Is a 20' vernier transit more accurate than a 30' vernier, (assuming all other things are equal).
Thank you,
N
Maybe.
There are many variables that come into play. Circle diameter, precision of vernier markings, overall quality (and condition) of the entire instrument (not mention adjustment thereof).
In theory... yes, in practice... maybe (maybe even probably).
Loyal?ÿ?ÿ
Assuming proportional improvement in the marking of the scales, then yes. Whether that matters depends on whether you are doing 200 ft lots or very long traverses and how many turns you do.
Repeating D&R angles on either scale can get you so much more accurate that the scale difference isn't a big deal.?ÿ The ultimate accuracy with multiple repetition will be somewhat sensitive to the visual acuity of the user, finesse of technique (ending with the last movement of the knob in the same direction, etc.), and such things as centering error and tripod stability.
I agree. In theory a 20" gun should provide an angle mean within or less than 10" with 3 properly repeated observations. The same observations with a 30" gun should provide an angle mean within or less than 15".
And as Bill pointed out, finesse of technique is paramount.
No, but it's more precise.
It seems from memory that the 20' unit also read seconds to 20", (direct), and the 30' model also read direct to 30". But, it's been a long time ago.
Nate
What matters is the leas count of the vernier, not the range of the vernier.?ÿ I thought transits were always named by least count. I missed that your OP described them the other way.
Spent a lot of time using vernier transits and winding up several repeat observations followed by winding down with the 'scope inverted for distant observations is fruitful and make a 30" more like a 10" gun if careful technique is followed.?ÿ
Plate accuracy, nearly neutral trunnion axis and general stability of the gun/tripod situation is paramount.?ÿ I was overjoyed when the T2 (glass plates) became available and a set of direct/reverse observations done quickly was good to 5" or less on long lines with appropriate targetry & weather.
Of course any observation over a few miles is done using GPS these days but a vernier transit when laying out construction or a 1/2 acre boundary with existing nearby solid control is acceptable to me.?ÿ In some cases it's faster to set up a gun, Ni2 level and be more accurate concerning critical points locally than showing up with GPS registered to control miles away for curb stakes and other stuff that requires +-0.02' accuracy vertically.?ÿ Same with EDMs. fantastic for the long shots but why in God's name would you setup for a 30' shot when a tape would be more accurate concerning centering, etc.??ÿ
Sorry, I'm old school but in recent years I've seen newly constructed parking lots & gov'mnt roads where the construction surveying sucked resulting in birdbaths, crazy bad curb returns, etc.?ÿ ?ÿIs this the new paradigm, where a GPS obsv from remote control that's good to +- a tenth and the vertical control is +-0.5' worst case is good enough for construction??ÿ I think not.?ÿ Of course your GPS construction guys will claim much tighter specs and more power to you.?ÿ But it can go horribly wrong; I've experienced the "GPS construction survey" where flatwork didn't work and some guy with a simple transit and and an Ni2 straightened it out.
S
There are a variety of vernier scales used on transits.
I am looking at an example I the Davis, Foote, and Kelly book of a 20-0-20 vernier that reads to a half minute 30".
Another is 10-0-10 reading to 20".
Kissam's book has a 15-0-15 reading to 20".
Within 4' over my right shoulder is a 1926 K&E with a horizontal circle divided into 1/4 degrees and left/right verniers divided into 45 parts (15x3).
I've always thought of it as a 20 second instrument (the smallest division on the main scale divided by the number of parts on the vernier).
It is an issue here also. Why with the tools available today there would be these issues is puzzling. Our robot will take care of almost anything as long as you are careful, there will still be the mismarked stake. C-2.75 gets marked C-2.57. It happens, but using GPS to layout tight grade is a non-starter.
Ours was a 1923 Adolph Lietz, San Francisco. My brother wanted it, so he's had it for about 15 yrs. It was a 20 minute vernier in the horiz, and 30 minute vert.
The focus rack and pinion has a worn out spot. You have to work it carefully, to focus on close stuff. 2 hands, one working the scope, and the other turning the knob, to get it through the bad spot!
We really put in the hours with that gun. It was my dad's parting wage, when we left CA, in 1977.?ÿ
Sentiments rise, as I remember...
N
?ÿ
If you turn 3 sets you can divide the expected error in s single set by 1.74. If you want to cut it in half you need 4 sets..
I've used both and had the same relative amounts when computing closing angles.
Most every transit I've run was easily read at 1/2 and/or 1/4 of their lowest markings. It entailed at focusing beyond the first where the 1st set of lines that were in alignment.
I've seen the transits that others were using that were so crusty and pitted from longtime use in industrial areas without being cleaned and could not understand why some people never cleaned or had their instruments sent to someone for a routine cleaning and lube job.
