Photogrammetry

Measurement Accuracy

Table of Contents

General

Photogrammetry differs fundamentally from other measuring methods when the achievable accuracies are considered: In principle, one can achieve any desired accuracy with corresponding effort. Or viewed the other way around:

If you know which accuracy you need, you have to keep a maximum shooting distance with a given camera/lens combination to achieve this accuracy, or with a given shooting distance, what focal length the lens and/or what resolution the camera must have to achieve this accuracy

Rules of the thumb

  • The accuracy you achieve with ELCOVISON 10 is at natural points about the pixel size at the object.
  • The higher the quality of the camera-lens combination, the better the images, the better the accuracy.
  • Fixed focal length lenses are faster than zoom lenses. With the same lighting conditions, you can therefore increase the f-number, the images become sharper and the accuracies better.
  • Do not zoom or refocus during photographing. If this is done, the corresponding images must be calibrated simultaneously by ELCOVISION 10 and the achievable accuracy may decrease. 

What is "Pixel Size at the Object"?

Let's assume you have a camera with 12 MPixel, i.e. approx. 4000*3000 pixels, resolution. You are photographing an object with an 4*3 m dimension.

Case 1:
You photograph it in such a way that it occupies approximately half the image content, i.e. approx. 2000*1500 pixels:

Object fills approx. half of the image content
Object fills approx. half of the image content

Pixel size at the object is now ca: 4m/2000 Pixel = 2mm



Case 2:
You are now getting closer or you have a lens with a higher zoom factor, the object is now full-frame on the image:

Object fills the entire image content
Object fills the entire image content

Pixel size at the object is now approx: 4m/4000 pixels = 1mm

In case 1 you will therefore have approx. 2mm accuracy, in case 2 approx. 1mm accuracy.

Accuracy at signaled points

If points are signaled with ELCOVISON 10 targets, an accuracy in the range of 0.1 - 0.01*[pixel size at object] is normally achieved at these points, i.e. approx. 10 to 100 times more accurate than with natural points.

Precision in aerial photography e.g. drone images

In the case of aerial photographs, a distinction is usually made between position accuracy and height accuracy. The reason for this is that you normally have mainly vertical shots and no oblique shots.

The position accuracy is identical to the pixel size of the object, and is mainly determined by the focal length and resolution of the camera.

For practical purposes, height accuracy depends almost exclusively on flight altitude. The camera resolution plays hardly a role here, provided it is a normal or high-quality camera.

Following some tables with the typical precision for cameras with different sensors for different focal lengths and flight altitudes, Position precision is in italic script

APS-C Sensor: approx. 28*19mm; 12 MPixel: 4000*3000 Pixel

AltitudeHeight prec. 8 mm Lens15 mm
Lens
20 mm
Lens
30 mm
Lens
10 m0.3 cm0.61 cm0.33 cm0.25 cm0.16 cm
15 m0.5 cm0.92 cm0.49 cm0.37 cm0.25 cm
20 m0.7 cm1.23 cm0.65 cm0.49 cm0.33 cm
30 m1.0 cm1.84 cm0.98 cm0.74 cm0.49 cm
40 m1.3 cm2.45 cm1.31 cm0.98 cm0.65 cm
50 m1.7 cm3.06 cm1.63 cm1.23 cm0.82 cm
60 m2.0 cm3.68 cm1.96 cm1.47 cm0.98 cm
80 m2.6 cm4.90 cm2.61 cm1.96 cm1.31 cm
100 m3.3 cm6.13 cm3.27 cm2.45 cm1.63 cm
120 m4.0 cm7.35 cm3.92 cm2.94 cm1.96 cm

APS-C Sensor: approx. 28*19mm; 24 MPixel: 6000*4000 Pixel

AltitudeHeight prec.8 mm
Lens
15 mm
Lens
20 mm
Lens
30 mm
Lens
10 m0.3 cm0.41 cm0.22 cm0.16 cm0.11 cm
15 m0.5 cm0.61 cm0.33 cm0.25 cm0.16 cm
20 m0.7 cm0.82 cm0.44 cm0.33 cm0.22 cm
30 m1.0 cm1.23 cm0.65 cm0.49 cm0.33 cm
40 m1.3 cm1.63 cm0.87 cm0.65 cm0.44 cm
50 m1.7 cm2.04 cm1.09 cm0.82 cm0.54 cm
60 m2.0 cm2.45 cm1.31 cm0.98 cm0.65 cm
80 m2.6 cm3.27 cm1.74 cm1.31 cm0.87 cm
100 m3.3 cm4.08 cm2.18 cm1.63 cm1.09 cm
120 m4.0 cm4.90 cm2.61 cm1.96 cm1.31 cm

Full format Sensor: approx. 36*24mm; 24 MPixel: 6000*4000 Pixel

AltitudeHeight prec.8 mm
Lens
15 mm
Lens
20 mm
Lens
30 mm
Lens
10 m0.3 cm0.51 cm0.27 cm0.21 cm0.14 cm
15 m0.5 cm0.77 cm0.41 cm0.31 cm0.21 cm
20 m0.7 cm1.03 cm0.55 cm0.41 cm0.27 cm
30 m1.0 cm1.54 cm0.82 cm0.62 cm0.41 cm
40 m1.3 cm2.05 cm1.09 cm0.82 cm0.55 cm
50 m1.7 cm2.56 cm1.37 cm1.03 cm0.68 cm
60 m2.0 cm3.08 cm1.64 cm1.23 cm0.82 cm
80 m2.6 cm4.10 cm2.19 cm1.64 cm1.09 cm
100 m3.3 cm5.13 cm2.73 cm2.05 cm1.37 cm
120 m4.0 cm6.15 cm3.28 cm2.46 cm1.64 cm

Full format Sensor: approx. 36*24mm; 36MPixel: 7360*4900 Pixel

AltitudeHeight prec.8 mm
Lens
15 mm
Lens
20 mm
Lens
30 mm
Lens
10 m0.3 cm0.43 cm0.23 cm0.17 cm0.11 cm
15 m0.5 cm0.64 cm0.34 cm0.26 cm0.17 cm
20 m0.7 cm0.86 cm0.46 cm0.34 cm0.23 cm
30 m1.0 cm1.28 cm0.68 cm0.51 cm0.34 cm
40 m1.3 cm1.71 cm0.91 cm0.68 cm0.46 cm
50 m1.7 cm2.14 cm1.14 cm0.86 cm0.57 cm
60 m2.0 cm2.57 cm1.37 cm1.03 cm0.68 cm
80 m2.6 cm3.42 cm1.83 cm1.37 cm0.91 cm
100 m3.3 cm4.28 cm2.28 cm1.71 cm1.14 cm
120 m4.0 cm5.14 cm2.74 cm2.05 cm1.37 cm