# To measure by dual‑prism offset

In a conventional survey, use this measurement method to coordinate a point that cannot be observed directly with a pole in a plumb position.

The use of a tiltable prism with the appropriate nodal offset will give accurate results regardless of the direction of pole tilt. Prisms that should not be tilted (such as the Trimble VX/S Series 360° prism) do not correct the vertical angle and slope distance for the difference between the optical center of the prism and the centerline of the pole.

1. As shown in the following diagram, space two prisms (A and B) apart on the range pole. The distance BC is known.

2. Tap and select Measure and then perform a station setup. See Station setup.
3. Tap and select Measure / Measure topo.
4. Enter the Point name and, if required, the Code.
5. In the Method field, select Dual‑prism offset.

6. Complete the fields as required.

   Enter a suitable Tolerance AB to generate a warning if there is a difference between the keyed‑in distance AB between the two prisms and the measured distance AB between the two prisms. Exceeding the tolerance could indicate that the entered distance AB is incorrect, or it could indicate pole movement between the measurement to prism A and the measurement to prism B.

7. Tap Measure. Take two measurements.

   The software calculates the obscured position (C) and then stores it as a raw HA VA SD observation.

   All raw observations are stored in the job file and are available for export.