Week 3 Flashcards
(23 cards)
What is levelling in land surveying?
Levelling is the procedure used for determining or comparing the difference in heights between points on the earth’s surface.
What is another name for the levelling process?
Transferring Heights
How are heights typically given in levelling?
Heights are normally given relative to a plane or datum.
What is the UK datum for heights?
The datum for heights in the UK is defined at mean sea level in Newlyn, Cornwall.
What are elevations in the context of levelling?
Heights relative to the UK datum are referred to as elevations.
What equipment is needed for a levelling survey?
A level and a levelling staff.
What does a level do in levelling surveys?
A level is a device that provides a truly horizontal line.
What is the purpose of a levelling staff?
A levelling staff is used for reading vertical heights.
What is the length of a typical levelling staff?
A typical levelling staff is 5 meters long when extended.
What is the first step in setting up a level?
Set up the tripod with the levelling head approximately level and securely attach the instrument.
How do you center the small bulls eye bubble when setting up a level?
Use the three foot screws to center the bulls eye bubble.
What is the Line of Collimation?
The Line of Collimation is the imaginary line that passes through the crosshair of the eyepiece and the optical center of the objective.
What is parallax in the context of levelling?
Parallax is the displacement or difference in the apparent position of an object viewed along two different lines of sight.
What is a Change Point in levelling?
A Change Point is where two readings are taken on the staff: a foresight (F/S) followed by a backsight (B/S) after moving the level.
What should be ensured about a Change Point?
It should be on a hard surface that does not allow the staff to move vertically between readings.
What is the procedure when the distance between benchmark and point is greater than 50 m?
Set up the instrument, take a backsight at a known point, move the levelling staff, and take intermediate sights if required.
What is the purpose of the two-peg test?
The two-peg test checks whether the line of collimation of a level is horizontal or if it needs adjustment.
What are gross errors in levelling surveys?
Gross errors include user errors such as wrong readings, wrong bookings, and not taking two readings at a Change Point.
What are systematic errors in levelling surveys?
Systematic errors arise from improper levelling of the instrument, malfunction, parallax, or incorrect staff extension.
What are random errors in levelling surveys?
Random errors can be caused by factors like wind, incorrect reading of bookings, or vertical movement of the staff.
How often should the two-peg test be carried out?
The two-peg test should be carried out regularly, e.g., once per week or two, depending on the frequency of level use.
What are the acceptable error limits for levelling according to Uren and Price?
1 mm per 20 m.
What are the acceptable error limits for levelling according to Wimpey?
4 mm per 50 m.
