Prism Constants in Survey Targets Explained

Have you ever used targets and total stations from different manufacturers (e.g. using a Leica Target with a Trimble instrument) and wondered why you’re having systematic errors in your data? Chances are you need to correct your constants.

One of the most common causes of error in surveys seems to be caused by the lack of understanding of prism constants. In today’s age of Land Surveying, the most commonly used targets contain a prism reflector that is used to return the signal back to the EDM of the Total Station. These prisms often have different characteristics defined by their construction which differs between manufacturers. In this article, I will try to explain:

• what a prism constant is and how it affects your EDM measurements
• how most manufacturers define their prism constants
• a list and comparison of the most commonly used prism and their constants and how to correctly use prism constants with instruments from other manufacturers (e.g. how to use a Leica prism constant with Trimble).

With so many manufacturers out there, this article would become too long and difficult to follow. Therefore I will be focusing on the most common equipment used throughout the world.

This article will focus only on Corner-cube prisms (reflectors). The flat reflective targets have no constant as the signal bounces directly off the target’s surface.

What are prism constants

To define what the prism constant is, we must first explain how a prism target works. In principle, the target reflector is a piece of glass that bounces off the signal in the same direction it came from. The image below shows how the incoming EDM signal is reflected in the same direction it came from.

As you can see, regardless of the point of entry of the signal, it will be reflected in the same direction where it came from. Without going into much detail, this type of reflector is the most commonly used due to the simplicity of the design.

How does a target constant come into play? Well, the constant is defined as the correction of the measured distance based on the physical and design characteristics of the prism or reflector in the instrument. This is calculated based on the type and length of the glass prism itself and the mechanical mounting of the prism on the target holder. To put this into perspective, the sketch below shows a 2D view of a prism. The magnitude of the prism constant is defined as the distance between the true axis of the prism holder (the point around which the prism tilts up and down) and the virtual axis (the theoretical turning point S₀).

True Axis represents the vertical axis of the target (i.e. centered on the point). Most commercially available prisms will always have the virtual axis behind the true axis, making the measured distance too long, thus requiring a negative correction for the distance (K is negative). Best case scenario is when virtual axis and true axis coincide, requiring no correction. In this case, there is a minimal effect on measurements of angles and distances caused by inaccurate prism alignment.

Prism constant vs prism offset

Although these terms are commonly interchanged, they technically refer to different values. Prism constant is often referred to as prism offset, however this might not always be the correct definition.

If you are referring to the prism glass, the constant can be referred to as the offset. However, when the context is the surveying prism (including the body of the holder), the prism offset is actually the height difference between the mounting point of the survey prism (survey target) and the centre of the prism glass. In this case, the prism offset is a vertical offset, rather than a horizontal one.

To avoid confusion, whenever discussing surveying prisms on this site, the prism constant is the value added to the EDM path, where as prism offset is the height of the prism.

How manufacturers define their prism constants?

Most big-name manufacturers (Trimble, Topcon, Sokkia, Pentax, and Nikon) will define all prism constants based on their physical characteristics mentioned above, making them absolute offsets. The only exception is Leica which defines the prism constant with reference to its standard round prism (GPH1 + GRP1). The absolute correction for this prism is -34.4mm, which Leica defines as a constant 0.0mm. It is essential to keep in mind that if non-Leica prisms are used with Leica instruments, the constant of the prism needs to be converted. Similarly, when a Leica prism is used with a non-Leica instrument, the same process applies, but in reverse.

Example – using a non-Leica prism with a Leica instrument

Consider we have a non-Leica prism with a manufacturer defined constant (absolute offset) of -17mm. In this case we use the following formula to compute the correction required in the Leica instrument:

Leica Offset (Kl) = Absolute Offset (K) + 34.4 mm

Replacing the value in the formula above will get us a value of +17.4mm, an adjustment that must be made on the Leica Instrument.

Leica Offset (Kl) = -17 + 34.4 = +17.4mm

Example – using a Leica prism with a non Leica instrument

In this case the process is similar, but in reverse. We need to subtract -34.4mm from the Leica defined offset (Kl):

Absolute Offset (K) = Leica Offset (Kl) – 34.4 mm

Considering a Leica prism with the manufacturer defined constant (Kl) of 0.0mm, we can compute the required value that needs to be used in the non-Leica instrument:

Absolute Offset (K) = 0.0 – 34.4 = -34.4mm

In practice, it is very common for surveyors to use prisms from different manufacturers and use the constants written on the body of the prism. Or, alternatively, switching from a Leica instrument to a non-Leica instrument, but not knowing the difference in prism constant definitions. This causes a large number of issues with topographical surveys, especially when creating control networks.

Most common commercially available prisms and their constants

The table below serves as a reference and contains the most common prisms used in the industry and their constants in the Absolute and Leica systems.

This list isn’t exhaustive, but hopefully it can serve as a guide when switching instruments or mixing target manufacturers.

What prism constant do I use?

As presented above, different manufacturers define prism constants in their own way. These can be broken down to two different groups:

• Leica
• Other manufacturers (Trimble, Sokkia, Topcon etc)

You should always use the prism constant as defined by the manufacturer of your instrument. The choice of what prism constant you need depends on both the instrument (total station) manufacturer, as well as the prism manufacturer.

If you’re unsure what prism constant you should use, use the flow diagram below to decide.

To see more surveying prisms and find out their characteristics such as prism constant, prism offset or mount type, please go to the dedicated page about survey prisms and targets.