Method 2: Cowley Automatic Level
When transferring levels over very long distances, the use of a spirit level and straight-edge can be very time-consuming and prone to error.
The Cowley Automatic Level (or just ‘Cowley’ as it is commonly known) is a simple and, by modern standards, somewhat old-fashioned type of levelling instrument. Despite not having the same sort of complexity as modern optical and laser levels, it is readily available to be hired for simple and/or one-off levelling tasks and remains a very effective levelling tool. Its accuracy is limited to around 30m in one direction but it can be rotated through 360 degrees, making it effectively capable of transferring levels between pegs that are 60m apart, when it is placed in the middle. This range should be more than adequate for most applications and projects.
Fig. 57 Transferring levels with a spirit level and straight-edge.
The equipment consists of three main components (see Fig
Fig. 58 Cowley Automatic Level, tripod, staff and target.
2. A tripod with a metal spike on top on to which the Cowley level is placed.
3. A graduated staff with an adjustable target that slides up and down the staff. The target is usually 450mm long and 50mm wide and is painted in a fluorescent orange, red or yellow paint so as to be easily visible.
The Cowley level is set up on the tripod and adjustments are made to the tripod legs to level
up the instrument by eye – it does not have to be perfectly aligned in order to do its job. Fig 59 illustrates the general operating procedure for a Cowley level, together with the potential results that will be seen through the eyepiece when looking at the target.
Note: the action of placing the head unit on the spike releases an internal locking mechanism that allows the mirrors to move freely. This is why the level must never be carried around when attached to the tripod as this can damage the internal mirror and prism system.
Once the tripod legs have been adjusted, the staff can be positioned on the site datum peg, or any peg whose level is to be transferred to other points on the site. Rotate the Cowley level until the target is in view through the eyepiece – it will appear as a spilt image of its two halves side by side. Slide the target up or down the staff until the two halves of the target coincide exactly with one another. Lock off the target with the screw at the back and then check that it is correct and has not moved whilst being locked off. The Cowley level and the target are now in the same horizontal plane.
The instrument can now be rotated through 360 degrees to transfer the level to other pegs established at any point within its range. The position of the target on the staff remains fixed and each new peg is driven into the ground to the required depth so that the view through the eyepiece reads level at every new peg.
Fig. 59 Cowley level operating procedure for transferring levels.
The most important sequence is to set-out, check and re-check before building work commences. It is vital to be absolutely sure that the dimensions are accurate and the location of the structure is correct, and it is not possible to check the accuracy of setting-out too many times! If setting-out work has to be left for an extended period, such as overnight, it should be checked again in the morning to ensure it has not been disturbed. Dimensions and diagonals should be checked every time ranging lines are removed and re-attached.
Successful setting-out cannot be carried out by one person. It requires at least two people but it is preferable to have three, due to the number of processes that have to be brought together at the same time.