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Mike Burkes wrote:
> Hi folks need your help! I realize photo is not very clear but I
> determine the " least count" of the vertical circle reading is 6 sec of arc.
> Is "least count" the correct term for the smallest reading?
> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=55807&item=7520967460&rd=1

I think a 6 second vernier on the vertical circle would be way out of
line for an instrument of this type. To me it looks like a 1 minute
vernier in the photo. It's a bit blurry, but there are clearly 10
graduations between "20" and "30" on the left half of the vernier.

Note that even the classic Parkhurst theodolite, used by the U.S. Coast
& Geodetic Survey for first-order triangulation, only had a 10-second
vernier on the vertical circle. (The horizontal circle was read by a
pair of microscopes with filiar micrometers, estimated to to .1 second.)

One of my surveying textbooks says a 1-minute vertical circle vernier is
usual on traditional American transits like that Gurley on eBay. The
horizontal circle may be divided finer. Also, that book agrees with your
usage of "least count" regarding the vernier graduations.

I don't watch Gurleys on eBay that closely, but by my recollection the $200
start price is pretty attractive. Granted, you'll never see the smart
bidders until after the auction closes (they'll "snipe" the item), but
*no* bidders after several days? I wonder if something is not kosher
about this transit.

   

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