level fieldwk 2012

SURVEYING

FIELDWORK

LEVELLING

GETTING STARTED

Extend the legs almost fully and place them securely on the ground.

Place the instrument case on the ground and open it.

Look at how the instrument is placed in the case (so you can put it away)

Carry the instrument carefully to the legs and secure it.

SETTING UP THE LEVEL

Centre the circular bubble using the three footscrews

CENTRING THE CIRCULAR BUBBLE

Move two footscrews in opposite directions (Both thumbs IN or OUT)

Move the third footscrew alone

Repeat iteratively

Levelling the Telescope

Centre the CIRCULAR bubble using foot screws at every set up. – (Approximates

horizontal)

Eyepiece

Focussing screw

Tangent screw

Foot screws

Focussing the Cross Hairs

Before commencing readings focus the cross hairs. To do rotate the eyepiece until a sharp image is obtained

STARTING TO READ Place the staff on a BM, and the level away

from it where you can see it and some of the points to be levelled. Centre both bubbles and read the staff.

STAFF READINGS

Once the main bubble is centred read the MIDDLE crosshair.

In some instruments the top and bottom crosshairs are very large.

Readings – 3.60 and 0.98

HOLDING THE STAFF

Place the staff on the mark, and hold it firmly with your fingers clear of the face.

A staff bubble may be used to keep it vertical.

POOR PRACTICE

Do not use the staff to support you.

Do not put your fingers across the front of the staff.

CONTINUING

Start with a BACKSITE to the BM then take INTEMEDIATE SIGHTS to as many marks as possible.

When you need to move the level to advance the work, a point must be nominated as a CHANGE POINT and a FORESIGHT taken to it.

SECOND INSTRUMENT SET UP

Move to the second instrument position making sure that you can see the CHANGE POINT.

Set up and level circular bubble. Read a BACKSIGHT to the CHANGE

POINT then continue with more Intermediate or Fore sights as required

FINISHING or “CLOSING” the RUN

Read to all points needed. The sequence is not critical – they can be done in any order.

To check the field work you MUST FINISH on a KNOWN POINT.

A second BM is ideal or RETURN TO THE STARTING POINT.

Either of these will CLOSE THE LEVEL RUN.

THIS is ESSENTIAL to Check your FIELDWORK

FIELDWORK CHECK

Sum the Backsight Column Sum the Foresight Column Calculate the difference between them

(ΔHfield) Calculate the difference between the R.L.s

of the points you stared and finished on (ΔHtrue)

If your work was perfect (ΔHfield) would equal (ΔHtrue)

If they are NOT equal, this discrepancy is called the MISCLOSE.

MISCLOSE Measurements in the real world are

never perfect so the misclose will have an allowed size.

Factors involved in creating the misclose– Length (number) of instrument set

ups – Precision required for work in hand

Formulae available to calculate the ALLOWABLE MISCLOSE.

If misclose is too big then large (or too many small) errors present so do the levelling again.