Surveying Targets & Prisms for Surveying & Construction Projects Video

Understanding the Differences between Reflective Targets and Survey Prism Systems

The use of survey prisms in construction and monitoring projects has a long reputation for accuracy and reliability on site. While prisms continue to be the tool of choice for many surveyors, reflective target systems feature some interesting advantages potentially beneficial to projects operating on strict budgets or those looking for an alternative.

Broadly speaking, the accuracy of well-built reflective targets is much more precise than what many projects require. Specifically, survey teams have enjoyed immense success with target systems on construction sites where distances tend to demand between  +/- 1mm and +/- 2mm of precision.

Let’s dive deeper into the advantages surveyors might find with a reflective target system rather than a traditional prism system. 

Outlining the variety of reflective target types

Reflective targets are broken into categories according to their relative sizes. These differences correspond directly to their measurement capabilities. The general range capability of a reflective targeting system is anywhere from two to 500 meters.

In addition to size differences, angled targets solve some of the problems associated with swiveling prisms when angles greater than 30-35 degrees require measurement. A long-standing problem with prisms has been the need for manual realignment in some circumstances.

This obviously becomes a serious issue when prisms are placed in particularly difficult to reach places. In addition, prisms used in high traffic areas are often removed from the site overnight if there is a substantial risk of theft, vandalism or exposure to the elements.

The significantly lower costs of reflective targets means separate pieces of equipment can be placed at every angle needed to make a measurement. Many times, survey crews are able to leave the targets fixed on-site throughout the entire project schedule as the inexpensive components are easily replaced or repaired if damaged.

Accuracy and on-site uses of reflective targets

Many survey operations don’t require measurements calibrated to the extreme degree of accuracy afforded by survey prisms. The +/-1mm and +/-2mm degree of accuracy afforded by reflective targets is perfectly acceptable for many projects found on construction sites.

Reflective targets are also an efficient option for monitoring projects where structures undergo constant measurement fluctuations as a result of constant use. Bridges are perhaps one of the most common examples of this.

When survey crews are dispatched to take measurements on the bridge itself, heavy use of the bridge from traffic can cause the bridge to move itself––making it impossible to take measurements within a millimeter of precision.

More benchmarks means better accuracy and measurement consistency

Another potential of reflective target systems is the ability to collect a much more complete set of benchmarks than what is possible using prisms. With one prism position and one corresponding measurement, surveyors can place 20-30 separate targets. With more reference points, a substantially larger amount of benchmarks will naturally result in a more accurate measurement.

In addition to accuracy improvements, more reference points give surveyors greater versatility with the total station. With an integrated data documentation system, you can quickly verify height, access, and other measurement criteria.