When it comes to land surveying, most total stations use purpose-built glass prism reflectors for the EDM signal. Usually, a typical total station has the capacity to measure a distance of up to 1,500 m (4,900 ft) with an accuracy of about 1.5 mm (0.059 in) ± 2 parts per million.
A prism is a corner cube or a reflector that comes attached to a surveying pole and is used as a target for distance measuring. They also include prism poles that are used to accurately measure the elevation of the existing ground by using a sight level that may include either a total station, a transit level, an automatic level, or a laser level.
Understanding Prism Constants and Offsets
A prisms constant is a property of a certain prism model that exists to define the relation of the distance measurement to the mechanical reference point of the prims. The prism constant is unvarying and should not have any influence on the measurement.
Most cube prims come with an offset due to the transmitted light beam that comes from the EDM. Due to these factors, it may take longer to enter and exit the prism. Common offsets that you will come across include 0, -17.5 mm, -30 mm, -34 mm, and -40 mm.
At the same time, land surveyors use a variety of different prisms that all come with their own unique properties and different characteristics.
360-Degree Prisms
360-degree prisms work similarly to a round prism with the main difference being in it providing a permanent and continuous reflection of the EDM signal. Additionally, they are much easier to use once a pole operator needs to move quickly from one point to another. Omnidirectional prisms are the best choice when it comes to using stations that have automated target recognition, as they don’t need constant readjustments.
A 360-prism consists of six corner cubes that are joined together into one. This unique type of arrangement allows the measuring beam to reflect the beam directly back to its source from any position of the prism set. Thanks to the even distribution of the prism, it is capable of resulting in similar accuracy and a similar offset from any side. This type of reflector can be also used for all types of robotic work done with a surveying pole.
Explore the table below for a quick overview of the key information we've gathered to enhance your understanding of the topic about 360 degree prisms.
 |
 |
 |
|
|---|---|---|---|
| Model | GRZ122 | ATP1 | VX&S Series |
| Brand | Leica | Sokkia | Trimble |
| Height Offset | 86.0mm | 100.0mm | 139.0mm |
| Offset - Constant | -11.3mm | 7.0mm | 2.0mm |
| Leica Constant | +23.1mm | ||
| Accuracy | +/-2.0mm | <5sec | |
| Mount | 12mm Leica Spigot |
5/8''x11 Thread |
5/8'' Thread |
Mini Prisms
Mini prisms are capable of providing high positional accuracy due to their small size. They come with a built-in leveling bubble and an attachable spike. This type of arrangement allows for pinpoint accuracy during any type of land surveying job. Besides increasing the accuracy rate, they work well in decreasing the possibility of errors.
Mini prisms are also cheaper than other types of prisms and are good for multi-purpose usage as they perform well in short to mid-range measurements. Another very important advantage of mini prisms is owed to their size as they are quite easy to keep in storage and don’t cause any trouble in transportation.
Explore the table below for a quick overview of the key information we've gathered to enhance your understanding of the topic about mini prisms.
 |
 |
 |
|
|---|---|---|---|
|
Model
|
|||
|
Brand
|
Leica
|
Leica
|
Leica
|
|
Height Offset
|
50.0mm
|
50.0mm
|
15.0mm
|
|
Offset - Constant
|
-34.4mm
|
-6.3mm
|
-4.40mm
|
|
Leica Constant
|
+17.5mm
|
+28.1mm
|
+30.0mm
|
|
Accuracy
|
+/-2.0mm
|
+/-2.0mm
|
+/-1.5mm
|
|
Mount
|
1/4'' Thread
|
5/8'' Thread
|
1/4'' Thread
|
Monitoring Prisms
Monitoring prisms usually installed on structures like buildings, bridge piers, tunnel walls, pavements, or railroad tracks. Once changes in the positions of monitoring prisms are noticed, it indicates movements in the structures. These types of prisms are essential for tachymetric structural monitoring.
In order for the highest accuracy to be achieved, the prisms needs to be positioned in the center symmetrical point. These types of prisms also make it possible for a large number of measuring points to be observed so the object is measured economically and precisely within the varying requirements.
