Equipment
The level, its tripod, the staff and the staff bubble are all precision items of equipment upon which the accuracy of the work is highly dependent. They shall be kept correctly calibrated, and be used and stored with care.
Levels shall be carried in vehicles in a padded box, case or shelf in addition to the normal case, and staves shall be kept in a canvas or plastic sleeve to prevent damage to the face and entry of dirt.
Levels
A level is basically a telescope attached to an accurate levelling device, set upon a tripod so that it can rotate horizontally through 360° . Normally the levelling device is a bubble, but modern ones incorporate a pendulum. There are three basic types of level, shown in figure 6.1 (from MWD, 1981)and described below:
A level is basically a telescope attached to an accurate levelling device, set upon a tripod so that it can rotate horizontally through 360° . Normally the levelling device is a bubble, but modern ones incorporate a pendulum. There are three basic types of level, shown in figure 6.1 (from MWD, 1981)and described below:
(a) Dumpy levels:
These are more basic levels often used in construction work. The telescope is rigidly attached to a single bubble and the assembly is adjusted either by means of a screwed ball-joint or by footscrews which are adjusted first in one direction, then at 90°.
(b) Tilting levels
This type of level is fitted with a circular bubble for preliminary approximate levelling and a main bubble which is attached to the telescope. For each observation (not setup) the main bubble is viewed through an eyepiece and the telescope tilted by a fine screw to bring the two ends of the bubble into coincidence.
This type of level is fitted with a circular bubble for preliminary approximate levelling and a main bubble which is attached to the telescope. For each observation (not setup) the main bubble is viewed through an eyepiece and the telescope tilted by a fine screw to bring the two ends of the bubble into coincidence.
(c) Automatic levels
This more modern type of level is now in general use. It has a compensator which consists of an arrangement of three prisms. The two outer ones are attached to the barrel of the telescope. The middle prism is suspended by fine wiring and reacts to gravity. The instrument is first levelled approximately with a circular bubble; the compensator will then deviate the line of sight by the amount that the telescope is out of level.
This more modern type of level is now in general use. It has a compensator which consists of an arrangement of three prisms. The two outer ones are attached to the barrel of the telescope. The middle prism is suspended by fine wiring and reacts to gravity. The instrument is first levelled approximately with a circular bubble; the compensator will then deviate the line of sight by the amount that the telescope is out of level.
Staves
The levelling staff is a box section of aluminium or wood, which will extend to 3 or 5 m in height by telescoping, hinging or addition of sections. One face has a graduated scale attached for reading with the cross-hairs of the level telescope. These faces can vary in pattern and graduation; 5mm graduations should be the maximum for accurate levelling of gauging stations. Many staves used these days are of aluminium because of its durability. However aluminium has a co-efficient of thermal expansion of :
The levelling staff is a box section of aluminium or wood, which will extend to 3 or 5 m in height by telescoping, hinging or addition of sections. One face has a graduated scale attached for reading with the cross-hairs of the level telescope. These faces can vary in pattern and graduation; 5mm graduations should be the maximum for accurate levelling of gauging stations. Many staves used these days are of aluminium because of its durability. However aluminium has a co-efficient of thermal expansion of :
0.000023m/metre of length/°C
and this can cause some potential inaccuracies. For instance, "Survey Chief" and "Brookeades" staves are standardised at 27°C, and in very cold weather these staves could be as much as 3mm too short over their full length. For low temperature work consult the temperature table for each staff which should be with its "instruction manual" or printed on the staff itself.
Staff bubbles:
These are generally a small circular bubble on an angle plate which is held against one corner of the staff to ensure that the staff is held in a vertical position. If the staff is not held vertical, the reading will be too large and may be significantly in error. A staff bubble shall be used at all times. If one is not available, the "chainman" (staff operator) shall rock the staff slowly back and forth about the vertical in a line towards the instrument. The observer notes the smallest reading which will occur when the staff is vertical.
These are generally a small circular bubble on an angle plate which is held against one corner of the staff to ensure that the staff is held in a vertical position. If the staff is not held vertical, the reading will be too large and may be significantly in error. A staff bubble shall be used at all times. If one is not available, the "chainman" (staff operator) shall rock the staff slowly back and forth about the vertical in a line towards the instrument. The observer notes the smallest reading which will occur when the staff is vertical.
The three main types of level
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