Free-Stationing 


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RichVanDam
Posts: 11

Joined: 1/9/2009
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Sloppy, where are you from? What are you studying? I'm very intrested to find out which usniversity has Trimble S6's! I actually worked with a agency engineer who had one, amazing piece of kit! Very intricate and highly accurate. The robotic features are very advanced!
I am very interested in this topic as i have spent over a year investigating it, how has this come about? please tell more as there are a lot of companies interested in the results i have obtained.
Send me a e-mail at richarddale_uk@hotmail.com and i'll send you well explained theory into free-station compared with conventional setting up including how the programs work.


I think its down to you the individual on what tolerances you are working to but i wasn't getting anymore than 1mm in the results i had undertaken from Free-Stationing. I was using Prism's set up over control stations however and taking the utmost of care which probably isn't usual whilst undertaking a major civil engineering project for instance. I.e. pogo stick / faster approach.

The only error i ended up getting whilst performing my studies was like i stated earlier in the zenith.
I'll elaborate a bit on the procedure:- I Actually set the total station over a control station - measured the height to the control station the backsighted two control stations - My theory being - What redudancy would this produce; as the total station should theoretically derive the actual co-ordinates of the control station it was set up over?. Whilst this was true in the Eastings and Northings, it was the Zenith that was out by 10 - 20mm. I did abandon this however becasue i wasn't quite sure where this error came from as my previous results did not highlight anything of this nature.

Yet, after speaking in depth with people on this board and as can be shown in the above i deduced that it was not down to the program / total station and later attibuted it potentially to taping errors measured from the control stations.


Are you asking the question how are total stations computes residuals?

Because the answer is as you stated - By equal adjustment - The more common approach.
Trimble S6 (you are one lucky student to have one of those i may add) can use Least Squared regression to calculate the most likely co-ordinates of each station.
You are in affect performing a "traverse" that is then adjusted in both cases i may add.
Just remember to watch the residuals after each free-station and also, sight a third station to check that the co-orindates match.

Have you played around with AutoCAD 3D Civil?
If you are looking to compare techniques of traverse adjustment based on free-station i.e. least squares regression, equal adjustment you can do so in there. I had a go with it last year but it kept giving out poor results, apprently its been all but sorted nowl not hard to get your head round either!
If your a student you can download a student edition for free also!


Richard Dale
  Tuesday, May 12, 2009 at 5:16:32 PM
sloppy
Posts: 8

Joined: 5/12/2009
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I am topping up from my Fdeng Civils to BSc in Kent.

I have used a Trimble not in Uni though. I have not come across any text books that offer a good clear, concise explanation into the theory of resectioning / free stationing. I have Bannister and Baker's Solving problems in surveying, but it just flies over it not offering me the detail I wan't. (Bearing in mind I am no genius when it comes to mathematics). I also have the added problem that I canot just take certain rules for granted, I like to prove them! lol.

As I have been studying Civils, my surveying studies are pretty basic. I appreciate least squares is an extensive area with lots of calculations but would just like to prove a 2 and 3 point resection using leats squares and comparing it to software calcs. Also I would like to compare setting out from a pillar by resectioning and by bearing orientation method.

I am sure you have satisfied your problem regarding zenith now, though I believe your error would be down to a combination of taping, angular and distance error in the machine.
 Say you had +-3",+-3mm EDM and then the added fact that the control stations were put in with a regular level. You would also have very small corrections to your distance on the EDM wave because of temperature and curvature of the earth.

I will drop you an e-mail, maybe between us we can come up with an answer that satisfies us both.


