Written by Robert Greenhalgh (BSc, University of Leeds, UK), a 3D measurement specialist with over 17 years of experience in reality capture technologies. His work bridges technical precision and practical problem-solving across complex environments. Credit to Lisa Wahl for technical support and quality control.
Accuracy starts with your first measured point. A high precision total station EDM remains the gold standard for surveying, and the mini-prism is a great tool for setting up the total station and performing stakeout – when used correctly.
Real-World Setup Challenges – How are errors introduced, and how can we reduce them?
Most surveyors will have experienced setting up a total station with fixed point locations in unfavorable—but necessary—positions due to accessibility, passing vehicles, or pedestrian traffic. Not every site lends itself to carrying a tripod and clamp to stabilize the prism while setting up, and time pressure during measurement can further complicate accuracy. It is therefore imperative to understand how variations across the leveling bubble impact the accuracy at each target height.
Here we isolate the variables to find the true impact of these small variations during measurement, ensuring we can quantify potential errors in the process.
The Test Setup
mini-prism at 100mm, stabilised in the tripod with clamp.
The test used single face measurements from the total station, comparing the results of the mini-prism when levelled and when positioned at the edge of the centering ring—reflecting the difference between a fully stabilized, levelled prism and real-world conditions where wobble and tilt are common due to wind, passing vehicles, or an unsteady hand.
The total station was set up on three fixed points, and then a test point 12 meters from the total station was measured. The mini-prism was stabilized throughout with a tripod and clamp, and directed at the total station to minimize the angle of incidence on the prism face. The mini-prism used is nodal, so effects of the angle of incidence should be negligible within normal ranges.
3D model representation of the test process with different prism heights
Measurements were taken at 100mm, 400mm, 700mm, and 1000mm target heights—achieved by inserting 30 cm sections in the pole. At each target height, the point was measured with the pole both levelled and unlevelled.
The test focused on the difference in measurement between the best-case scenario (bubble perfectly centered) and the typical variance seen as slight wobble affects the pole (bubble at the edge of the centering ring). It is assumed that a diligent surveyor striving for the best available accuracy would always be able to maintain the leveling within the centering ring.
(left) Levelled mini-prism (right) Unlevelled mini-prism
Results: How Much Error Does It Introduce?
Below is a tabulated list of the results, comparing the reference position with the inclined test value. The easting and northing errors were calculated together into a single linear value.
Target Height (mm) | Horizontal Error (mm) |
100 | 0.0 |
400 | 1.0 |
700 | 4.5 |
1000 | 7.1 |
At each target height, the measured point height remained within 1 mm between the levelled and unlevelled conditions.
Key Takeaways and Recommendations
An unlevelled mini-prism can introduce horizontal errors into the measurement process—these are reduced to negligible levels at the lower prism heights tested. Therefore, where possible, the lowest prism height should be used to minimize the effects of wobble on the measured position.
Should a higher target be needed for visibility reasons, such as uncut grass or other obstructions, then use of a tripod and clamp for stabilization is recommended. It is known that horizontal errors in total station networks build up more quickly than vertical errors, so any steps taken to mitigate these errors are key to ensuring reliable data.
The second takeaway is that the measured height is affected much less by an unlevelled prism, even at the higher prism height. This means that for measurements where height is key but the XY position is not as critical, the surveyor has a little latitude regarding the leveling but can still be confident of a precise height reading.
Equipment Used
- Trimble S7 3” total station, calibrated to within specification by Allterra West (Backnang, Germany)
- Trimble TSC7 controller running Trimble Access
- Nodal mini-prism with 30mm constant
Credits
Test and report by Robert Greenhalgh.
Lisa Wahl
Lisa Wahl (MEng, Karlsruhe, Germany) brings extensive experience in infrastructure and fibre network planning to the challenge of strengthening survey and documentation processes for a digital future. From fieldwork to final deliverables, her strong CAD expertise enables the seamless integration of survey data and reality capture technologies—resulting in robust, future-ready client solutions.
Robert Greenhalgh
Robert’s expertise spans laser scanning, drones, GPR, mobile mapping, and traditional surveying—applied across diverse sectors including energy, construction, pharmaceuticals, and architecture. Choosing the right tool for the job has consistently proven critical to project success.
Robert has delivered projects in the UK, Austria, Denmark, Germany, and Switzerland, gaining valuable insights into best practices for maintaining measurement accuracy. His career has included roles with hardware vendors, distributors, and surveying service providers, giving him a well-rounded and practical understanding of the industry.
Today, he draws on this diverse background to dig deeper into the why and how behind the what.
