In a story worthy of any quiet Friday afternoon, certain UK news organisations and social media users lost their minds recently over a picture of a “giant” rat found near a playground in Hackney, East London.
The rat, claimed to weigh more than 25 pounds (11kg), was found by a pair of tradesmen before the story and picture were distributed to news organisations by UK news agency SWNS.
“This is the largest rat I’ve ever seen in my entire life,” said gas engineer Tony Smith, who took the photo. “I’ve got a cat and a Jack Russell and it was bigger than both of those put together.
“I’d say it was about four foot (120cm).”
Let’s have a look at this rat then.
So, this giant rat was found in north London near a playground. Said to be bigger than a four year old child. Ewwww pic.twitter.com/1byR5mF2jF
— Kay Burley (@KayBurley) March 11, 2016
But four foot? Let’s think about this.
If the happy gentleman holding the rat up to the camera– a Mr. James Green, according to the story – is six-feet (180cm) tall, the rat would be as long as the distance from his head to his knee. Which it certainly appears to be, but only with a complete disregard for the role perspective plays in how large the rat appears.
Oliver O’Brien, a senior researcher and data scientist at University College London, has thrown the weight of physics behind his estimate that the rat is probably about two feet (60cm) long.
The “giant rat” is two foot rather than the “four foot” claimed in the press. Perspective trick. Here’s my working: pic.twitter.com/DYGE3hPDSz
— Oliver O’Brien (@oobr) March 11, 2016
“You’re adjusting for the length of the man, which I’ve done as a [yellow] line, and you’re adjusting it twice to account for the fact that the rat is much closer to the camera,” O’Brien told First Draft. “To do that scientifically you have to have equivalent measurements for the same distance away from the camera.”
He broke down his calculations for First Draft.
There are three planes of perspective to the photograph: there’s Green himself, the hand holding the grabber, and then the rat. Each are a different distance from the camera.
“If you look at the picture, not only has he pointed the grabber forward… but actually the hand that the other end of the grabber is in is also much closer to the camera than the man himself,” said O’Brien. “So there’s a double trick going on here.”
The yellow line represents Green’s height, an estimate of six feet or 180cm, and converting the length of the yellow line to pixels allows us to figure out the ratios between the different planes of perspective.
Green’s left hand is the same distance from the camera as his body, but his right hand is closer. Assuming the fingers on each hand are the same length, the difference between the two purple lines will tell us how much bigger his fingers appear in the second plane, where his right hand is.
The rat is closer to the camera again. So using the straight line of the grabber and the difference in its width in pixels at the point of his hand and at the point of the rat, we can again estimate the third, closest plane of perspective, which holds the rat.
“You can assume those red lines in real life are parallel to each other, but of course, because of perspective they’re not parallel,” continued O’Brien. “So it gets larger according to the camera. But actually the distances in real life are the same.”
Taking the length of the rat in pixels, represented by the green line, we can compare the different ratios to calculate the length of the rat, based on the assumption Green is 180cm tall.
Still with us? Here’s the maths:
180cm x 0.814 (the ratio of Green to the rat) x 0.615 (the ratio of left hand to right hand) x 0.667 (the ratio of right hand to rat) = 60.1cm
“It’s all very rough and ready and the measurements are very imprecise,” O’Brien said. “But my estimate is in the right ball park. It’s probably between 1.5 feet and 2.5 feet, but it’s certainly not two-thirds the height of the man.
“If it really weighed two stone (11kg) there’s no way he would be holding it in that position in the grabber, even if he was a very strong guy.”
Residents of Hackney are understandably alarmed, but Dr Dougie Clarke of the University of Huddersfield believes the rodent to be an African Pouch Rat, which are often kept as pets.
“These are highly intelligent and extremely large rodents – so big they are often kept on a lead,” he told the Daily Mirror. “They are very popular as pets, so this one may have escaped from a back garden nearby.”
Thankfully, Hackney Council have seen the funny side but are looking into the matter anyway.
— Hackney Council (@hackneycouncil) March 11, 2016
First Draft contacted Green’s employers to try and get a first-hand account of the monster rat but were told both he and the man who took the photo were out on assignment, and shouldn’t be speaking to the press anyway.
In this instance, it seems a lot of people could take a lesson or two from Father Ted about perspective.