Changing the Prism Constant in X-PAD

Edited: 20200528_DL - Updated, screenshots added, different prisms added

Created: 20190912_PH

Important Background:

Prism Constant = Prism Offset = Addition Constant

Why we have to deal with it:

The distance from the total station telescope to the prism is measured by the time that the laser beam takes to go from the telescope to the prism and back again. This is the ‘time of flight’.

Distance = Time of Flight x Speed of Light

However, light is slowed down going through the glass in the prism, causing the distance to be measured as longer than it actually is. This is compensated through the use of the Prism Constant, and naturally it is -ve to reduce the measured distance to the actual distance. Some prisms manufacturers offset the back of the prism (which is the measured point) towards the instrument so that they have a 0.0 mm or any other desired prism constant.

There are generally two different approaches to prism constants depending on manufacturers.

• Absolute prism constants (used by majority of instrument manufacturers) – these are the fundamental constant of the prism irrespective of which instrument is being used
• Leica prism constant – these are the constant that applies when the prism is being used with a Hexagon's Geomax or Leica instrument

The reason Leica Prism Constants is different from absolute prism constant is because there is a pre-installed offset of 34.4mm in all Hexagon's instrument firmware/software.

These are formulas describing the differences in constants:

Leica Prism Constant = Absolute Prism Constant + 34.4mm

Absolute Prism Constant = Leica Prism Constant – 34.4mm

If a prism has a 0.0 prism constant you need to know whether it is Leica Prism Constant = 0.0 or

Absolute Prism Constant = 0.0

How to change the prism constant in X-PAD

You can reach the prism constant screen from any place where you see the prism icon: