Jun3-12, 08:06 AM
I am trying to understand the closing error of a Level Survey in civil engineering.
I have carried out a survey of a highway, I started the survey on a Temporary Bench Mark (TBM), I took several readings of point of interest and several change points. I ended the survey back onto the original TBM.
I have read that a simple calculation to check if an arithmetic mistake was made is to use the following equation:-
∑Back sight - ∑Foresight = Last Reduce Level – First Reduce Level
My first question is this:- am I correct in thinking that if the left hand side is equal to the right hand side then NO arithmetic mistake was made. If however one side was not equal to the other then an arithmetic mistake was made when reducing the levels.
My second question is this:- if after reducing the levels and applying the above formula the values for the left and right side was 0 then does this mean that there was no arithmetic error AND there is no misclosure error???
My final question is this:- in my survey the final reading was 150. After which I carried out a series of change points so that I could close back to my TBM. I chose to use the same change points to fly back to my TBM as I did when I began the survey, I did this so that I could compare my readings. But I have noticed that when I compare my change point reduce level values the values differ i.e.
Survey Station Initial RD Level Fly Back Reduce Level Diff
TBM 10000 9999 -1
40 (CP) 9100 9105 5
80 (CP) 9673 9678 5
120 (CP) 10019 10019 0
What I do not understand is if my misclosure is just -1 which is the difference from my start TBM reduce level and my end TBM reduce level then why do we not factor in the difference between the initial change point reduce levels and the fly back change point reduce levels??
Attached my survey data, can anyone help????
Thank you for all your help.
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