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### The Process of BackSighting using Leica or Any Total Station

Please correct me if I'm wrong.

Taking backsight observation is a process of aligning your horizontal circle. This is to make sure of the coordinate system consistency in the entire project.

I guess that there is a unique case that you don't need to perform backsight observation for as long as you're very sure of backsight point information. Setting up fresh a total station is having its horizontal circle not correctly oriented. Horizontal angle difference or angle correction can be derived between correct orientation based on stored coordinates and disoriented horizontal circle reading. The correction value can be applied to all subsequent observations. In this case, no backsight observation is practically needed. This means that if a total station supports this, a great deal of work is eliminated.

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#### Replies

• The original reason for a "resection" ,was to come off of two points that you could not occupy  ,the purpose was not to be,orienting ones instrument,it was to compute the coordinates of the point under the instrument. then the process of normal COGO should take place.I would like to know how the Total Stations that is set up over a point can calculate coordinates to a new point ,without having a point are line of direction as a reference .Sure you do not have to Zero the gun,but it must have a Back site, But why would you not want to Zero it,then all these angles,have to be computed in the math from what was in the instrument at the time of the back site,to the angle of the shot point.This make Raw Data look like a Jigsaw Puzzle ,best practice is to Zero the Gun or use line azimuth or True Azimuth on Back site input,This way if problems occur and you also have to rotate or Translate,things will be much easier to solve.Just Imagine a compass ,with no circle readings,where is North,This is what you would have.

How does this Total Station ,know where North, are anything else is located .

No matter what programs you are using or methods , the best solutions is to stick to Basic,these methods have been used and proven down thru the years ever since Surveyors have surveyed.

And that is my opinion ,but every body has one.

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• what you seem to be talking about here is resection. I think Leica have a process with their instruments which I think they call free station. I'm not sure of the purpose of this as they also have different setup techniques available when setting station.

My personal view would be to always take a BS/RO unless line of site control has been destroyed.

record in both faces also, seems that a lot of surveyors have stopped doing this but I find the vertical control is much more robust

• It is not resection or free station I am referring to.

The process of backsighting is mainly a two-folds work: 1. orienting your occupied station, and 2. checking occupied elevation/level. The second item can be done without bothering to know the orientation. The first item is basically orienting the horizontal circle reading of the occupied station with that of the parent station. Of course, locking/unlocking of horizontal circles are a thing of the past. It is now a semi-automated process using total stations. The theoretical basis for this is to zero-in the difference between forward and back azimuths. With total station's onboard capability to automatically calculate values, my opinion is I don't need to orient the occupied station to conform to its parent station. During start-up, a total station provides arbitrary orientation. Comparing that arbitrary orientation with the computed parent-to-child direction/azimuth, correction value can be readily achieved. With that correction value known, I don't need to perform backsighting..

This is just a humble opinion.

• Good Luck with that.....I must be missing what you are trying to say here but no BS, no orientation.....You have to orientate yourself at some point or each station will be on a pivot point. At set up if you select 'known backsight' the bearing of the line will appear.....you must sight your BS at this point and record it or you can't proceed. If you record this point without aiming at the BS your orientation will be out. You don't have to record a new distance and bearing but you must hit 'set station' or the instrument won't let you proceed. At that point aiming at the wrong thing would be bad.....I would also suggest recording all BS information again as a mean.

I have just read your post again.....Are you assuming that as the instrument knows the BS bearing that by recording the reading on any orientation the instrument will automatically set the BS to the correct bearing?

That would certainly be interesting but I'm pretty sure that's not how case. At the very least you have to sight the BS and hit 'set orientation'

as I said good luck with that. Try it tomorrow and let us all know how you got on.

Leica 'when it has to be right'

• Government Professional

Remember that at least two points are needed to conect an observation from one station to another, so it doesn´t matter if one of those points is a backsite or not, if you have two points in common you are done.

The best option (least error option) would be: one point (actual station) and the other (backsite) the last station. In this case the two points were stations and each one was observed directly from the other.

• Some data collector software calls the non-backsight method as a "resection", where you set the instrument in an erroneous place and sight a minimum of two points with known coordinates, it just triangulates the instruments coordinates based on the Side-Angle-Side method triangulation ... and the strongest resection is placing the instrument as close to the vertex of a 90 degree angle to the two points being sighted. This method is sloppy in my opinion and can cause a lot of trouble that setting up and sighting between two points eliminates, but often saves time.

It does also orient vertically, but this is just as error-filled. And I don't trust the vertical accuracy any more than 400' on a cool day with no obstructions or heat waves.

You can also set up and sight between two points without using "0" for your backsight, you just have to calculate each angle and makes things tougher.
• it also aligns vertically, i don't see how much work this would really save? and would introduce another level of error?
• If the only purpose of backsighting is to orient the horizontal circle and be able to check the elevation of the occupied station then I guess it is theoretically workable for as long as you're very sure of backsight point information. You're also right that this method promotes blunders in work.
The amount of work to save on not performing backsight is very evident.

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