ISO 12858-3:2005规定了大地测量中使用的三臂的最重要要求，即仪器主体与其底座的连接。 然而，ISO 12858-3:2005中的要求并不保证仪器主体与不同制造商的三臂完全互换，而是给出了夹具系统的详细规格，以确保可靠的安装和夹紧，而不影响其性能和用途。目前使用的三叉枝主要有两种类型，在ISO 12858-3:2005中分类为W型和Z型。 ISO 12858-3:2005适用于用于水准仪、经纬仪、航向仪、GPS设备、EDM仪器以及与目标、反射器、天线等组合的三臂。

ISO 12858-3:2005 specifies the most important requirements of tribrachs used in geodesy for the connection of the instrument's body with its base. The requirements in ISO 12858-3:2005, however, do not guarantee the full interchangeability of the instrument's body with tribrachs of different manufacturers, but give the detailed specifications of the clamp system to ensure the reliable mounting and clamping without prejudicing their performance and their usefulness. There are mainly two types of tribrachs in use, categorized in ISO 12858-3:2005 as Types W and Z. ISO 12858-3:2005 is applicable to tribrachs which are used for levels, theodolites, tacheometers, GPS equipment, EDM instruments and in combination with targets, reflectors, antennae etc.