Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie

Total Station Accuracy

Options
  • 03-06-2011 1:57pm
    #1
    robbiefat
    Closed Accounts Posts: 21 ✭


    Can a surveyor answer this question.
    How accurate is setting out from a resection(free-station) compared to the conventional method?
    The total station uses a program called free-stationing whereby it utilises the sine rule and depending on the total station model least squres also, which is open to very small errors. Are these errors significant?
    Any input would be appreciated.


Comments

  • Options
    #2
    REFLINE1
    Registered Users Posts: 377 ✭✭REFLINE1


    robbiefat wrote: »
    Can a surveyor answer this question.
    How accurate is setting out from a resection(free-station) compared to the conventional method?
    The total station uses a program called free-stationing whereby it utilises the sine rule and depending on the total station model least squres also, which is open to very small errors. Are these errors significant?
    Any input would be appreciated.

    It depends whast you are trying to do?
    Set out a building? Long distance traverse?
    What errors are u reading? whats the angle between u & the 2 stns?
    Distance between the two stns relative to the furthest point ur setting out? etc etc-more info plz !


  • Options
    #3
    Max Moment
    Registered Users Posts: 252 ✭✭Max Moment


    IMO once there is a good angle between the two stations being used for the re-section, and the errors obtained are very small i.e 1 or 2mm at most for each station, this is as accurate as setting up the conventional way. The more stations you use for the re-section the more accurate the set up will be.

    But as Refline asked, what is the distance you are setting out relative to the stations i.e if you get a large error from the re-section, and you are setting out a good distance away, the error will be multiplied leading to inaccuracies.


  • Options
    #4
    robbiefat
    Closed Accounts Posts: 21 ✭robbiefat


    Max Moment wrote: »
    IMO once there is a good angle between the two stations being used for the re-section, and the errors obtained are very small i.e 1 or 2mm at most for each station, this is as accurate as setting up the conventional way. The more stations you use for the re-section the more accurate the set up will be.

    But as Refline asked, what is the distance you are setting out relative to the stations i.e if you get a large error from the re-section, and you are setting out a good distance away, the error will be multiplied leading to inaccuracies.

    Thanks to both of you for answering, it was a question i have been meaning to ask for a long time not specific to a particular job. I wonder if you may know why some surveyors input the instrument height as 0 when using the re-section?


  • Options
    #5
    Eurocodus
    Registered Users Posts: 21 ✭Eurocodus


    If you are setting out/ surveying with regard to E, N, Z the total station height and prism height should be imputted.
    If you are setting out with elevation controled by rotating lasers that is to say E, N then total station and prism height are not required.


  • Options
    #6
    Max Moment
    Registered Users Posts: 252 ✭✭Max Moment


    robbiefat wrote: »
    Thanks to both of you for answering, it was a question i have been meaning to ask for a long time not specific to a particular job. I wonder if you may know why some surveyors input the instrument height as 0 when using the re-section?


    I always input the instrument height, hi (i.e height of instrument above station) as zero for a free-station as you are trying to determine the co-ords of where the actual instrument is at from two or more known points(not from a known station under it for a normal set up).

    For example, if you entered hi as equal to +1.000m for a re-section instead of zero, you would be telling the instrument that you want to compute the co-ordiantes of a station 1m below the central axis of the instrument. You would still get the correct E & N so you need to be careful.

    Hope this helps


  • Advertisement
  • Options
    #7
    boreds11
    Closed Accounts Posts: 14 ✭boreds11


    Whats the longest backsight you would do for accurate setting out say conc frame or steel and to what accuracy should you get?

    Also do people do traverse adjustments and face left face right with either manual bowditch or least square in autocad when setting up the primary control points on big building projects? or is the practise in the real world just set new stations and survey them?


