Nothing to do with time but...
Technically speaking, in surveying a bench mark is a vertical control point. In
playing with Geocaching I've located bench marks that were placed in the 1930's
and never found again (until I did). There were often a hundred feet or more
from where the description had them. (Great fun BTW)
See: https://www.geocaching.com/mark/
On 5/2/2016 9:13 AM, Chris Albertson wrote:
A man with a GPS knows where he is. A man with two GPS' not not sure.
I've always wanted to walk my self-survey GPS over to a brass USGS
benchmark and see it the GPS matches to benchmark location. OK, I've done
this with a hand held GPS and gotten readings within about 10 meters.
But before spending a lot of time removing the lat 10cm of error I'd do a
test at the nearest BM that is not in the middle of a street.
What has stopped me from doing this is that a few years ago I had to have
my lot lines surveyed. They got to better then 1/10 of a foot at each
corner and shot some brass markers into the concrete. Google can see my
house's roof ridge lines and the concrete so I can work out the exact
location of the roof mount antenna to within maybe 18 inches. It seems to
agree with the survey as long as everyone uses WGS84.
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