Levelling
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Levelling

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its not complete, but i hope this can help u ^^

its not complete, but i hope this can help u ^^

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Levelling Levelling Document Transcript

  • Introduction& methodology The task of this exercise was to carry out a looped levelling survey using the rise and fall booking method around the CSM building. Our group consisted of Christopher Crabb, Alex Levantis and Robin Watson. The weather conditions were overcast with a fairly pervasive mist throughout. We chose the corner of a drain cover closest to the campus buildings to act as our first Backsight(BS) and last Foresight (FS), the level we determined for this point witha handheld GPS was 124m AOD. From here we proceeded to take one Intermediate Sight (IS) for each setup of the level, before taking a FS onto our first Change Point (CP) where the staff remained whilst the instrument was moved to its next setup position. The difference between each sight and the previous IS or BS was then worked out and applied to the RL level of the last point as either a rise or fall.This was repeated for 11 different set-ups. As can be seen on Fig 1., where practicable we kept our sights to the magnitude of 20-25m to reduce collimation error, with an IS on a memorable location between these two CPs. Each of us took turns to read the level, book the results and hold the staff. Results Fig 1 showing the location of our survey stations, change points and intermediate sights.
  • Point Name 1 Back Site 2 BS (M) IS (M) Corner of wall ESI Building White wall below 3 camera Surveyors Mark 4 "JLI" by Mast Survey mark "TI" 5 by disabled Above 'FH' on 6 yellow drain Lampost side by 7 triangle key Left corner of 8 drain Corner of lower 9 curb by reception "00" in 2008 on 10 plaque Base of double 11 lampost 12 Drain by bike rail Drain Kite Mark, 13 by designated North corner of 14 drain Drain SE shipping 15 container 16 Rise (M) Fall (M) Reduced Level (M) 1.856 Remark 124.000 BM 1.551 0.045 0.305 1.611 0.893 1.999 1.625 0.602 115.585 CP 2.018 2.435 113.567 IS 0.095 113.472 CP 1.550 0.659 0.402 1.926 116.948 CP 119.294 IS 0.055 119.349 CP 3.978 0.714 115.022 IS 2.346 0.509 1.969 116.473 IS 0.888 2.715 4.380 116.099 CP 0.374 2.620 3.060 116.990 IS 0.891 2.513 3.985 119.460 CP 2.470 3.874 120.326 IS 0.866 2.983 121.572 CP 1.246 3.005 121.926 IS 0.354 2.139 124.245 CP 2.319 2.718 0.513 124.305 IS 0.060 2.364 2nd pipe west of car registration sign 17 Curb Corner Drain by Nitrogen 18 Tank Driain by 19 Nitrogen Base of private 20 property sign Return to initial 21 BS Sum of the recordings: FS (M) 123.327 IS 0.624 1.687 0.222 123.105 CP 0.609 123.996 FS 19.306 Σ BS - Σ FS = Σ Rises – Σ Falls = Last initial RL – First RL 123.387 IS 1.078 19.302 0.282 11.425 11.429 0.004 -0.004 -0.004 0.004 Fig 2.Booking sheet with arithmetic checks. On completing our survey we originally calculated our final error to be 0.051m, on examining the booking sheet, it was noticed that one fall (point 3) had been treated as a rise and this was responsible for 0.047m of theerror. The booking sheet shown at Fig 2.is the revised version. Our final error was then -0.004m; well within the allowable misclosure where m=5mm m√11=0.0165… Likely sources oferror include wind buffeting the staff,especially at its fullest extension.The mist certainly made sighting onto the staff difficult at certain points. Changing the position of the staff on the CP is another possible source of error. Use of sights of unequal lengthsalong steep slopes may have exacerbated any collimation error present in the instrument,other calibration issues may also have affected our results. Reading errors, or inconsistencies in the method of estimating to the nearest 1mm between the three people taking those readings may also contribute; booking errors would certainly have caused an error outside the allowable misclosure (0.051m) had this not been identified during our arithmetic checks (Uren & Price, 2010: 43, 54-55).
  • Contouring exercise The next part of the assignment involved the recording of a series of levels in a rough grid across a slop. The position of these spot heights was then to be worked out using the Stadia Tachometry method. For each point a bearing was recorded, along with the upper and lower stadia readings, and a reading from the central crosshair. The difference between these points multiplied by 100 gave a horizontal distance to the point being levelled. A=100d, where d = the difference between the upper and lower stadia. These coordinates and their heights are shown in Fig 3. Our coordinates were then calculated with the sin/cosine rule and plotted in fig 3. ΔN =cosθ°A, ΔE = sinθ°A where θ=the bearing of the point and A=the distance to the point calculated by the stadia tachometry method above; Results Fig 3.Location of spot heights with Point 19’s adjusted position in red.
