I was impressed a few months ago by the unexpected evidence I got from video of a site taken from a drone.  Impressed to the extent that I flew another site a few days ago, but this time looking for the “unexpected” evidence.  The experience confirmed the importance of filming the site of a forensic investigation from a low flying drone.

A few months ago I videotaped a compact site – couple of 100 feet square – for record purposes.  I did this for 35 minutes from several 10s of feet. On looking at the video I saw a feature of interest relevant to the problem I was investigating.  It was not evident when I walked across the site in an earlier visit.

Talking with the property owner later I learned that the feature seen in the video marked a characteristic of the ground that was very important to understanding the problem I was investigating.

A few days ago, I video taped a long, narrow site – 1,500 ft. x 200 ft. – from an altitude of several 100s of feet as well as 10s of feet.  This time, however, I looked for a feature in the video that I had seen on the ground at one end of the long site.  It was relevant to the problem I was investigating.  I found it, then studied the video for the occurrence of the feature elsewhere along the length of the site and noted these locations. This was sort of the reverse of the situation at the compact site.

Put another way, a few months ago I identified a feature of interest in a video, and had it confirmed on the ground.  A few days ago I identified a feature of interest on the ground and had its location elsewhere on the site confirmed with a video.

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This process is called terrain analysis in engineering:

• You identify the different features at the ground surface appearing in aerial photographs,
• note the characteristics of the subsurface implied by the shape and look of each of the different features,
• then assess how these subsurface characteristics relate to one another and to the problem you’re investigating.

For example, how does the strength of the subsurface soils relate to the foundation failure of a building, bridge or highway?  Or the subsurface drainage characteristics relate to the location of fuel oil spilled on the ground some time in the past?

I’ve done terrain analysis for years using aerial photography often taken from 6,000 feet – a long way up.  It has been useful – but not at all, at all like the detail you get from low level drone photography.

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The drone videotaping at the compact and long sites was similar.  Take orientation video from the four sides of the site from a high altitude then close up video of different features at a low altitude.

I’ll study the video of the long site in greater detail then select optimum places on the ground – representative and cost effective – for collecting subsurface data and solving the problem I’m investigating.  The data won’t really be “unexpected”.  I’ll get what I expect to get – the depth to the water table, and water and soil samples for testing.

The unexpected evidence a few months ago reminded me about the value of aerial photography in forensic engineering investigation, and the greater value today of low level, drone photography.  I was also reminded to be on guard against the tyranny of the obvious, to expect the unexpected in forensic work.