  • Options
    #8
    robbiefat
    Closed Accounts Posts: 21 ✭robbiefat


    Thanks Max Moment very helpful.
    Correct me if im wrong, from what i gather it is best to enter the instrument height as 0 because the elevation will be incorrect if the measured instrument height is inputted. What if i want to create a station under the instrument could it have an incorrect elevation?
    Thanks


  • Options
    #9
    Max Moment
    Registered Users Posts: 252 ✭✭Max Moment


    boreds11 wrote: »
    Whats the longest backsight you would do for accurate setting out say conc frame or steel and to what accuracy should you get?

    Also do people do traverse adjustments and face left face right with either manual bowditch or least square in autocad when setting up the primary control points on big building projects? or is the practise in the real world just set new stations and survey them?

    I generally try to keep the backsights as far away from the works as possible but would set up the instrument as close to where I was setting out (IMO maximum of 50m but It generally depends on how well you can see the targets).

    Personally I just set new stations. I carry at least three stations when surveying in a new one to minimise errors. Might just be a bad habit though!


  • Options
    #10
    Max Moment
    Registered Users Posts: 252 ✭✭Max Moment


    robbiefat wrote: »
    Thanks Max Moment very helpful.
    Correct me if im wrong, from what i gather it is best to enter the instrument height as 0 because the elevation will be incorrect if the measured instrument height is inputted. What if i want to create a station under the instrument could it have an incorrect elevation?
    Thanks

    You have me thinking on this one now Robbie - I know on the Leica model I use if I carry out a free-station directly over a new station and entered the instrument height at that point instead of zero, the free-station calculated will give the height of the newly surveyed station and not the instrument. If zero was entered, the free-station will give the height of the instrument.

    I'm not sure if this is specific to this model though?

    The best way to check this is to set up a free stations separately using the two instrument heights (one being zero) and then survey or stake out a control point and see if the correct level of the control point is measured.


  • Options
    #11
    robbiefat
    Closed Accounts Posts: 21 ✭robbiefat


    That sounds like the best way of finding the solution. Thanks once again Max Moment


  • Advertisement
  • Options
    #12
    kc20010101
    Registered Users Posts: 3 ✭kc20010101


    Hi lads,

    Its been a long time since I used a total station but need to help out a friend if i can. Just want to make sure of a few things. I have a Topcon 226. Basically I need to survey a simple site, set up control points and be able to set up again at a later date to set out.

    I am trying to remember but havent had a chance to try it out yet. If I set up in a random spot and call that the occ pt. say 100,100, 0. when i import that to CAD and have fixed points such as corner of an existing house of other fixed points, the co-ordinates shown on cad could be used as station co-ordinates. Would that be correct??

    Also the process for re section and occ pt for setting out would be great. Cant find a step by guide anywhere.

    Thanks in Advance for any replies.


  • Options
    #13
    Dr_Bill
    Registered Users Posts: 295 ✭✭Dr_Bill


    robbiefat wrote: »
    Can a surveyor answer this question.
    How accurate is setting out from a resection(free-station) compared to the conventional method?
    The total station uses a program called free-stationing whereby it utilises the sine rule and depending on the total station model least squres also, which is open to very small errors. Are these errors significant?
    Any input would be appreciated.

    The quality of the resection will depend ultimately depend on the quality of the original control points. Any errors in the original control will be transferred into the resected point.

    When coordinating a point using a resection, two points will give you a coordinate X,Y,Z with no check, three or more points used in the resection will give you redundancy and what you should ideally be aiming for provided site conditions allow of course.

    Depending on the software the position will be determined by least squares. As a rule the backsight should always be longer than any point to be set out. For example if your angular error is 20" at 100m and your only setting out to 50m the error is 10" and hence may not be significant. Conversely if used a 20m backsight and set out at 100m with a 10" error then your error will increase with distance so it is never desirable to use a short backsight.

    Typically a 100m-150m would be fine for setting out RC frame or longer if available and site conditions allow. So long as follow the rule above you won't go too far wrong.

    In terms of the differences between say setting up over a known point, measuring the backsight and turning an angle to a free point, from experience maybe 1-2mm provided all the equipment is within adjustment and in good order.

    The key to a decent resection is decent balanced geometry with the points well spaced from different directions.


Advertisement