  • POI Point Name Back sight taken from 'FH' on yellow drain levelling point 1 BS (M) IS (M) 1.801 FS (M) HPC (M) Angle (Deg) Angle (RAD) Upper Stadia Lower Stadia Distance Easting Northing 3 4 33 0.576 1.895 1.706 18.900 10.2936778 15.8508737 1.953 120.129 103 1.798 1.978 1.926 5.200 5.06672434 -1.1697455 120.210 91 1.588 1.897 1.847 5.000 4.99923848 1.813 2 120.281 1.872 First row (near CSM) 122.082 Level (M) 120.269 79 1.379 1.838 1.787 5.100 5.00629864 0.97312588 -0.087262 1.756 120.326 68 1.187 1.783 1.729 5.400 5.00679281 5 First row (ending at tree) 1.721 120.361 59 1.030 1.749 1.693 5.600 4.80013688 2.88421322 6 Second row (at Tree) 1.876 120.206 65 1.134 1.908 1.844 6.400 5.80036984 2.70475688 1.974 120.108 74 1.292 2.004 1.943 6.100 5.86369635 1.68138787 2.014 120.068 84 1.466 2.043 1.983 6.000 5.96713137 0.62717078 2.064 120.018 92 1.606 2.093 2.034 5.900 5.89640588 2.183 119.899 101 1.763 2.214 2.153 6.100 5.98792582 -1.1639349 2.413 119.669 100 1.745 2.448 2.378 7.000 6.89365427 -1.2155372 2.359 119.723 91 1.588 2.394 2.324 7.000 6.99893387 -0.1221668 2.339 119.743 81 1.414 2.377 2.301 7.600 7.50643139 1.18890193 2.318 119.764 72 1.257 2.357 2.280 7.700 7.32313518 2.37943086 2.269 119.813 64 1.117 2.310 2.227 8.300 7.45999058 3.63848052 2.535 119.547 70 1.222 2.579 2.491 8.800 8.26929506 3.00977726 2.718 119.364 77 1.344 2.763 2.675 8.800 8.57445657 1.97956928 2.859 119.223 84 1.466 2.904 2.816 8.800 8.75179268 0.91985048 2.938 119.144 91 1.588 2.993 2.892 10.100 10.0984617 -0.1762693 2.996 119.086 100 1.745 3.040 2.951 7 8 9 10 Second row (ending at CSM) 11 Third Row (starting at CSM) 12 13 14 15 Third Row (ending at tree) 16 Fourth Row (starting at CSM) 17 18 19 20 Fourth row (ending at Tree) 8.900 2.0228756 -0.205907 8.764789 -1.5454688 Fig 4. Stadia Tachometry booking sheet with outlying result highlighted. Note conversion to radians for use in Excel. An obvious outlier in the results appears to be due to a reading/booking error, as the IS does not lie in the middle of the upper and lower stadia readings as it should do. The IS for point 19 should be equidistant between the upper and lower stadia; 2.9425 rather than 2.938 as booked. Lower Stadia = 2.892 IS = 2.938 Upper Stadia = 2.993 Lower Stadia – IS = -0.044, and IS – Upper Stadia = -0.055 We would expect these two differences to be equal. If we adjust the upper stadia by the difference between these two differences and recalculate the coordinates, then we can now mark the point at 9m from the level, 8.998629256 -0.157071658. 2.993 – 0.011 = 2.982 so 100(2.982 – 2.892) = 9m so cos91x9 = -0.157071658 and sin91x9 = 8.998629256
  • As shown in Fig 3.this point is far closer to the actual progression of points across the slope. Of course the exact booking error cannot be known, but it seems reasonable that an extra 0.010m was added, probably by reading the wrong 0.010m block on the E type staff used. Alternative levelling methods An electronic laser level would have eliminated our booking error of 0.047m and any other potential errors from booking or reading the staff, laser levels can also be used accurately over greater distances (up to 100m). Poor light or weather would affect the ability of the electronic level to sight on the barcoded staff but these conditions affect all methods of levelling. Perhaps the only significant disadvantage of the electronic level is the increase in price compared to the advantages it affords; competent surveyors with a systematic approach to reading and booking can complete a survey in a comparable period of time with an automatic level (Uren and Price, 2010: 41). Levels can be calculated trigonometrically using a theodolite. Trigonometric levelling can be prone to error as gravity is not used as a control element; sighting and reading of vertical angles must be absolutely precise or errors in calculation become exacerbated as the slope distance increases (Uren and Price 2010, p175). Another limitation is that the horizontal distance between the survey station and the point being levelled must be known or calculated, again increasing the possibility for errors due to inaccurate sighting. A survey of this kind involves far more calculation than direct levelling with an automatic level, with the corresponding time required and increased chances of booking error. However, the trigonometric method is useful (and indeed may be the only possible method) in circumstances where the points to be levelled are inaccessible, such as mountaintops, or overheard power cables. Levelling with an EDM or total station is entails using the same calculations involved with trigonometric levelling, only carried out automatically by the instrument itself. The EDM allows this method to be used where the distance to a point is not known by accurately measuring the slope distance. The same errors resulting from inaccurate sighting still remain however, the instrument must be sighted exactly in the centre of the prism, and it is advised that numerous readings be taken and any anomalies discarded, thus adding to the length of the survey. Total stations are far more sensitive than automatic levels and atmospheric conditions require that the instrument be given a chance to acclimatise to the ambient temperature, and low and high levels of light or ground shimmer can require mathematical adjustments to be made to the readings . Levelling with a total station is an effective way of obtaining heights for a topographical or walkover survey (Uren & Price 2010: 174). Alternative methods for a contouring survey could involve carrying out a detail survey with other methods (i.e., total station, GPS or even chain survey), then simply adding spot heights with an automatic level and the height of collimation method. Indeed this is the most common method within the archaeological profession and one I have some experience of. As can be seen there are advantages to using survey equipment other than an automatic dumpy level, but these all have the potential for errors and a greater complexity than the direct levelling method. An automatic level is economical, simple and robust where other methods may be complicated, delicate or expensive. Bibliography Uren, J and Price, B. 2010, Surveying for Engineers, Fifth Edition, Palgrave Macmillan, Basingstoke.