Photogrammetry rules of thumb

I’ve uploaded a CloudCompare file of some fieldwork I did last year to my website here. It uses the UK national LiDAR inventory data, mentioned in the post here. I think it espouses lots of the fundamentals discussed here, and is a good starting point for thinking about network design.

80% overlap

This dates way back, and I’m unsure of where I heard it first, but 80% overlap between images in a photogrammetric block with a nadir viewing geometry is an old rule of thumb from aerial imaging (here’s a quick example I found from 1955), and carries through to SfM surveying. I think it should likely be a first port of call for amateurs doing surveys of surfaces, as it’s very easy to jot down an estimate before undertaking a survey. For this, we should consider just camera positions orthogonal to the surface normal (see this post) and estimate a ground sample distance to aid us with camera spacing from there.

1:1000 rule

This has become superseded in recent years, but is still a decent rule of thumb for beginners in photogrammetry. It says that, in general (very general!), the surface precision of a photogrammetric block will be around 1/1000th of the distance to the surface. Thus, if we are imaging a cliff face from 30m away, we can realistically expect accuracy to within 3 cm of that cliff. This is very useful, especially if you know beforehand the required accuracy of the survey. This is also a more stable starting point than GSD, whose quality as a metric which can vary widely depending on your camera selection.

Convergent viewing geometry

Multi-angular data is intuitively desirable to gather, with the additional data comes additional data processing considerations, but recently published literature has suggested that adding these views has the secondary effect of mitigating systematic errors within photogrammetric bundles. Thus, when imaging a surface, try and add cameras at off angles from the surface normal in order to build a ‘strong’ imaging network, to avoid systematic error creeping in.

Shoot in RAW where possible

Whilst maybe unnecessary for many applications, RAW images allow the user to capture a much great range of colour within an image, owing to the fact that colours are written on 12/14 bits rather than the 8 of JPG images. Adding to this, jpg compression can impact the quality of the 3D point clouds, so using uncompressed images is advised.

Mind your motion

Whilst SfM suggests that the camera is moving, we need to bear in mind that moving cameras are subject to blur, and this is sometimes difficult to detect, especially when shooting in tough conditions where you can’t afford to look at previews. Thus, you can pre-calculate a reasonable top speed for the camera to be moving, and stick to that. We recommend a maximum of 1.5 pixels in GSD over the course of each exposure given the literature and as advised by the OS.

Don’t overparameterize the lens model

Very recently, studies have suggested that overparameterizing the lens model, particularly when poorer quality equipment is being used without good ground control, can lead to a completely unsuitable lens model being fit which will impact the quality of results. The advice – only fit f, cx, cy, k1 and k2 parameters if you’re unsure of what you’re doing. This is far from the default settings in most software packages!

Conclusion

I had a few more points in my long list, but for now these 6 will suffice. Whilst I held back on camera selection here you can read my previous camera selection post for some insight into what you should be looking for. Hope this helps!

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