Overview
ISO 12858-3:2005 - "Optics and optical instruments - Ancillary devices for geodetic instruments - Part 3: Tribrachs" specifies essential requirements for tribrachs used to connect geodetic instrument bodies to their bases. It defines two main tribrach categories, Type W and Type Z, gives mechanical dimensions and nomenclature examples, and sets performance expectations (mounting/clamping, corrosion protection, levelling features). The standard does not guarantee full interchangeability between different manufacturers’ instrument bodies and tribrachs, but it standardizes the clamp system to ensure reliable mounting and operation.
Key topics and requirements
- Tribrach types: Classification and nomenclature for Type W and Type Z tribrachs with illustrative figures and dimensional guidance.
- Mechanical dimensions: Tables specify centring/pitch diameters, pivot dimensions and other mechanical parameters for reliable interfacing (detailed in the standard).
- Clamp system: Detailed specifications for clamp knobs (Type W) and clamp screws (Type Z) to ensure smooth, damage-free fixation of the instrument pivot or fixing star.
- Connection to tripod: Requires a 5/8 in nut thread in the instrument base plate per ISO 12858-2 (tripods).
- Torsional rigidity: Tribrachs must absorb operational torsion without lasting deformation. The standard describes a practical test method-apply torque and measure residual horizontal displacement.
- Levelling/foot screws: Foot-screw mechanisms must allow smooth, fine levelling operations.
- Circular level: Provision for a visible circular level on the tribrach is allowed and recommended for easy levelling checks.
- Corrosion resistance: Materials and surface treatments must resist corrosion.
- Marking: Minimum marking includes manufacturer name or trademark.
Applications and users
ISO 12858-3:2005 applies to tribrachs used with:
- Levels, theodolites, tacheometers, EDM instruments
- GPS/GNSS receivers, reflectors, targets and antennae
Primary users:
- Survey instrument manufacturers and designers
- Surveyors and geodetic technicians specifying compatible mounts
- Procurement and quality-assurance teams in surveying and construction
- Calibration laboratories and test houses verifying tribrach performance
Related standards
- ISO 12858-2:1999 - Tripods (interfaces such as 5/8 in thread)
- ISO 12858-1 - Invar levelling staffs (part of the same series)
- ISO 9849 - Geodetic and surveying instruments - Vocabulary
- ISO 2768-1 - General tolerances for dimensions
Keywords: ISO 12858-3:2005, tribrachs, geodetic instruments, Type W tribrach, Type Z tribrach, surveying tribrach standard, torsional rigidity, tripod interface, clamp screw, levelling.