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

Brooke Clarke brooke at pacific.net
Wed Sep 14 18:37:41 UTC 2011

Hi Chris:

And there's the problem that the BM is moving.

Have Fun,

Brooke Clarke

Chris Albertson wrote:
> 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
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