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Hie fellow surveyors. I have faced another challenge again and need your advice. I don't know if anyone here has used the same machine, I am using an Optron Focus robotic total station for setting out on a construction site. My challenge is with fixing benchmarks(heights). After a resection using my control points i get an RMS of 1mm to 3mm for the Z values. From that same setup I have fixed benchmarks for levels on site. If i use a dumpy level to check these benchmarks using the main control points, the benchmarks fixed used the total station are out by varying values of up to 40mm. This is a problem now since the tolerance is only 2mm for the levels. I have now resorted to using a dumpy level rather to give benchmarks. I have tried to check the instrument settings but everything looks alright. I only adjusted the earth Curvature and Refraction Coefficient to zero so that there is no adjustment for the effects of the earth curvature and refraction to the zenith angle. Still to no avail. The interface of an Optron Focus is just the same as that for trimble

Please assist.

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  • Hi Killivin,Good to here from you,There will come a time, were a resection is your friend, we have all faced finding two or more points, that a rod was the only access or you have to even set a new point on line with that point at a O/S dist to it. A fence or any other reason,this happens. I do not like to add O/S to a shot , that,s your decision , mine i do not do it,to a traverse or side shots, setting this, i then have a new point. If this is tying some object,like a fence corner etc.. , that is fine. But a resection can be your friend, when you need it.Try this around your control points or your work point that you are occupying  set you a TBM on some concrete or whatever after you did your back site TS , shoot this TBM, and record it, This lets you know other than a back site check your elevations are good , plus it's recorded in your notes, that you checked into this TBM, before you took any other shots, just try it.

  • Land Surveyor
    Hie Fellow Surveyors

    Thank very much for all your ideas. I can't disagree with you regarding sound surveying practices. I had actually resorted to setting up on a known point since I was having a problem with the resection since it's solution is not always consistent depending with the resection angles when setting up. Setting up on a known point has proved to bring out heights more in agreement with the automatic level.
  • Billy,
    Couldn't have said it better myself. You are exactly right of course. It should be understood that sound surveying practice & procedures should always be used. I have never used resection for a vertical solution but have used it many times for establishing horizontal coordinates.
    • Kudos, to you Charlie, and kudos to Michael too and  Killivien, you are wise in asking, you have been giving and showed a lot of good advice and History, so soak it all in.

  • Hi Everyone, Charlie and Michael,Both of you make Very good points, and good advice, specially the Resection Thing. and the use of a level and the D and R. Too many times they are use for the wrong reason. This program was not put there as a go to solution to just set up.  The original reason for a "resection" ,was to come off of two or three 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, too many times i have seen this done. A resection has a closure just like a traverse, some of these program will fit the points as best fit, especially the Elevation off both points or three point resection Data. This was not designed  as a fast solution to your every day survey it was made to be use with inaccessible points. This is not a short cut,but a tool to be used to solve coordinates under the  Gun. I prefer to do mine manual with  a calculator, then i get to choose,what i thing is the best points solutions,it does take a little more time, but i decide what coordinates are under my gun.

  • The only question I have is if you have established the elevation of your benchmark, why are you using a resection program to establish the height of your instrument? The precision of a solution obtained by resection is dependent on the strength of the solution & is often less than a direct observation by a good instrument in proper adjustment. This is not to say that it is not a good practice to check your results against other marks. Two or three mm is likely the mathematical calculation of the probable error of data feed into your instrument. You can increase the strength of the solution using established mathematical & surveying principles. There may not be anything wrong with your instrument.
    I don't know what type of site you are working on or the requirements for the precision of measurements but 2 mm is about as close as you are going to get to a true orthometric height using GPS or most anything else. Of course, relative elevations on a construction site can be better than that. How do the elevations obtained with the total station compare with those obtained with your level?
    • From my experience those tolerances pertain to the finished work withing itself, in other words you can be 1cm off the primary benchmark, but in the fixed works you are building it should be tighter.

      This would normally in the US be 3rd order class C unless specified, which allows I believe 0.04' in a mile run about a 1cm /KM +/- .  

      Proceedure: your best bet is to set up in the middle of the benchmark and control point, I would use the same rod if possible to eliminate HI error, and use a autosighting instrument such as a robotic or motorized unit, Do this early before heat waves affect your sighting.

      Read foresight and backsight in direct mode, and write down

      read "    "    "   in reverse mode, check the differences and if not too much mean them.

      you can also just read the foresight D&R then the backsight D&R or vice versa.

      if the D&R are very difference you need to calibrate the instument, but nevertheless meaning should take out the error.  

      Do the same thing for any other TBM's you might have set.

  • Hi Killivien,In that collection i saw some instruments made in South Africa.I have also used Dumpy's that were as big as 2 feet in length  and very powerful magnification ,but these are very heavy ,but great for crossing wide Rivers, we always called them Gap levels,  as in a wide Gap, they were good for building a long  Bridge over something,  glad you enjoyed .

  • Hi Killivien, I thought i would send you a picture of a old Dumpy Level. To my surprise when i did a search on line, What came up was not a Dumpy Level that i was familiar with, And this search also showed Dumpy levels for sell in South Africa, What i saw was not a Dumpy at all, instead it was a modern automatic level.It seems some parts of the world call all spirit levels Dumpy, This is News to me,Dumpy levels where some of the First levels used in Surveying, they are more like a Telescope with a level on it, most have four leveling screws like a Transit of yesterday.In 1832, English civilengineer WilliamGravatt

    is who invented it . Back then they used a Y level ,this was a big improvement to it

    The term dumpy level endures despite the evolution in design. A dumpy level is an older-style instrument that requires skill to set accurately. The instrument requires to be set level (see spirit level) in each quadrant to ensure it is accurate through a full 360° traverse. Some dumpy levels will have a bubble level intrinsic to their design which ensures an accurate level. So if you are using a Auto  Level,  i now understand why you called it Dumpy.Also there are some that are still made and other that are still in use.They are also called builders levels, that is why i told you that carpenters mainly used them,but here is a pic. of a old Dumpy level,do not under estimate there accuracy of those made in the 1940 thru 1960 , these will surprise you how well they work.

    http://www.sage.unsw.edu.au/currentstudents/ug/projects/f_pall/html...

    • Land Surveyor

      Hie Billy

      Woow!!! I did check it out. It was some level, 4screws and 3 bubbles for levelling the instrument. Guess the term dumpy endures because it appears the one I referred to as a dumpy is in fact an automatic level, and seems like everyone generally calls it a dumpy.

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