Understanding Face Left and Face Right Angles

The terms "Face Left" and "Face Right" originally refered to which side of the telescope the vertical circle of a theodolite lay when the telescope was reversed by rotating it 180?ø about its tilting axis. It the vertical circle was on the left side of the telescope from the observer's point of view at the eyepiece, that was the Face Left position. An alternate terminology is "Face I" and "Face 2".

The purpose of measuring an angle on each of those two different telescope positions or faces was to cancel certain errors resulting from the fact that the tilting axis may not have been perfectly perpendicular to the turning axis of the instrument and that the line of sight of the telescope (its "axis of collimation" was not exactly perpendicular to the tilt axis.

Any good surveying textbook will discuss errors of these types and how they are canceled in how observations are taken.

See Kent posted at same time. Some duplicate info
The saying goes back to the origins of Theodolites.
It has nothing to do really with the direction you are reading. Anticlockwise or clockwise.
Setup, sight backsight on 'face left', site foresight. Flip telescope and rotate theodolite (total station) site foresight again (you are now on face right) then resight backsight.
Record each reading.
Some sight back and fore, then flip telescope and resight back then sight fore again.
Backsight 0?ø0'0ƒ?? foresight 156?ø25'36ƒ??
Flip and reread foresight 336?ø25'12ƒ??
Re read backsight 179?ø59'36ƒ??
Take mean face left and face right (included angles) and theoretically that's your true included angle. (those are a bit sloppy and good instrument should be far better)
In past with old instruments we'd stack up readings. Observe back then fore, then hold that reading and release bottom clamp and set that to backsight, read foresight etc.
That uses different parts of the circle to supposedly take out instrument errors due to poor circles.

I pinched the following of a site.
(i) Face Left: When the Vertical Circle of a Theod is situated on the left of the observer and the observation or reading taken with this face is called Face Left Observation or Reading.(ii) Face Right: When the Vertical Circle of a Theod is situated on the right of the observer and the observation or reading taken with this face is called Face Right Observation or Reading

Richard and Kent explained FL and FR pretty well. I always called it direct and reverse (D & R), same thing as FL and FR.

The other thing you mention is angles right and angles left. When I used to use an optical theodolite (i.e. manually read circles) to run control, i would turn half of my sets (for example, 8 sets for second order class I) as angle right (BS-FS-flop-FS-BS). Then I would do the other half of the sets as angle left (FS-BS-flop-BS-FS). The average of each of these should sum to 360 within a few seconds. It was just another way to avoid blunders in reading and recording.

John Hamilton, post: 430343, member: 640 wrote: Richard and Kent explained FL and FR pretty well. I always called it direct and reverse (D & R), same thing as FL and FR.

The other thing you mention is angles right and angles left. When I used to use an optical theodolite (i.e. manually read circles) to run control, i would turn half of my sets (for example, 8 sets for second order class I) as angle right (BS-FS-flop-FS-BS). Then I would do the other half of the sets as angle left (FS-BS-flop-BS-FS). The average of each of these should sum to 360 within a few seconds. It was just another way to avoid blunders in reading and recording.

Isn't 'angle right' increasing to right? And an 'angle left' increasing to the left? regardless of BS. I've never seen a theodolite that read 'angle left'.

Switching BS to FS is 'closing the circle', still angle right.
(Don't want to confuse the new guy.)

Yes, it is closing the horizon, and of course the circle reads only one direction. But we called it AL and AR because we were measuring the angle in a different sense (FS first, then BS). So maybe it is not the clearest terminology. Once I converted the "AL" back to BS-FS by subtracting from 360, it was an "angle left" from the BS.

Larry Scott, post: 430346, member: 8766 wrote: I've never seen a theodolite that read 'angle left'.

Maybe I'm misremembering -- it's been about 37 years -- but doesn't the T16 read angles both left and right?

Edit: I guess the ones we had were T16-ED models. From the http://www.wild-heerbrugg.com/theodolites2.htm#T16&apos ;">Virtual Archive of Wild Heerbrugg:

The T16-ED has the extra facility of a horizontal circle with double numbering (360?ø circle only), allowing angles to read or set out either to the left ("North to West") or, in the normal fashion, to the right ("North to East").

One also, as in many discussions across continents, have to account for local lingo.
I'm equally at home with F1, F2 or FL F2.
Also the language of theodolites, jigger, guns not to mentioned terminology for observing principles and so it goes.
We talk about the same things, just the language sounds strange.

Richard-I have always referred to a total station or an optical theodolite as a "gun". Causes some weird reactions sometimes when non-surveyors hear you talking about using a gun and shooting some distances.

About terminology...I had a non-surveyor one time who was running a GPS receiver for me tell the office people that we were out "turning corners".

Military instruments usually have double etching. Orient the magnetic instument to the direction of fire. Sight the scope of another instrument (usually a gun site) and read the angle left. The gun puts that number in his site and shifts the whole cannon until he is sighting the magnetic instrument.
Repeat the process for the remaining cannons and voila. Everyone can hit the same target...

Jim Frame, post: 430350, member: 10 wrote: Maybe I'm misremembering -- it's been about 37 years -- but doesn't the T16 read angles both left and right?

Edit: I guess the ones we had were T16-ED models. From the http://www.wild-heerbrugg.com/theodolites2.htm#T16&apos ;">Virtual Archive of Wild Heerbrugg:

I was about to mention the T-16 before reading this. Never used it for production but did have a chance to use it in school in 1987. So 30-years for me.

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Steward Souten, post: 430334, member: 12714 wrote: Hello all

I am trying to understand face left and face right observations using a total station.

Many Thanks
SS

https://www.pearsonhighered.com/program/Ghilani-Elementary-Surveying-14th-Edition/PGM91929.html

rfc, post: 430367, member: 8882 wrote: https://www.pearsonhighered.com/program/Ghilani-Elementary-Surveying-14th-Edition/PGM91929.html

just got this book thanks