Collecting Input Data

Wanted: XYZRGB Pointclouds

In .ply format. See usage tips for more details.

LIDAR

If you have access to LIDAR hardware expensive enough to do full-colour scans, you shouldn’t need my help operating it. Have fun, fortunate user!

Photogrammetry

Photogrammetry, sometimes called Structure-from-Motion (SFM) is the technique of reconstructing a 3D scene from multiple 2D images.

With a quadcopter to make landscape-scale scanning practical, this is a viable and reasonably-priced option for research, forestry, and citizen forest monitoring programmes.

The tutorials on flightriot.com are a good overview of the whole process.

Flight paths

The key property of a flight path for photogrammetry is that every part of the scene must be visible in multiple images, from multiple angles. Images should have a roughly one-third overlap in each direction.

Note that while it is tempting to use a fisheye lens for a wider field of view, the avoiding image distortion is much more important. The less reprojection must be done in software, the better! Also ensure that the shutter speed is fast enough to avoid motion blur, and that geolocation metadata is of reasonably high precision.

  • The classic option is a simple grid pattern with the camera facing directly downwards. This generally works well at low altitude; coverage of horizontal surfaces is good but the sides of trees are often missed.
  • Grid patterns with an angled camera (eg 45 degrees) work very well, but require many more photographs - essentially flying the pattern multiple times in different directions. One is similar to a nadir grid; two would be better, and I suspect three would give very good results.
  • An ‘orbit’ pattern works well if only a single landscape feature is of much interest. Ensure a variety of angles (including altitude) and distances - without this the reconstruction will likely fail.

Software options

There is a wide variety of photogrammetry software out there; this section only mentions those useful at landscape-scale. Feel free to contribute examples!

I do not know of any photogrammetry packages with a license that meets the Open Source definition, which is frustrating for would-be developers and non-research users. I encourage anyone interested to start or support a project to fill this gap.

Software License Comments
VisualSFM Free for noncommercial use, unmodified redistribution Works well with limited compute power available; low cost.
CMPMVS Research use only (free). Requires unusually powerful hardware (eg 4GB+ video memory).
Pix4D Sells expensive commercial and costly educational licenses. Very expensive, but excellent results. Trial version works, except exporting files!

Suggested settings

The various software options are quite distinct, but a few tips carry over.

  • Performance

    3D reconstruction is more computationally intensive than any common desktop application. Look at all the options; some shave hours off the running time.

    • Ensure that GPU acceleration is enabled
    • Test a subset of your data, and run the full set overnight
    • Reserve 1GB RAM and a CPU core, if you want to run anything else
  • Completeness of reconstruction

    Trees look very different from slightly different angles, so some settings tweaks are useful to ensure they are in the scene as well as the ground. Remember that forestutils will drop spurious ground points later.

    • Generally a point is constructed if it is recognised in 3 images. Turn this down to 2 images.
    • Use reduced image scale for the keypoint reconstruction stage.
    • Use full image scale (or even more) for point densification.
    • Turn desired point density to maximum.