[time-nuts] nuts about position

[jimlux]

On 4/25/18 7:46 PM, Brooke Clarke wrote:
> Hi J:
> 
> I had a number of survey stakes I placed using a manual transit and tape 
> measure and hired a local surveyor to tell me where they were and also 
> tell me where my GPS antenna was located.
> 
> He setup a GPS antenna on one tripod and a (Trimble?) combined GPS-total 
> station on another tripod and ran a cable between the two.  After some 
> time (tens of minutes or ??) he used the theodolite to sight my stakes 
> and the GPS antenna.  I got a report back in a week or so.  Total cost a 
> few hundred dollars.
> 
> I'm in the process of looking at how accurate the GPS is in my new LG G6 
> phone.
> 

Yep - A typical total station is good to "a few seconds" (some single 
digit mm at 100 meters) in angle. Distance is usually pretty accurate 
1.5 mm + 2ppm.

So over a typical 100m sort of size (several acres) a total station can 
provide relative positions to better than a cm, but probably not better 
than a mm.

A good optical theodolite can do *maybe* an order of magnitude better, 
in terms of the basic measurement, but then there's other confounding 
factors that might dominate.

For instance, are you sure you're holding the prism/target *precisely* 
over the mark? And just what *is* that mark.

Precision metrology in any field is fascinating, but a rabbit hole down 
which one can fall very, very deep.