Reflectorless total station and EDM: range, accuracy and prism use
Key takeaways
- An EDM (electronic distance meter) measures distance with modulated light, using phase comparison or time of flight.
- Reflectorless mode measures to walls, rock faces and structures without a prism, typically to several hundred metres or more depending on the surface.
- Prism mode reaches longer ranges and better accuracy, so it remains the choice for control, traverses and monitoring.
- Reflectorless accuracy depends on surface colour, angle of incidence, laser spot size and edges, not just the published specification.
- Distance accuracy is quoted as a constant in millimetres plus a scale error in ppm, which grows with distance.
What is a reflectorless total station?
A reflectorless total station is a total station whose distance meter can measure directly to a natural surface without a prism or reflector. The instrument aims a laser at a wall, kerb, rock face or beam, detects the small amount of light scattered back and calculates the distance.
Also called a prismless total station, it still works with prisms when needed. Most modern instruments switch between prism, reflective sheet and reflectorless modes from the keypad or screen, so one instrument covers both tasks.
The main benefit is access. Surveyors can measure points they cannot reach safely or practically: building façades, bridge soffits, tunnel profiles, high-voltage areas, busy roads, stockpile faces and the inside of excavations.
What is an electronic distance meter in surveying?
An electronic distance meter in surveying is the component that measures slope distance with light or radio waves. In a total station, the EDM is built into the telescope and shares its line of sight, so angles and distance are measured to the same point.
EDM surveying replaced tapes and chains for most distance measurement because it is faster and more accurate over long distances. Modern total stations use electro-optical EDMs with infrared or visible red laser light.
How EDM works
Two measurement principles are common:
- Phase comparison: the EDM sends a continuous beam modulated at known frequencies and measures the phase difference of the return. Several frequencies resolve the whole number of wavelengths. This method gives high precision and is traditional for prism measurement.
- Time of flight (pulse): the EDM sends short laser pulses and times their return. Distance equals the speed of light times the round-trip time, divided by two. Pulses work well on weak reflectorless returns and at long range.
Many instruments combine the two or use signal-processing hybrids. For a broader introduction to the instrument, see what a total station is and how it works.
Reflectorless total station range and accuracy
Reflectorless range depends on how much light the surface returns. Published ranges are measured against a standard grey card, typically 90 % reflectivity, in good conditions; dark, wet or angled surfaces give much shorter ranges.
Accuracy is written as a mm + b ppm. The constant (a) applies at every distance; the scale part (b) adds b mm per kilometre. For example, 5 mm + 3 ppm at 200 m gives 5 + 0.6 = 5.6 mm. Reflectorless specifications are typically a few millimetres worse than prism specifications on the same instrument.
| Mars Geosystems model | Reflectorless range | Prism range | Angle accuracy / notes |
|---|---|---|---|
| STSA9 Android total station | 1500 m | 5000 m (single prism) | 2"; Android, dual 5" touch screens |
| TS1000 dual-axis total station | 1000 m | 7500 m (single prism) | 2"; 5 mm + 3 ppm reflectorless, 2 mm + 2 ppm prism |
| STS2 total station | 1000 m | 5000 m | Dual-axis compensation, laser plummet |
The TS1000 dual-axis total station shows the typical gap between modes clearly: 2 mm + 2 ppm to a prism versus 5 mm + 3 ppm reflectorless.
What affects reflectorless EDM accuracy?
Reflectorless measurements are more sensitive to the target than prism measurements. A good surveyor checks the conditions at each point, not just the instrument specification.
- Surface colour and texture: white and light grey surfaces return well; black, wet or glossy surfaces return weakly or reflect the beam away.
- Angle of incidence: steep grazing angles spread the laser spot and lengthen the measured distance.
- Laser spot size: the beam widens with distance. On edges and corners, part of the spot can hit a surface behind, mixing two distances.
- Obstructions in the beam: branches, wires, rain, dust or passing vehicles can return a false distance.
- Atmospheric conditions: temperature and pressure change the speed of light in air; entering them applies a ppm correction.
- Instrument calibration: EDM constants drift; field and laboratory tests such as those in ISO 17123-4 verify performance.
When in doubt, take a check measurement to the same point from a second setup, or measure a nearby surface at a more favourable angle.
