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03/04/2019

BLK3D: Accuracy Best Practices

By g.capitano

BLK3D measuring accuracy

The most significant feature of the Leica BLK3D is how the device can measure within photos. However, like with most other measurement devices, you'll need to consider a number of factors when attempting to take the most accurate measurements. Below is a detailed overview of best practices for achieving the highest accuracy when measuring with the BLK3D, and if you'd like, you can also check out technical specialist Andy Fontana's video tutorial on measuring with the BLK3D on YouTube.

Leica BLK3D Accuracy Best Practices

The accuracy of a BLK3D in-picture measurement is influenced by four variables:

  1. Distance to the object being measured.
  2. The baseline between the 3D images used to calculate the measurement points.
  3. The orientation of the camera to the object being measured.
  4. Light conditions, structure and texture of the object.

First and foremost: the closer you are, the more accurate your measurements will be.

BLK3D accuracy table

For reference, the following four images were taken at the distances referenced in the chart above:

7.5 meters away

Distance to object (m, ft)
7.5m, 24.6'

Field of view (m, ft)
11.2m x 6.3m
36.7' x 20.6'

5 meters away

Distance to object (m, ft)
5.0m, 16.4'

Field of view (m, ft)
7.4m x 4.2m
24' x 14'

2.5 meters away

Distance to object (m, ft)
2.5m, 8.2'

Field of view (m, ft)
3.6m x 2.1m
12.0' x 6.9'

1 meter away

Distance to object (m, ft)
1.0m, 3.3'

Field of view (m, ft)
1.4m x 0.8m
4.5' x 2.7'

Due to the BLK3D’s wide field of view, we recommend keeping the distance to your object below 7.5 m (25’). An image taken from 7.5 meters will have a 11m (35’) wide field-of-view. At 7.5m distance, the measurement deviation can range between 10 – 35mm (0.1% - 0.4%).

Double up for best results

The gray line in the accuracy table above is for a "single shot." Beyond 2 meters, the accuracy of a single shot degrades rapidly as distance to the object increases.

The orange and green lines show how much accuracy can be improved using the “multi-shot” technique. single vs multi-shot

To take a multi-shot:

  1. Take your first 3D image.
  2. Take a step sideways.
  3. Take a second 3D image of the same area.

The side step only needs to be approximately 10% of the distance to the object (5m distance ~ 0.5m side step). A second set of 3D images provides the BLK3D with a longer baseline, improving the accuracy of the photogrammetry algorithms used to calculate measurements.

The second 3D image adds only a moment to the capture process and significantly improves the accuracy of any measurement taken with the BLK3D. The multi-shot capture method is essential when creating 3D models.

Favorable versus unfavorable conditions

The graph above features two multi-shot specifications: Favorable (green line) and Unfavorable (orange line) conditions.

Accounting for the differences in favorable versus unfavorable conditions is twofold. Firstly, the position to the object and secondly, it is a study of contrasts; the more contrast in light and color that you have between the line you are measuring and the background, the better. And when it comes to contrast, lighting is the most important aspect of the environment that you can potentially control. And when it comes to contrast, lighting is the most important aspect of the environment that you can potentially control.

If you are indoors, we suggest using the built-in LED Flash. In darker areas, an attachable LED light is available.

Adding light and correctly adjusting the exposure will yield an image with better contrast and produce the best possible results.

Last but not least

Measurement points within the middle 50% of the 3D image can be more accurate than “edge to edge” measurements. For best results, try to keep the area that you are measuring in the middle 50% of the screen when you capture an image.

And, if you think about it, getting the best results from the BLK3D is similar to how we get the best results from a smartphone camera in different situations.

For example: when taking a picture of a document with my mobile phone, you wouldn't stand halfway across the room and hope that you can zoom in enough to read the words on the page. You'd want to get up close and orient your phone in the same direction as the page, so that everything is straight and clear for when you need to read it later.

This is exactly how you can think about framing shots with the BLK3D. There is no doubt that you will be able to get results from images taken from further away, but the closer you are, the more accurate the results will be.

Tips for getting the highest accuracy when capturing 3D images:

  1. Get the proper amount of light on your subject. Activate the built-in LED flash or the optional LED lamp accessory. Avoid under and overexposure.
  2. Always take a second 3D image (multi-shot) of the area of interest. You need only step to the side 0.5m (1-2’), or 10% of the distance to the object you are measuring. This capture process only takes a few seconds, and make sure that all the objects you want to measure are in all the shots you take.
  3. If you have a large area to measure, break it down into a few smaller areas. You will get better resolution, sharper details, and better accuracy. Image capture is very fast and the BLK3D has 64GB memory and fast USB-C download, so you don't need to worry about running out of storage.
  4. For critical measurements, dedicate a separate multi-shot. The best accuracy is obtained by keeping the measurement in the center 50% of the image field-of-view and orienting yourself perpendicular to the object of interest. For the highest accuracy, the integrated laser electronic distance meter (EDM) of the BLK3D will typically measure within +/- 1mm. A laser EDM measurement can be attached to an in-picture measurement in the field using the Tools menu on the BLK3D.
  5. A stable device produces the best 3D images. When accuracy is paramount use a tripod, monopod, and/or the built-in timer function to stabilize the device.

 

 

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