Levelling Procedures

a)  Setting up:

•  Backsight and foresight distances should be approximately equal to avoid any errors due to collimation, refraction or earth curvature.

•  Distances must not be so great as to not be able to read the graduations accurately.

•  The points to be observed must be below the level of the instrument, but not lower than the height of the staff.

(b)  Elimination of parallax:

Parallax is the apparent movement of the image produced by movement of the observer's eye at the eyepiece. It is eliminated by focusing the telescope on infinity and then adjusting the eyepiece until the cross-hairs appear in sharp focus. The setting will remain constant for a particular observer's eye.

(c)  Booking:

•  level books or loose-leaf levelling sheets shall be numbered and indexed in a register.

•   details of the site, work, date, observer, chainman, booker, weather, wind, instrument and any other relevant items shall be entered.

•  enter the first observation (which is on a known point) in the Backsight column, and sufficient detail in the Remarks column to identify it. Enter the point's R.L. zero from the site register or plate on the BM, etc.

•   enter all other points on subsequent lines as intermediates except the point chosen as the foresight. Identify them in the Remarks column as above. Enter the foresight on a further line in the Foresight column.

•   change the instrument to the next setup. Enter the following backsight on the same line as the previous foresight but in the Backsight column.

•   repeat the above procedure at each setup on the outward run then reverse it to work back to the starting point on the return run. The furthest point out is treated as for all other change points.

Reducing the levels:

Two methods are in general use; the "rise and fall" method and the "height of collimation" method. The latter reduces levels relative to the instrument height. As it has inferior in-built checks it should not be used and will not be covered here. The "rise and fall" methods shall be used for reduction of all site levelling. Reduction shall be carried out on site before packing up to ensure that the levelling has been done correctly.

• calculate the rises and fall between successive points and book them in the appropriate column (one can determine whether each shot is a rise or fall by the following rule of thumb: a higher value on top denotes a rise; a higher value on the bottom denotes a fall)

•  add up the backsight and foresight columns for the entire traverse and note the difference between them; this is the close

•  add up the rises and falls for the entire traverse, and compare the difference between them with the difference between the backsights and foresights; they should be the same.

•  carry the reduced levels in the R.L. column down the page by adding or subtracting the appropriate rise and fall values to the successive values of R.L. The final value of the original starting point will differ from the original value by the amount of the close.

If the levelling has been done correctly and all arithmetic reductions are correct, the differences between total backsights and foresights, total rises and falls, and starting and finishing R.L.'s should be the same. This difference is the close; and for site inspection purposes it should be within ± 2mm or ± 6mm, depending upon which water-level standard is being followed, ± 3mm or ± 10mm.

Level books
All levelling shall be booked in either level books or levelling sheets which shall be retained as permanent records. Level books shall be numbered so that they can be referenced on station history and inspection forms. They should be stored in fire-proof storage as for original record. They should also include an index. Levelling sheets shall be filed in time-sequential order in site files, and also need to be in fire-proof storage as for level books.