Measuring the distance between 2 points - challenge

[Caerleon]

Thought I would post this challenge, it was something on a test paper I did years ago in the army. So here it is, you want to measure the distance between 2 points, lets use a real example here - there's a tree on one side of a river you want to cross & need to get a line over the river, therefore understanding the width of the river would be useful.

The kit provided to measure the distance is: 4 large ground stakes & 12 meters of rope (clue: this rope is not meant for going across the river).

Presume there'll be some who know this straight away but its an interesting problem for those that don't.

Happy solving

 

[widu13]

Simplest method- crow into the water with rope, across water, measure rope.

 

[Caerleon]

1 out of 10 for effort

 

[hughlle1]

trigonometry?

 

[John Fenna]

Three, Four, Five ....

 

[bopdude]

If the ropes a given length then as John says, the 3,4,5 method could be used as long as we're not getting into cm and mm's

Sent from my SM-G900F using Tapatalk

 

[widu13]

Step measuring will do it, but you don't need a rope or stakes. The crow is still the easiest way.

 

[bopdude]

Step measuring will do it, but you don't need a rope or stakes. The crow is still the easiest way.

OK, I give up, what is the crow method ?

 

[mousey]

Are you on the same side of the river as the tree?

Edit///

I'm assuming you don't want to get wet in order to measure the distance either...

 

[JohnC]

I think the tree is a red herring... I recall someone doing this on a TV show, it involved trig and geometry....

 

[mousey]

I think the tree is a red herring... I recall someone doing this on a TV show, it involved trig and geometry....

Having a fixed landmark on the opposite bank would be helpful.

EDIT///

Oh, and also having something to measure angles with would be handy.

 

[dewi]

is each side of the river parallel?

 

[JohnC]

oh yes, to get a straight line.. Im thinking of the height of the tree! doh!

 

[mrcharly]

I think the solution involves 3,4,5, lining up stakes with a tree and remembering that a right-angled triangle containing one 45degree angle has two sides of the same length.

 

[dewi]

If both sides of the river is parallel, it should be easy.

Directly across from the tree, stick a stake into the ground.

Fold the rope into 3 equal lengths so each is 4 metres... lay one end of the rope to the stake you've just put in the ground.

Walk away from the stake along the river bank until you get to 2/3rds of the ropes length. Put another stake in the ground. Carry on walking away from the first stake and the second for the remaining 1/3 of the rope and put in another stake.

Now walk away from the river bank at 90 degrees to the last stake you put in... and when the tree and the 2nd stake are in line, put another stake in the ground.

Using the rope, lay it at the 3rd stake.... take it across to the 4th stake. Whatever that distance is, the river is double that distance... and knowing the rope is 12m, you can fold it to get measurements of 6m, 4m, 3m, 2m, 1m and so on so you can accurately measure between stake 3 and 4. Double that measurement. You now know the distance across the river.

 

[mousey]

I don't see why you'd need parallel banks to do what you've describe there dewi?

Neater than my idea and doesn't need as much maths to accomplish or a way to accurately measure angles. [apart from 90]

 

[widu13]

OK, I give up, what is the crow method ?

A crow (sprog) is an Army term used for the (usually) youngest in age and service. They are the one's given the "choice" jobs. Currently a particularly choice job is to test for chemical agents by unmasking (taking off their gas mask) first whilst everyone else "observes them for signs and symptoms" of the agent involved.

Therefore, the crow method is always the easiest method.

 

[Caerleon]

Dewi your just about there in the logic and also those that stated trigonometry:
Put a stake in the ground opposite the tree - using a 3/4/5 triangle from that stake march out a given distance (say 10 paces) at exactly a right angle to the first stake), place the second stake in the ground then march a further 10 paces place the 3rd stake in the ground, 3/4/5 triangle again march away from the bank at right angle to the 3rd stake. Once you see a direct line between you the second stake & the tree then place the final stake in the ground. The distance between the 3rd & 4th stake is the distance between the 1st stake & the tree.

 

[dewi]

A crow (sprog) is an Army term used for the (usually) youngest in age and service. They are the one's given the "choice" jobs. Currently a particularly choice job is to test for chemical agents by unmasking (taking off their gas mask) first whilst everyone else "observes them for signs and symptoms" of the agent involved.

Therefore, the crow method is always the easiest method.

Unless you're the crow

 

[widu13]

Dewi your just about there in the logic and also those that stated trigonometry:
Put a stake in the ground opposite the tree - using a 3/4/5 triangle from that stake march out a given distance (say 10 paces) at exactly a right angle to the first stake), place the second stake in the ground then march a further 10 paces place the 3rd stake in the ground, 3/4/5 triangle again march away from the bank at right angle to the 3rd stake. Once you see a direct line between you the second stake & the tree then place the final stake in the ground. The distance between the 3rd & 4th stake is the distance between the 1st stake & the tree.

Also known as Step Measuring.