Measuring the density of a random lump of soil sample is not as easy as you might think, according to New Mexico State University.

NMSU Brungard-soil-sample-photogrammetry

Weighing it is simple – use a set of scales.

But turning this weight into density requires the volume of a formless lump to be measured – you might need to know that the lumps soil naturally crumbles into are called ‘peds’.

Traditionally, apparently, volume measurement required carefully coating the ped in wax so that the amount of water it displaced could be found. Laser scanning is a newer, if expensive alternative.

With an idea that photogrammetry – combining simple photos mathematically – might be a cheaper alternative to lasers, New Mexico researcher Colby Brungard presented the ped volume problem to a group of students.

“When I gave the guest lecture, I explained my idea and asked the undergraduate students if any of them had an interest in taking on the project,” said Brungard. “Michael Whiting volunteered, and did the research.”

Whitting used a phone as a source of high-resolution photos, and 3d-printed a hand-cranked turntable and phone holder to control distances and angles accurately.

“When they compared the smartphone system to laser-scanning or wax-dipping, the final measurements were nearly identical. That accuracy proved true across five different types of soil peds,” according to the university. “It took just 15 minutes per sample, while a laser scan can take an hour and a half.”  This said, off-line computer time was needed to analyse the photographs but “nonetheless, the method was simple, fast, cheap and accurate”.