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Hello Courtney. If you wanted to have the individual sextant measured, that is 
possible but as far as I know, there are very few places doing it. One of the 
major names in the business (i.e. Robt. White & Son out of Boston) could 
probably send it out for measurement, or a manufacturer might be willing to 
perform re-measurement for you.

Sometimes it is also a question of whether you think the possible error 
matters. My Cassens & Plath was bought second-hand without an individual 
card. But C&P were quick to correspond and state the instrument error is 
guaranteed to such a small range, that it could be effectively ignored (at 
least for me).

From the make and model you should be able to find a published spec for 
manufacturing accuracy if this is a recent model or a well-known one. You can 
then translate that into a potential of position error, and once your 
sightings are placing you within that amount of error, first worry how much 
is from the sextant.

A point of view which I hope does not horrify too many list members.

Here in the US, our Geological Survey maintains a list of survey benchmarks. 
The same information must be available for you up in Canada. When I wanted to 
see how accurate I was or wasn't, I checked the list for a local benchmark 
near a public beach, so taht I could walk up to the spot, know that "I am 
here" according to the best survey data possible, and then compare the 
sextant readings to it.

You would think that was unneccessary and I could have just measured my 
position from a map. Nope. The various maps (topo quad, nautical) were from 
different bases and literally from one to the other, some positions changed 
by 1/3 mile if you just measured them from the maps. I figured started from 
the basemark itself was easier than messing around with rulers at that point.

   

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