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How to Get Perfect Prism Alignment

surveying prisms

Widely accepted as reliable accessories, surveying reflectors and prisms are not often considered as a potential source of measurement error. However, obtaining high levels of accuracy and reliability requires the consideration of all possible effects on measurement, including the prisms. In particular, round prisms must be aligned with the line of sight of the instrument within a certain tolerance.

For high-precision measurements, this dependency is clearly shown in the figure below. An acceptable deviation would be +/-10°.

prism alignment

When pointing to a prism that is not perpendicular to the instrument’s line of sight, it is difficult to point to the prism’s actual center. This is because the refractive index between air and glass is different and can cause further error in the horizontal measurement.

This refraction occurs in daily life as well. The same effect appears when conducting tilted measurements of a pencil in a glass of water observed from different angles, for example. The image on the left appears different because the refractive index differs from air to water. But observing the prism perpendicular to its front surface allows the true recognition of its position, as shown in the right image below.

The effect of refraction on prism alignment
 
With most of today’s instruments, gun sights ensure an accurate alignment with the instrument’s line of sight so that the operator can do a quick and easy check within seconds by simply aiming the prism to the instrument.

gun sights ensure an accurate alignment with the instrument’s line of sight
 
If the prism is not aligned with the instrument’s collimation axis, the standing axis of the prism pole will not coincide with the apparent prism center and the operator would have to aim to the apparent prism center. Because the prism body is symmetrical, the effect of vertical misalignment is the same as for horizontal misalignment.

While today’s tools make it easy to achieve proper prism alignment, a quick check can help ensure accuracy and measurement quality.

You can find a wide selection of prisms and accessories in the APE Online Marketplace. For more troubleshooting tips, browse the APE Blackboard or contact our knowledgeable experts.