Any Scale Bar Will Do Part II

Forensic investigation of vehicle collisions typically involves road surfaces and both Simon and José will practice photogrammetry from time to time in real world conditions. Closing a road “just because we like to practice” does not go down well with local residents but thankfully there is a closed section of road near to where Simon lives

Thats the good news. The bad news is its right next to an active airport and is used by the military to train drivers – but not at weekends – the place is empty. With permission from air traffic control this area is a perfect place to fly a drone and survey tarmac, road junctions and roadside vegetation etc.

Somewhere under the snow is a road

Road collisions can happen in the worst conditions. Snow normally accounts for a lot of low-level accidents but with reduced speed most do not require investigation. Nevertheless its still good to know what works, what needs technique, and what is just impossible.

Snow is rare in the south of the UK – maybe just one or two days a year – so after a fresh fall Simon thought it perfect time to get some practical experience in a scene full of soft snow and harsh lighting.

No Scale Bar? No Problem!

The military had conveniently left behind some ISO containers. Anything with a known dimension will work as a source for a scale bar so why not scan the container as well as a snow-covered road? Unscaled models are fine for VR game assets but real world scaling adds far more value so it makes sense to work with this mindset.

ISO containers with known dimensions. Not at all interesting but great training aids

Two short flights in the cold were enough to cover the container and a good section of the snow covered road. The first lesson was a reminder bright snow will fool the camera light meter and under expose everything by 2/3rds (or thereabouts).

The second lesson was the dark shadow cast by the container itself. Feature and tie point detection was sparse in this area and this is reflected in the overall confidence view of the mesh:

The harsh shadow extending north east cast by the container results in lower mesh confidence

ISO for Scale

The great thing about standards is their predictability. Manufacturing tolerances aside an ISO 20′ container has very predictable dimensions – good enough in this instance use them as reference values for our scale bars.

The result is never going to win awards but that’s not the point here. Practicing the art of photogrammetry under all conditions builds skill and knowledge, something that gets passed on to students in our courses.

Early February will see the release of our Metashape Professional Edition for Forensics & Surveying online course. One police force here in the UK has felt confident enough to sign up their forensic vehicle collision examiners so our deadline for finishing the content is fixed.

Naturally its endorsed by none other than Agisoft themselves, for which we are truly grateful – more news on this course will be published soon.

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