BTEB Amin Survey Exam common mistakes
BTEB Amin Survey Exam · Understand the traps and how to avoid them.
Recording chainage in the field book as a running total that is written only after the tape is lifted, so the last measurement of each chain length is silently dropped or double-counted.
Candidates treat the field book as a fair copy rather than a legal record and reconstruct entries from memory at the end of the line; the habit comes from classroom exercises where the tape never actually moves.
What to do instead
Enter every measurement at the instant it is taken, in ink, directly in the field book, and cross-check the sum of the offsets and chainages against the total chain length before leaving the station. Adopt the discipline that nothing is written twice and nothing is erased.
Booking whole-circle bearings without reducing them to the correct quadrant, so a line reported as 310 degrees is later plotted as 50 degrees or vice versa.
The candidate has memorised the quadrant rules as text but never practised converting observed bearings to reduced bearings under time pressure, and the arithmetic sign errors are invisible on the booking sheet.
What to do instead
Reduce every bearing immediately after booking, writing both the whole-circle bearing and the reduced bearing with its quadrant letter side by side, and verify that the reduced bearing never exceeds 90 degrees.
Accepting a closed traverse whose angular misclosure exceeds the permissible limit and distributing the error anyway, instead of re-observing the suspect angles.
Candidates want to finish the computation and fear that re-observation means repeating the whole set; they also confuse the permissible misclosure for a theodolite with that for a compass.
What to do instead
Compute the misclosure before any adjustment and compare it with the stated permissible value for the instrument and number of angles. If it fails, identify the angle most likely in error by re-observing that single station, then adjust only after the misclosure is within limit.
Applying the two-peg test result by adjusting the collimation error at the eyepiece end rather than at the diaphragm, or skipping the test entirely and assuming the level is in adjustment.
The procedure is learned as a sequence of steps without understanding that the diaphragm is the only user-adjustable element; candidates also assume a hired instrument is already adjusted.
What to do instead
Perform the two-peg test at the start of every levelling exercise, record the two readings and the computed error, and correct at the diaphragm screws by the calculated amount, then repeat the test to confirm the error is removed.
Ignoring curvature and refraction when reducing long sights, or applying the combined correction with the wrong sign, so benchmark elevations drift by tens of millimetres over a long line.
The correction is small per sight and easy to forget, and the sign convention (curvature subtracts, refraction adds, combined correction subtracts) is memorised without derivation.
What to do instead
Keep sight lengths balanced and short wherever possible, and when a long sight is unavoidable compute the combined correction as 0.0673 times the square of the distance in kilometres, applied as a subtraction from the staff reading, and show the correction as a separate column in the reduction table.
Orienting the plane table by back sighting to a single well-defined point and then treating the orientation as fixed for the whole day, so later rays are rotated relative to earlier ones.
Candidates confuse orientation with centring and believe one back sight at the start of the survey is sufficient; the table is also nudged when sheets are changed or the tripod is moved.
What to do instead
Re-check orientation by back sighting at every new setup and, where a third known point is available, confirm by the two-point or three-point problem before plotting any rays; clamp the table and record the check in the field notes.
Computing area by the average ordinate rule when the offsets are unevenly spaced, or mixing trapezoidal and Simpson's rule within one chain of offsets.
Candidates match a rule to the shape of the formula rather than to the spacing of the offsets, and Simpson's rule is applied to an odd number of ordinates without noticing the requirement for an even number of equal intervals.
What to do instead
Inspect the offset spacing first: use the trapezoidal rule for any spacing, Simpson's rule only for an even number of equal intervals, and the mid-ordinate rule only when offsets are at regular intervals. State the rule used and the number of intervals beside the computation.
Taking cross-section levels at irregular intervals and then applying the mean area or prismoidal formula as if the intervals were equal, producing a volume that cannot be checked.
The formula is applied mechanically without checking its precondition, and the candidate does not keep a sketch of the cross-sections to see where the ground changes abruptly.
What to do instead
Fix a constant interval along the centre line before levelling, take extra sections only at breaks in the ground and treat them as separate segments, and compute each segment with the formula appropriate to its own equal spacing, tabulating area, interval and volume per segment.
Setting out a right angle or a building corner by eye or by a single tape triangle without checking the diagonal, so the structure is out of square by the time the foundations are marked.
Candidates trust the 3-4-5 triangle but mis-chain one side, and they omit the closing check because the corner looks correct on the ground.
What to do instead
Set out with the 3-4-5 method or a theodolite, then always close the figure by measuring both diagonals of the rectangle; the diagonals must be equal within the specified tolerance before any peg is accepted.
Submitting field notes and a plan with no date, no instrument identification, no benchmark reference and no signature, and with the reduced levels traceable to no known datum.
Candidates treat the drawing as the deliverable and the notes as scrap paper, and they assume the marker knows which benchmark was used.
What to do instead
Adopt a standard title block and field-book header recording project, date, observer, instrument number, weather, benchmark and its assumed or known reduced level, and sign and date every sheet before leaving site; state the datum on the plan itself.