Prisms, Tripods & Staff
Survey accessories are the prisms, tribrachs, tripods, poles, staff and ranging rods that carry the instrument and the target. They are routinely treated as an afterthought, yet a worn tribrach or a bent pole introduces more error into a survey than the difference between instrument grades.
Where surveys actually lose accuracy - the cheap parts holding the expensive instrument.
ALSO CALLED: survey accessories · reflector prism · tribrach · levelling staff · ranging rod · prism pole · survey tripod
Typical specifications
Class-level figures. Individual models vary, so treat these as the range to check a quotation against rather than a guarantee.
- Prism constant
- Must be set correctly in the instrument, or every distance is biased
- Tribrach
- Optical or laser plummet; adjustment drift is a common hidden error
- Tripod
- Wooden tripods are more thermally stable; aluminium is lighter
- Prism pole
- Straightness and a working bubble matter more than the brand
- Levelling staff
- Aluminium or fibreglass for site work; invar for precise levelling
- Ranging rods
- Alignment and sighting aids, not measuring devices
How a prisms, tripods & staff works
01
Set the prism constant
Different prisms have different offsets. If the instrument's constant does not match the prism in use, every distance in the job carries the same silent error.
02
Check the plummet
A tribrach plummet drifts out of adjustment with handling. Rotating the tribrach through 180° over a mark reveals it in under a minute.
03
Keep the pole vertical
A pole out of plumb by two degrees at two metres displaces the point by roughly seven centimetres - larger than the instrument's error by an order of magnitude.
04
Look after the staff
A staff with a worn foot, a bent section or a loose lock reads short or long consistently. It is the cheapest item in the kit and the easiest way to spoil a levelling run.
What it is used for
- Instrument and target setup on every survey
- Traverse and control work with forced centring
- Levelling runs and benchmark transfer
- Setting out and alignment
- Teaching correct field practice
Who uses it
- Survey crews
- Contractors
- Engineering colleges
- Local body works departments
How to choose a prisms, tripods & staff
Match the prism to the instrument
Confirm the prism constant and that it can be set on your instrument. Mixed prisms across a crew are a classic source of unexplained distance discrepancies.
Buy the tribrach properly
Forced-centring traverse work needs matched, adjustable tribrachs. This is the accessory where saving money costs the most accuracy.
Tripod material
Wood for stability on precise and hot-weather work, aluminium for weight on long walks. Whichever you pick, check the head is flat and the clamps hold under load.
Staff type versus the work
Fibreglass or aluminium for general levelling; invar only where precise levelling accuracy is genuinely required and a digital level is being used.
What it costs
Accessories are individually inexpensive but add up to a meaningful share of a working kit, and they are the items most often quoted vaguely. Ask for an itemised list rather than 'with standard accessories'. To confirm: accessory price list
What Axisline does with a prisms, tripods & staff
FAQ
Prisms, Tripods & Staff - common questions
Why does the prism constant matter?
The prism constant compensates for the extra path the light travels inside the glass. If the value set in the instrument does not match the prism actually being used, every measured distance carries the same offset - commonly 30 mm - and the error is invisible until it shows up as a closure failure.
How often should a tribrach be checked?
Before any control or traverse work, and after any knock. The check takes a minute: set up over a mark, note the plummet position, rotate the tribrach 180° and look again. If the mark moves, the plummet needs adjustment before the work proceeds.
Does a bent prism pole really affect accuracy?
Substantially. A pole leaning two degrees at two metres height moves the recorded point roughly seven centimetres horizontally - far larger than the total station's own error. A working bubble on a straight pole is one of the cheapest accuracy improvements available on any survey.
Related instruments
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