R3111 Feb ‘19
a) Define the term ‘offsets’ as used in linear surveying; [2 marks]
b) Illustrate, using a diagram, a method for recording offsets in a field book. [5 marks]
c) Define THREE types of reading that are taken from a staff in the rise and fall method of level
surveying using a levelling instrument. [3 marks]
This question relates to the following part of the syllabus:
Outcome 3. Understand a range of basic surveying techniques.
3.1 Describe the linear surveying of a site using appropriate equipment.
Definitions: base lines, trilateration, off- sets, tie lines, running measurements.
Methods used including conventional recording.
Equipment to include: tape measures, pegs, ranging poles, field book, compass.
3.2 Describe the level surveying of a site using appropriate equipment.
Definitions: datum level/benchmark, back sight, intermediate sight, foresight, reduced level,
change point, line of collimation.
Methods used including conventional recording by the rise and fall method.
Equipment to include: optical (Quickset/automatic) level and tripod, staff, booking sheets.
For part a) the examiners’ comments suggest a definition of offsets as ‘Lateral measurements at
right angles to a base line, made in order to plot positions of points on a line or individual objects’.
Whenever you are asked to include a diagram in an answer make sure that it is clear and well
labelled. Also make sure that it is a suitable size. You are given about ⅔ of a blank page for part b), so
make sure your diagram isn’t tiny and difficult to read. It’s a good idea to practice drawing simple
diagrams for topics such as this.
The examiners’ comments state that the following needed to be included for full marks:
- The base line drawn vertically on the page (represented as two parallel lines drawn with a space between for recording measurements). It should be labelled, e.g. A-B, at each end.
- Direction of work is clear, either from the labelling of the base line, e.g. line A-B with A at the bottom of the page, or progressive measurements (between the two drawn lines) from bottom to top.
- Examples of running measurements (e.g. 5.1m, 7.2m. 11.5m)showing distance along baseline where offsets join it.
It is also pointed out that taking measurements from regularly spaced points along the line aren’t
the best way of carrying out a linear survey.
To plot a particular object, it is measured from the object back to the base line. The shortest distance back to the base line will create a right angle. For plotting a curving bed it is important to measure back from the apex of the curve.
A greater number of offsets, or more closely spaced offsets are useful for a very tight curve, whereas
a gentle curve doesn’t need so many for accuracy. This could be shown on your diagram.
The examiners’ comments give three clear definitions for the types of readings for part c). These are:
‘Backsight; first reading after positioning the instrument, intermediate sight, reading between backsight and foresight, and foresight, last reading before moving the instrument’.