Wall Stations

Calibrating the Total Station

The total station is a critical component to any survey traverse. It must be in good calibration with its angle measurements, its ATR (automatic target recognition) and its EDM (electronic distance measurer).
The angles and ATR can easily be checked by observing to a target on both faces and observing the difference between angles from each face. Any large difference should be adjusted out by running the calibration routine built into the instruments software.
The edm should be checked by measuring a distance or several distances over a known baseline or between two known stations. Comparing the distance measured to the actual will give the measured distance error if any. The distance offset is rarely out but can be adjusted in an instrument, this must be done by a qualified Leica service technician. Be sure to measure the temperature and pressure and apply these values to correct any ppm errors.
Underground mines usually have constant temperatures so a generic temperature can be set. If temperatures vary throughout a mine then measure these and understand what differences these variations can make to your day to day surveys and any resurveys.

Prism and Stem

prism The prism and stem should be in good visual condition, with the prism free of chips, scratches or any marks that could obscure sighting and measuring. The stem should be straight and connected securely into the prism. A fast and simple check of alignment is to sight and measure to the prism while positioned in a wall station sleeve, and then rotate the prism 180 degrees on both axis and repeat the measurement. If you a measure a difference then further inspection is required click here.

Tribrach

The tribrach is often overlooked as a source of error when it comes to traversing, but it is usually the most likely offender, if you are seeing poor results in horizontal angles turned.
It is critical that the tribrach is in good adjustment and it should be serviced regularly. It is also common practice to send an instrument in for its annual service and just send the tribrach that is attached to the instrument, but not the rest of the target set.
If a tribrach is in poor adjustment it can directly effect the accuracy of angles turned. A tribrach that has worn foot screws or a loose fitting bottom plate can create a small rotation that is not measured by the instrument sitting in it. This rotation will create errors.
A simple test to check your tribrach, is to hold it by the top and bottom sections in each hand and try and rotate the tribrach about itself. A well adjusted tribrach will be solid or will have a slight rotation that will flex straight back. A poorly adjusted tribrach will have slop between these two parts. Remember you only require a tinny amount of movement to create errors of tens of seconds in an angle turned.

Tripod

As with the tribrach above the next source of error is usually the tripod. The tripod needs to be in good adjustment. This is something that all surveyors can do themselves.
Tripods require regular maintenance and should always be kept and only used when in good condition.
A tripod when used in the mining environment should be considered a disposable item and replaced regularly. They cost only a few hundred dollars and carry a precision instrument worth tens of thousands of dollars.
To adjust a tripod simply tension up the allen key bolts. These bolts hold the folding (hinge) mechanism that allows the tripod legs to fold in and out. Tighten these bolts to create a small amount of friction against the leg movement.