DIVISION OF NATIONAL MAPPING

FIRST ORDER LEVELLING

INSTRUCTIONS AND SPECIFICATIONS

Accuracy

The two levellings of each section between permanent bench marks shall not differ by more than 4√K mm (0.004√K metres) where K is the distance in kilometres between bench marks measured along the levelling route.

Allowable discrepancies between backward and forward runs of first order leveling are shown in the attached table for distances of from 0.1 to 20.0 kilometres.

Collimation

A collimation test shall be carried out each day before levelling commences.

The collimation error of the level should not exceed 1.0 mm over a distance of 50 metres.

This interval of 1.0 mm is equal to 0.02 staff units (i.e. 2.0 units on the micrometer drum).

Length of Sights

The length of any levelling sight should be such as to permit the certain reading of the staff and shall never exceed 40 metres. At 40 metres the stadia interval on the staff is 8.0 staff units.

The lengths of foresights and backslghts shall be measured by stadia and as far as practicable shall be of equal length. The total length of backsights should in no case differ from the total length of foresights between bench marks by more than 15.0 metres. 15 metres is represented by a staff interval of 3.0 staff units.

Each stadia wire need only be read to the nearest 0.1 of a staff unit (i.e. to the nearest small graduation on the staff).

Readings

Observation and recordings of the centre wire should be made to the nearest small graduated division of the micrometer (i.e. 0.00005 metres).

Observations and recordings of the stadia readings should be made to the nearest small graduated division of the staff.

The sequence of readings shall be:

Backsight stadia

Backsight left face

Foresight left face

Foresight stadia

Foresight right face

Backsight right face

The difference between the readings of the left and right faces of the staff should fall within the range of 60.645 to 60.655 staff units.

Temperature and Refraction

The temperature of the air shall be recorded together with the time of reading at the commencement of each new page of observations.

All sight lines shall clear the intervening ground between the level and the staff by at least 0.30 metres (i.e. 6.0 staff units).

Staff readings of less than 6.0 staff units will not be accepted.

Conversion of Staff Readings to Metres

Staff interval in metres        =    Staff reading / 20

Distance in metres              =    Stadia interval / 0.20

Units of Measurement

Staff Unit = 5 centimetres   = 0.05 metres

Smallest staff graduation    = 5 mm  = 0.005 metres

1.0 micrometer unit     = 0.5 mm =  0.0005 metres

0.1 micrometer units (smallest micrometer graduation) = 0.05 mm    = 0.00005 metres

Use of Micrometer

The last movement of the micrometer drum to bring the cross hairs of the level onto the staff graduation should be in a clockwise direction.

Change points

Two types of change point mark will be provided. The three pronged round plate should only be used on firm ground where there is no grass (e.g. consolidated road shoulders, concrete, gravel roads). This plate should not be used on soft ground or in grass or on bitumen surfaces. The tapered pin is for use in soft ground and on grassed surfaces. It should be driven into the ground, using the "dolly" provided, until the flat surface at the top of the pin is resting firmly on the ground.

Care should be taken to ensure that the spherical surface on which the staff is placed is not damaged when placing or driving the change points.

The staff base guide plate should be used when holding the staff on either the change point plate or pin but this guide plate must be removed when holding on a bench mark.