Reference Scale

The Reference Scale tool works exactly like the scales printed on maps or floor-plans. This calibration tool assumes that you are observing a flat object that is captured with the camera parallel to the object. The Reference Scale tool does not handle perspective distortion, but on the other hand, it is simpler to use than the Perspective Reference tool.

It works well if you are measuring scans of flat drawings, maps, or if you take care that the camera is parallel to the object surface. It is also useful for special applications like processing microscope images.


Figure 1 shows an example image using the Reference Scale tool. When capturing the image, a ruler was placed into the image to which the Reference Scale (striped vertical line on the left side) has been aligned. Based on this scale, ImageMeter can compute the dimensions of the hand automatically. Note that the camera was placed parallel to the object and that the 2cm wide stripes on the Reference Scale align nicely with the ruler.

Figure 2 shows using the Reference Scale for carrying out measurements on a floor plan drawing. In this case, the reference scale is aligned to the 5m reference size provided by the floor plan drawing in the bottom left corner.

Fig. 1: Reference scale is aligned to the ruler on the left side. The dimensions of the hand are computed automatically.
Fig. 2: Reference scale is aligned to the reference length printed in the bottom left of the floor plan. Click on the image to magnify.

Capturing Images Correctly

The crucial constraint when using this tool is to capture the image correctly. Hold the camera straight when taking the picture like shown in Fig. 3. Do not hold it at an angle with respect to the object (Fig. 4).

Fig. 3: Hold the camera parallel to the object you want to measure.
Fig. 4: Do not hold the camera at an angle as this will result in perspective distortions.

The difference between holding the camera correctly and incorrectly are shown in the captured images in Fig. 5 and Fig. 6. No perspective distortion is apparent in Fig. 5, all squares have the same size. On the other hand, Fig. 6 is perspectively distorted. Squares further away are depicted smaller than those in the front. You cannot use the Reference Scale calibration with the latter (however, you can use the Perspective Reference for these cases).

Fig. 5: Checkerboard pattern captured with correct camera position.
Fig. 6: Checkerboard pattern captured with unsuitable camera position. The perspective distortion cannot be handled by the Reference Scale calibration.

Usage Advice

As always, try to use a reference length as large as possible, because this reduces the errors resulting from a slightly inaccurate placement of the reference scale or camera lens distortions. For example, in Fig. 5, instead of using the width of one checkerboard square as the reference, it would be better to take the whole width (across 9 squares and setting the reference length 9 times the size of a single square).

If you use, e.g., a ruler as the reference object, check that also the intermediate segments of the reference scale match accurately the image (you can adjust the number of segments in the reference scale styling options). If they do not align well, the camera position probably was not well aligned or the camera has significant lens distortions.

When the reference scale is selected, a grid with distance steps equal to one reference scale segment is displayed. This grid can also be used to align the reference scale more accurately to the image in some cases.