Regards
S
  Thursday, May 14, 2009 at 6:22:18 AM
Amberftw
Posts: 11

Joined: 1/2/2009
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Hello,
I am amased at this thread. You guys talk about some in depth calculations. I am also a student, but in Land surveying. I never thought of it but I guess I am lucky. We have 2 trimble S-6 that we work with. Anyway, I wanted to get a better idea of what you guys are talking about when it comes to the resection. I have done a resection in class with a calculator and thats it. I haven't done it out in the feild  yet. I see you talking as though you do all the resection calcs in the instrument itself? And what is least squares? Mathmatically I can take two know points and calc the the third, I just use a calculator.Am I just not at the level of you guys yet?
Amber
  Sunday, May 31, 2009 at 11:47:14 AM
sloppy
Posts: 8

Joined: 5/12/2009
Reply  Re: Free-Stationing Flag »  Reply »
http://books.google.co.uk/books?id=aLhW8Eq2W2sC&lpg=PA43&ots=If9jnhjGo6&dq=resection%20surveying&pg=PA46


page 46. I am struggling with deriving the formula eq.1.51. Can someone help me understand it?

Regards

S
  Tuesday, July 28, 2009 at 10:14:20 AM
sloppy
Posts: 8

Joined: 5/12/2009
Reply  Re: Free-Stationing Flag »  Reply »
Amber,

Sorry for the late reply. I am studying civils and this has only cropped up as I need to understand it for my own sanity. I am going to use it for my dissertation.

Regards

S
  Tuesday, July 28, 2009 at 10:15:24 AM
Djerem
Posts: 1

Joined: 3/22/2012
Reply  Re: Free-Stationing Flag »  Reply »
Hi,

You put zero on elevation when free stationning because the machine calculates directly the point at the center of the TS. When you measure then data using these coordinates, the survey points are so calculated with Hi=0.

Rgds,
  Thursday, March 22, 2012 at 5:23:41 AM
BenR
Posts: 2

Joined: 7/13/2012
Reply  Re: Free-Stationing Flag »  Reply »

Howdy

The total station will calc it's position, but if you are doing topo surveys and lots of resections, it can be a good idea to measure the instrument height.

Depending on how you take your data, you may end up with the instrument setups scattered through out your points list, and if you have lots of points, you may miss one or two and then you will end up points that are about 1640mm (for me) too high. 
Not a massive issue, but it is just one more step in the data processing that can be removed by simply measuring the instrument height.

To see what errors your 2 point resestion produces, set up over a known point and then do a two point resection, compare the instrument position to what it should be. I did this yesterday, just to add to my frustration. Ofcourse it helps a lot if your control is good.

Generally, for resections with more then 2 points, I'll take errors of less then 10mm, confirmed with a check shot. This seems to be industry standard on most mine/wharf/pipeline sites I've worked on.

  Friday, July 13, 2012 at 6:17:32 PM
MapTack
Posts: 9

Joined: 10/11/2012
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Dear Richvandam,

part 1

It might be helpful to reduce the process of free-stationing to its constituent elements - the practical, the geometric & the mathematical:=

The Practical:
The repeatability of the target/instrument is essential. Setting up an instrument and observing a prism pole is not ideal. Reflective targets fixed to permanent structures are great. Trig pillars of concrete with mount-on targets will also fit the bill. Tripods with attached tribrachs and suitable prisms are good provided that the target axis height is properly determined. If tripods are to be reset at a different time then accurate millimetre positioning of the targets, xyz is necessary (repeatability). If a prism pole is used then best to use a bipod for stability. Point the pond bubble on the pole towards the instrument and observe. Point the pond bubble away from the instrument (turn 180 deg) and observe again. Any bubble error will be cancelled by reversal and estimation of the bubble centring will be somewhat improved. Calibrate the rod for heights (worn point, unmatched prism target &c). Make sure your prism constants are OK._

To determine the co-ords of the targets set up the instrument and take rounds of angles on both faces & with different zeros with multiple distances from a well defined local reference object of known or assumed orientation. Determine the cross-hair elevation by setting the theo to 90 deg and read a strutted levelling rod on a known or assumed datum point. Alternatively measure the distance to the levelling rod and read a series of vertical angles to various points on the rod and back compute the axis elevation of theo. From this axis elevation compute the target elevations and positions of the “known” stations using the angles and distances.  This results in accurate “known” target positions from which to free-station.

If you set a station point under the theo measure the axis height with a tape measure or better still a centring rod. If you level on to the ground stn with a level and rod from the datum point you will be able to compare your ability to measure the height of the instrument above the station with the tape measure. If the target stations are not accurately surveyed then your free-stn will be flawed to some greater or lesser extent.

