[time-nuts] USGS Benchmark accuracy...?

[Chris Albertson]

On Wed, Sep 14, 2011 at 9:52 AM, Jose Camara <camaraq1 at quantacorp.com> wrote:
>
>        Hopefully it is better than the ones that denied Georgia a piece of
> the Tennessee river - just about one mile off...  Interesting topic in "How
> the States got their shapes" on History channel.

In one sense a BM's position is dead-on perfect.   The marker only
says "my name is "X" and I am at the position marked by this cross."
By definition it can't be in error.

Then you look up in some database and find the last surveyed location
of the BM.  Now that might have an error or not.  The location of the
marker near me is at the fraction of an inch level now but apparently
in the 1940's it was way off by several feet because everyone's
property lines are wrong and the city is making people get new surveys
if they pour any new concrete foundations.

So the accuracy of the same BM can change over time as it is
re-surveyed using better methods


Chris Albertson
Redondo Beach, California