School of Surveying and Spatial Information Systems

The University of New South Wales

Evaluation of the Reflectorless EDM Instruments Nikon NPL-350 and NPL-821

by Cameron Johnson

Supervised (and edited) by J. M. Rüeger

October 2003


A 'reflectorless' EDM instrument is an electronic tacheometer (‘total station’) that has the ability to measure as an electronic distance meter (strictly to reflective targets) and as a reflectorless distance meter (to non-reflective targets). Reflectorless EDM technology uses phase measuring or pulsed lasers to measure to targets of a reflective and non-reflective nature. The range to a passive target can be anywhere from 2 m to 100 m and up to 5000 m to a glass prism. The reflectorless electronic tacheometers Nikon NPL-821 (s/n 130133) and NPL-350 (s/n 050350) were tested and evaluated. A calibration of the instruments, using glass and acrylic reflectors, was also performed (on the Main Walkway Baseline UNSW and the Cyclic Error Testline No. 2). The most important results are shown here. Due to time restrictions, the Nikon NPL-350 was omitted from some of the testing.


Measurement of Beam Width at Different Distances

Knowing the width of the measurement beam allows for better pointing with the instrument to non-reflective surfaces. I determined the beam widths of the NPL-821 (s/n 130133) and the NPL-350 (s/n 050350) by performing horizontal and vertical scans across a stepped timber target (comprising 2 wood blocks fastened together) at 2 m, 30 m, 60 m and 90 m in the reflectorless mode. The left side of Fig. 1 shows the target used for measurement.

Table 1 shows the values of the horizontal and vertical beam widths of the Nikon NPL-350 and NPL-821. The values are shown in mm and angular units. Nikon states that the measurement beam is round and its width, at any distance, is the diameter of the circle on the target reticle of the telescope (found to be approximately 5' 00"). Based on my results, this does not seem correct. However, the circle on the reticle of the telescope is still a guide since the beam is never larger.

Figure 1: Target measured to for the beam width determination(on the left) and Tiltable/Rotatable target used for testing (on the right)

Range (m)

Nikon NPL-350 Beam Width

Nikon NPL-821 Beam Width






5' 12" (3.0 mm)

5' 10" (3.0 mm)

5' 04" (2.9 mm)

5' 11" (3.0 mm)


2' 27" (21.4 mm)

3' 09" (27.5 mm)

3' 12" (27.9 mm)

3' 35" (31.3 mm)


3' 12" (55.9 mm)

3' 04" (53.5 mm)

2' 45" (48.0 mm)

3' 00" (52.4 mm)


Not measurable

Not measurable

2' 36" (68.1 mm)

2' 35" (67.6 mm)

Table 1: Horizontal and vertical beams widths for the Nikon NPL-350 and NPL-821


Measurements to two Targets at Different Distances

I determined when and where a change in measured distance, compared to that of the predicted distance, occurs to two non-reflective targets when measuring in the reflectorless mode using the Nikon NPL-350 (s/n 050350) and NPL-821 (s/n 130133) at 2 m, 25 m and 50 m. At these distances, the instrument was pointed in such a way that 50% of Target 1 was in view and 50% of Target 2. At each range, Target 1 remained stationary at the fixed distance whilst Target 2 was repositioned at 100 mm intervals until a maximum separation of 2 m was reached (21 steps of measurement). This was carried out, with the instrument focussed on Target 1, and repeated, with the instrument focussed on Target 2. Figure 2 is the chart for the 50 m range of measurements using the Nikon NPL-821. At 2 m (close range) focus on the stationary target returned distances of 2 m and focus on the moving target returned distances increasing at 100 mm intervals. This occurred because small changes to the focus significantly altered the vision observable through the telescope and as a result proved that both instruments are using focussed distance meters. At the middle and longer range, the measured distances tend to converge back to the distance of the fixed target, as shown in Figure 2 once the two targets are separated by >1.7 m. Nikon suggests that if the amount of reflection from a passing object is almost the same and the difference in distances is less than 3 m between the correct and false target, then an error to measurements may occur. It was concluded that this phenomena was occurring at longer ranges.

Figure 2: Chart for two target measurement for the Nikon NPL-821. Target 1 at 50 m.


Range Testing

A maximum and minimum distance to acrylic and tape reflectors, passive and coloured targets was tested for using the Nikon NPL-821 in the reflectorless mode. The minimum distance to all items was approximately 1.6 m which is shorter than the minimum focusable distance of 2 m. Three acrylic reflectors were measured to: 10 mm diameter clear acrylic reflector, 50 mm x 50 mm Nikon target sticker (silver) and a A4 size Leica Disto Target Plate (brown). The maximum distance recorded to each of these was >586 m, which is the longest straight line distance measurable on UNSW Kensington Campus. In the 'prism' mode, the NPL-821 was able to read up to 200 m to these items. The passive targets measured to included: brick, concrete, timber, PVC pipe and Styrofoam. The maximum distance to the majority of the items was within 80 m - 100 m. The Styrofoam was the only exception, allowing 140 m. However, this item was white, which I believe attributed to its maximum distance being the longest. The maximum distances to the coloured targets were all fairly consistent averaging at approximately 130 m. Yellow and white cardboard allowed the greatest distance (145 m ­ 150 m) whilst black was the shortest (120 m).


Effect of Angle of Incidence at Different Distances

Using the NPL-821 (s/n130133), I determined how much a measured distance changes when a target is tilted and rotated at different angles of incidence (±60° maximum), horizontally and vertically. The right-hand side of Fig. 1 shows the tiltable/rotatable target upon a tripod with the timber backing plate and protractors fitted. The protractors were used to determine the degree of tilt or rotation at ±1°. Figure 3 is a chart of the vertical tilt of the target at 100 m using the Nikon NPL-821. As shown in Figure 3, the measured distances in face left and face right differ by approximately 30 ­ 35 mm at ±40°. The Nikon NPL-821 measures longer distances in face left when the upper part of the target hitless away from the instrument. Hence the measurement beam in face left is tilting upwards above the line of sight.

Figure 3: Chart for measurements to tilted target using the Nikon NPL-821 at 100 m.


Recommendations and Conclusions

My testing was generally concerned with the Nikon NPL-821, on the basis of it being the newer of the two reflectorless instruments owned by the School of Surveying & SIS, UNSW. Overall, I found the Nikon NPL-821 slow in measuring distances (time taken between sent and received signals), when measuring in the ‘prism’ and ‘reflectorless’ as well as in ‘precise’ (measurement to 0.1 mm) and ‘normal’ accuracy (measurement to 1 mm) modes. The slow motion pointing at a 1" angular setting was poor and unstable; I believe the NPL-821 would be better suited to a 5" angular specification. However, I found the optics and electronics to be excellent. After the testing of the Nikon NPL-821, I suggest that the following options are considered when using this instrument:



Further Information

For more information, please contact                                                                                                     


Assoc. Prof.  J. M. Rüeger (Supervisor)




School of Surveying and Spatial Information Systems

University of New South Wales




Phone: +61-2-9385-4173

Fax: +61-2-9313-7493



Mr. C. S. Johnson