I presume that surveyors take a 0.000 axis height because of the relatively inaccurate measurement of the instrument above the stn by tape measure & as further heighting is computed from the elevation determined for the collimation then an inst height is not necessary unless establishing a station point under the instrument. It actually doesn’t matter in a sense as the poorly measured tape determination gives an eccentricity to the station elevation whereas the axis height from free stationing gives an accurate instrument elevation. If the reduced elevation of the free-station ground point is important then accurate measurement of the theo axis above the ground stn is obviously required.

The Geometrical:
As with all surveying techniques poor geometry will dilute or spoil the process. Acute or oblique angles are to be avoided. Free-stationing as with triangulation or traversing will suffer from bad conditioning. For this reason as many target stations as possible should be located throughout 360 deg  as is practical for re-locating by this method. Targets may become obscured by site equipment, spoil heaps &c. during the course of construction for instance.



MapTack
  Friday, October 12, 2012 at 9:56:14 AM
MapTack
Posts: 9

Joined: 10/11/2012
Reply  Re: Free-Stationing Flag »  Reply »
Dear Richvandam,

part 2.

The Mathematics:
The least squares solution has been described by Isleno & without doubt redundancy is essential to have confidence in the results. The program may permit the user to ascribe weights to the angles & distances. When the best fit for the free-station has been computed it is of comfort to the surveyor to back compute the elements i.e. compare the join distances from the computed position with the distances measured in the field and derive the dD (delta horizontal dist.)- compare the dH’s (elevation deltas) for each station from the mean instrument axis elevation and the field measured height difference with the given Z value for each target station & compare the horizontal angles from the derived x,y of the resected station, the computed Hz angles from the x.y of the targets/free-stn and the field measured Hz angle. This gives the surveyor a feeling for the goodness of the elements that he measures in “field“ terms.

A more useful function for comparison of field measured Hz angles is to determine the unknown mean orientation element of the theo. Since the bearing to each target has been calculated and the field measured target horizontal angles measured clockwise from the zero graduation of the theo., then it is possible to determine a number of bearing values from grid north of the zero graduation of the theo. The spread of the various values from the mean gives an excellent appreciation of the strength of orientation. The mean value may be used to calculate the instrument angle to be set to a point to be fixed in space(stake-out) or to be surveyed.

Having gone through all this it may seem somewhat cumbersome but free-stationing is very fast and particularly flexible and especially comes into its own in awkward situations in quarries, opencast works and congested sites but requires setting up the system to suit in the first instance and having the appropriate bits and pieces for the job.

Hope the foregoing is of some help!

MapTack
  Friday, October 12, 2012 at 12:03:49 PM
MapTack
Posts: 9

Joined: 10/11/2012
Reply  Re: Free-Stationing Flag »  Reply »
Part 3.…

Re: Trimble Total Station.

I have not used the new models Trimble T/S for Free-Stationing but I have used the Geodimeter 605 & the Trimble 5605 Dr-200+ for this application with an external data collector.

The 5 arc second instrument is capable of accuracy beyond its classification principally because of the dual-axis compensator, the circle being scanned throughout its full 360 deg. (rather than being sampled at discrete points) & a very useful D-Bar function. The D-Bar function takes a continuous mean of a series of distances, vertical & horizontal angles.

The inst. presents these means to the RS-232 port as a single-face distance, vertical & horizontal angle on demand (press record on the data collector). In addition if required the inst will double face the angles, presenting these also as a mean single-face polar observation. In practice the user waits until the mean values of D, VA & Hz stabilise & any incoming variations do not influence the individual mean of any of the respective measured values. External influences - shimmer or wind principally contributing to variations.

When utilised in this manner the free-station co-ords X,Y,Z come in to 0 or 1 millimetre each time. If the inst is in good adjustment a single face delivers the identical accuracy. Thus a free-station fix is achieved in minutes with the minimum of human interference.

The draw back in the instrument is that the D-Bar mean values are lost if the lock on the target is lost during a target acquisition.

MapTack
  Saturday, October 13, 2012 at 9:04:10 AM
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