When to use a prism instead of reflectorless mode
Use a prism whenever accuracy, range or reliability matter more than convenience. Reflectorless mode is a productivity tool, not a replacement for prisms in control work.
| Task | Recommended mode | Why |
|---|---|---|
| Control networks and traverses | Prism | Best accuracy, well-defined centring over marks |
| Long distances | Prism | Range of several kilometres versus hundreds of metres |
| Deformation monitoring | Prism | Repeatable target with a fixed constant |
| Stakeout of points on the ground | Prism | The pole marks the exact ground position |
| Façades, soffits, ceilings | Reflectorless | Points cannot be reached with a pole |
| Tunnel and shaft profiles | Reflectorless | Fast measurement of many surface points |
| Hazardous or restricted areas | Reflectorless | No one needs to stand at the target |
Prism type matters too. Always set the correct prism constant in the instrument; using the wrong one adds a fixed error to every distance. Our accessories guide to survey tripods, tribrachs, prisms and poles explains prism constants and setups.
Practical tips for EDM surveying without a prism
Good reflectorless results come from a disciplined field routine. These steps reduce the most common errors.
- Enter current temperature and pressure so the ppm correction is right.
- Confirm the instrument is in the correct mode before every measurement; a prism constant applied in reflectorless mode creates an offset.
- Aim at the centre of flat surfaces rather than edges and corners.
- Avoid steep angles to the surface; choose a setup that sees the target face-on.
- Watch for obstructions between instrument and target, especially vegetation and cables.
- Check a few points with a prism or tape to confirm results on unfamiliar surfaces.
- Follow laser safety guidance; check the laser class of the reflectorless beam under IEC 60825-1 and avoid aiming at people or vehicles.
Choosing a reflectorless total station
Choose a reflectorless total station by matching range, accuracy and workflow to your typical jobs. Most building and engineering work needs a reliable reflectorless range of a few hundred metres and 2" angles; longer ranges help in mining, tunnels and large structures.
- Reflectorless range: 1000 m or more covers most façades and open-pit faces under good conditions.
- Angle accuracy: 2" suits engineering and construction; tighter specs suit control and monitoring.
- Compensator: dual-axis compensation corrects small levelling errors automatically.
- Software and data transfer: onboard programs, Bluetooth, USB or Wi-Fi export.
- Durability: check the IP rating and battery life.
The STSA9 Android total station offers 1500 m reflectorless range, 2" angles and dual-axis compensation, while the STS2 adds a laser plummet and 32 GB storage. If you are deciding between instrument types, see theodolite vs total station. Mars Geosystems ships from one unit; request a quote with your range and accuracy needs.
Frequently asked questions
How far can a reflectorless total station measure?
Reflectorless range typically runs from a few hundred metres to over 1000 m on a bright surface, depending on the instrument. Published figures assume a highly reflective target, often 90 % reflectivity, and good conditions. Dark, wet or angled surfaces reduce range considerably, so plan for less than the specification on real sites.
Is reflectorless measurement less accurate than using a prism?
Usually, yes. Reflectorless accuracy is typically a few millimetres worse than prism accuracy on the same instrument, and it is more affected by surface colour, angle and edges. For example, a typical specification might be 2 mm + 2 ppm with a prism and 5 mm + 3 ppm reflectorless. Control work should use prisms.
What does EDM mean in surveying?
EDM stands for electronic distance meter or electronic distance measurement. It is the device, built into a total station, that measures distance using modulated light or laser pulses. The EDM compares phase or times the returning signal to calculate the slope distance to a prism or, in reflectorless mode, to a surface.
What is a prismless total station?
A prismless total station is another name for a reflectorless total station. It can measure distances directly to surfaces such as walls and rock without a prism, using a laser EDM. Most such instruments also measure to prisms, giving the surveyor a choice of mode for each point depending on access and accuracy needs.
What does mm + ppm mean in EDM accuracy?
EDM accuracy is quoted as a constant error in millimetres plus a scale error in parts per million. The constant applies at every distance, while the ppm term adds that many millimetres per kilometre. A specification of 5 mm + 3 ppm therefore means about 5.3 mm at 100 m and about 8 mm at 1000 m.
Can a reflectorless total station measure through glass or foliage?
Not reliably. Glass may reflect, refract or pass the beam, so the instrument might measure the glass, something behind it or nothing. Leaves, branches and wires in the beam path can return a false short distance. Always aim with a clear line of sight to the intended surface and check doubtful points.
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