Not totally serious & boring here, but not continuing in the vein of the puns either, how did they do that? If I wandered off into the local fields & tried to make any snow picture that could only be seen from the air, I bet it would turn out rubbish as I wouldn't be able to see the shapes to know if they were correct
So I was thinking if you make something that big on the ground, how do you know you've got it right? Do you think someone drew it on a piece of A4, blew it up on a photocopier until it was that big, then laid it on the ground as a template & trampled down the snow / drove a tractor around until they had that "design"
Or was someone in the air (hmm... a heli pilot maybe who could get off in those conditions) & radioing down instructions..."Make the left one a big bigger..." etc
The only other possible explaination I could come up with was that it was truely left by extra-terrestrials who thought it meant something important having seen it several times when they reconnoitered? Obviously their chosen landing site for a sample of eathling culture was the railway arches in Leeds down by the river & canal.
"Hey Zlarg, I wonder what this means?"
"Yes, Fluj, we keep seeing it a lot so it must be important to the earthlings"
"What about we write it somewhere so the earth beings know we mean well"
On second thoughts they would also have drawn a picture of an empty bag with an "M" on it too so that can't be right