Explore the table below for a quick overview of the key information we've gathered to enhance your understanding of the topic about monitoring prisms.
|
Model
|
ADP109 | ||
|
Brand
|
Leica
|
Leica
|
Metricop
|
|
Height Offset
|
90.0mm
|
64.0mm
|
128.0mm
|
|
Offset - Constant
|
-25.0mm
|
-34.0mm
|
-15.0mm
|
|
Leica Constant
|
+8.9mm
|
+0.0mm
|
+19.4mm
|
|
Accuracy
|
+/-1.0mm
|
5arcsec
|
5arcsec
|
|
Mount
|
L-bar
|
5/8'' Thread
|
M8
|
Circular Prisms
Circular prisms are considered reflectors that are used to reflect the EDM beam from a total station. Full-size reference prisms are capable of providing great accuracy at high ranges and can also be used to calibrate a monitoring network. A standard prism also comes with <5 seconds, making it really accurate for high-range measuring.
Explore the table below for a quick overview of the key information we've gathered to enhance your understanding of the topic about standard circular prisms.
 |
 |
 |
|
|---|---|---|---|
| Model | GPH1P | GPR121 | GPR111 |
| Brand | Leica | Leica | Leica |
| Height Offset | 86.0mm | 86.0mm | 86.0mm |
| Offset - Constant | -34.4mm | -34.4mm | -34.4mm |
| Leica Constant | +0.0mm | +0.0mm | +0.0mm |
| Accuracy | +/-0.3mm | +/-1.0mm | +/-2.0mm |
| Mount | 12mm Leica Spigot | 12mm Leica Spigot | 12mm Leica Spigot |
Reflective Targets
Reflective targets help to provide measurements over long distances. These can be as simple as adhesive targets and as complex as pole-mounted target systems. Adhesive targets come with a reflective surface or a white and black reflective target pattern. They are very easy to use and can be attached to most surfaces and should be available in different sizes in order to accommodate different types of land measuring situations. We can't say that reflective targets are prisms, but they definitely provide a similar, and also very accurate measurement in the other hand.
In order to suit various types of monitoring situations, reflective targets need to come in a variety of colors, so they can be visible during daylight or in various types of conditions.
Explore the table below for a quick overview of the key information we've gathered to enhance your understanding of the topic about reflective targets.
|
Model
|
TR01 | ||
|
Brand
|
Metricop
|
Metricop
|
Metricop
|
|
Dimension
|
40x40mm
|
20x20mm
to 60x60mm
|
R - 60mm
H - 86mm
|
|
Distance
|
10-150m
|
10-150m
|
10-150m
|
|
Mount
|
Ø 6mm
|
Self -Adhesive
|
M8
|
|
Color
|
Yellow
|
Silver
|
Yellow
|
|
Material
|
Delrin - Plastic
|
Plastic |
Delrin - Plastic
|
Conclusion
In land surveying, various prisms are employed to ensure accurate measurements and monitoring. From 360-degree prisms that provide omnidirectional reflection to mini prisms that offer high positional accuracy, surveyors have a range of options to choose from.
Monitoring prisms are essential for structural monitoring, while circular prisms excel in high-range measurements. Reflective targets, while not prisms, also contribute to the accuracy of measurements over long distances. Understanding the different types of prisms and their applications is crucial for choosing the appropriate tools for any land surveying project.
Types of Prisms for Land Surveying FAQ
Here are a few more interesting facts about the types of prisms used in land surveying.
What are the different types of survey prisms?
There are a few types of prisms that surveyors usually use. Mini prisms are very versatile as they can be used for both short and mid-range measurements, plus they are cheap and reliable. With 360 prisms, all angles have a shared center, making them also suitable for multi-station networks.
Dual prisms are less expensive than 360 prisms but need to be aimed at every total station in a network. They are also very reliable and come with shared centers.
How accurate is a survey prism?
The accuracy of a survey prism is usually determined by its physical location in the canister or holder. Most prisms have an accuracy of 1 mm or less.
What is a prism pole used for in surveying?
Prism poles are used to help measure the elevation of a specific ground point with the help of sight levels.
What is a mini prism in surveying?
Mini prisms are attached to static surfaces or prism poles in order for them to act as focal points for various EDM types of equipment, accounting for improved accuracy while surveying.
