The reconstruction coming from the structure from motion process is always up to an unknown scale factor. This means that to scale the reconstruction metrically and make measurements, you need to introduce at least one distance constraint or control points constraints. Control points with absolute coordinates positions can be used as constraints during the bundle adjustment to enforce the scaling and the position of the model during the structure of motion itself. Using control points constraints is a much accurate solution that can be done in Zephyr Pro and Aerial.
While control points and control distances can be only used in the Pro and Aerial version of Zephyr, you can still scale the reconstruction metrically also directly in the Free and Lite version by introducing one known metric distance constraint. Here's how:
- Suppose you know measurements in meters for the 3D reconstruction. In the following example, I suppose to know that the door base is 1.5 meters.
- Open the quick measurement tool
- Click begin and pick the first and last point of the distance you want to measure:
- In this case, the measured distance is 3.60539. Since we know the value of the "real" distance in metric space, it means that the reconstructed object would need to be scaled by a 1.5 / 3.60539 = 0.41604 scale factor.
- Open the gizmo tool
- Manual input the scale factor 0.41604 and click "ok":
The world is now scaled metrically and you can now take metric measurements with the quick measurements tool:
Please remember that this method cannot be used for accurate scaling and measurements, but it's a quick trick to get rough measurements also from the lite and free version.