Bihar Special Survey Amin common mistakes
Bihar Special Survey Amin · Understand the traps and how to avoid them.
When reducing a level field book, subtracting the backsight from the foresight (or adding where subtraction is required) so that reduced levels drift away from the benchmark, and only discovering it at the arithmetic check.
Candidates memorise 'BS minus FS' as a slogan but do not internalise that Rise = later staff reading minus earlier staff reading and Fall is the reverse, so they lose track of which reading came first in the column and apply the rule mechanically.
What to do instead
Before touching numbers, write the height-of-instrument chain on rough paper: HI = RL + BS, then RL = HI − FS or HI − IS for every subsequent reading. Practise ten full reductions from a benchmark, and after each one verify both the arithmetic check (ΣBS − ΣFS = last RL − first RL) and that no reduced level is absurdly above or below the benchmark.
Confusing whole-circle bearing (WCB) with reduced bearing (RB) when converting between the two, especially in the second and third quadrants, and then computing included angles from the wrong values.
The quadrant formulas (RB = 180° − WCB in SE, WCB = 180° + RB in SW, etc.) look similar, and candidates who have only rote-learned the table swap the direction of conversion under exam pressure.
What to do instead
Sketch the four quadrants with north at the top every single time and mark the line before converting. Drill the rule as: from WCB to RB, measure the acute angle from the nearer meridian (N or S); from RB to WCB, add the quadrant offset (0°, 180°, 180°, 360°) in NE, SE, SW, NW respectively. Check that the answer lies in the quadrant your sketch shows.
Using the wrong formula for local attraction correction - applying the 'interior angles' method to a compass traverse that should be corrected by the 'included angles' method, or accepting a bearing as correct merely because the needle reading looks plausible.
Both methods appear in the same chapter, and candidates rarely practise identifying which one the question demands; they also forget that local attraction is detected by comparing fore and back bearings of the same line.
What to do instead
First compute the difference between FB and FB of the reverse direction; if it is exactly 180°, that line is free from local attraction and its station is your reference. Correct every other bearing by the included-angle method station by station, and finish by checking the sum of interior angles against (2n − 4) × 90°.
Mis-stating the principle of plane table surveying as 'the table is levelled and oriented once at the first station', and therefore selecting the wrong method (radiation, intersection, traversing, resection) for a described field situation.
The three conditions - the table must be levelled, centred over the station, and oriented - are learned as a list without the crucial point that orientation must be maintained at every subsequent station, and the method names are memorised without their geometry.
What to do instead
Learn each method by the number of instrument stations and the data it yields: radiation for one station with all points visible; intersection for a point that cannot be occupied; traversing for a connected framework; resection (three-point problem) to fix the instrument station itself. For each, state which of the three conditions is being exploited.
Treating the theodolite's face-left and face-right readings as interchangeable, so that the mean of the two is taken without checking that the difference is close to 180°, and then reporting a horizontal angle that still contains collimation error.
Candidates focus on the final angle and skip the preliminary check that the instrument is in adjustment; they also forget that the mean of FL and FR eliminates collimation error only if the two readings genuinely differ by 180°.
What to do instead
Always record FL and FR separately. Compute FR − FL; if it departs from 180° by more than about 20″, note the collimation error and rely on the mean. Take the mean as (FL + (FR ± 180°))/2 and state it explicitly in the working.
Computing the area of an irregular plot from a chain survey by applying the trapezoidal or Simpson's rule to offsets that are not equally spaced, or by using the average-ordinate rule where the offsets include a curved boundary.
The three area rules are memorised as formulas without their preconditions, so candidates pick whichever formula comes to mind rather than the one the offset spacing permits.
What to do instead
First list the offsets and their chainages and check spacing. Equal spacing with an even number of intervals: Simpson's rule (odd number of ordinates). Equal spacing otherwise: trapezoidal. Unequal spacing: sum of individual trapezoids between consecutive offsets. State the spacing check as a line of working before substituting.
In cadastral questions, mixing up khasra, khata and plot numbers, or describing the Bihar land-record terminology of a survey map without distinguishing the field (khasra) number from the khata (holding) number.
The terms come from revenue practice rather than textbook theory, and candidates prepare them from memory lists without a mental picture of how a khasra number identifies a parcel while a khata number identifies an owner's holding across parcels.
What to do instead
Build a one-page glossary in your own words: khasra/plot number identifies the individual parcel on the map; khata number identifies the cultivator's holding and may group several khasra numbers; the jamabandi/record-of-rights links the two to the raiyat. Test yourself by drawing a small village map with three khasras under one khata and describing it aloud.
Reading a chain or tape measurement to the wrong precision, or recording the chainage of an offset from the wrong end of the chain, so that plotted points fall on the wrong side of the survey line.
Candidates concentrate on the offset length and treat the chainage as an afterthought, and they do not fix the convention that chainage is measured from the starting station of the line, not from the nearer end.
What to do instead
Adopt a fixed recording habit: station name, chainage from the start of the line, offset length, and side (left or right in the direction of progress). Add the two offset legs to check the cross-check rule (the sum of the two offsets from a point on the line equals the chainage difference when the point lies between them).
Losing marks on Bihar-specific general awareness by answering from generic Indian geography, for example giving a river or a division name that belongs to another state, or confusing a Bihar administrative division with a district.
The Bihar syllabus rewards local detail, but candidates revise only national-level facts and assume that any well-known Ganga tributary or any district name will do.
What to do instead
Prepare a compact Bihar sheet organised by division (Patna, Tirhut, Saran, Darbhanga, Kosi, Purnia, Bhagalpur, Magadh, Munger) with the districts under each, the principal rivers with their north/south bank position, and the state's boundaries. Verify each entry against a current official source before committing it to memory.
In reasoning questions on direction sense and seating arrangement, drawing no diagram and trying to hold the arrangement in the head, then answering the second or third sub-question from a stale mental picture.
Candidates treat reasoning as a speed section and skip the ten seconds of sketching, which works for the first inference but collapses when the question adds a condition.
What to do instead
Draw a compass rose and place every person or move on paper before reading the options. For seating, fix the facing direction at the top of the page and mark left and right accordingly. Re-read the final condition and update the diagram before selecting the answer.