down to the nearest hundredths
> > of a foot. That is a resolution of about 3 mm.
>
> That's the precision of numbers on the map; but the accuracy of locating a
> house in the field with a total station is 0.1 foot, or 30 mm. The new GPS
> device which my boss bought to replace a damaged station has centimeter
> accuracy (and this is one of the rare times I've heard him say anything in
> metric), but locating a corner of a house is still, I think, going to be
> less accurate than locating an iron.
Yes, I agree. That's why I used the word "resolution". Differential GPS
can
provide some tremendous internal consistency, I am told by those who have
used it --- good "closure" --- but I have no experience with it. It seems to
me that the technique is still going to be quite limited by the accuracy of
establishing a base station and baseline. I think that those still depend on
monuments which are still rated above general GPS location service. Am I
correct?
And of course, that can be a problem. A neighbor of mine in Tennessee
took up
the late night hobby of moving boundary line rocks and creating some extra
line marks on trees. Fortunately he was stopped before he got to the
surveying monuments.
> Back to my question: since surveyors already use decimals, does that mean
> that they will be less confused by centimeters than clothing-makers are?
I have no doubt of that, Pierre. My Tennessee surveyor seemed quite at
home
with that and said that his software could work in meters (or centimeters) as
well as in feet. Interesting guy; he started his professional life as a real
estate lawyer and got jealous of the time his hired surveyors got to spend in
the field. So he changed careers.
Jim
--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE
http://www.cofc.edu/~frysingj
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