Steep hills and gradients.

[FastBarry]

When I first started driving, hills were measured on road signs as roads having a gradient of "1 in 4" or "1 in 6" etc.  I knew what that was all about

It was changed some years back to a %. i.e. 20% gradient or 60% gradient. I never did understand what that was all about, I think it was meant to be easier.

So, what does it mean. Which is steeper, and what is it a percentage of ?

[Jay70]

I know what you mean Fastbarry.  I think to convert you just change the percentage to a ratio and put it in its lowest form. i.e 10% would be 1 in 10.  20% would be 1 in 5.  25% would be 1 in 4 etc. 

[rja211077]

spot on Jay70, that's exactly how it works. As for the reasoning or merit behind it, I don't know.

[Sequin]

It tells you the change in height over a fixed distance ie a 1 in 4 would be a rise of 1 metre every 4 metres horizontally, or a 25% gradient.

100%, or 1 in 1 would be a rise of 1 metre for every horizontal metre, or a 45 degree slope

[FastBarry]

Sequin, if I am thinking right, wouldn't a 100% gradient be a vertical cliff face ?  As I understand it, a "1 in 4" would be 25%. So wouldn't a "1 in 2" be a 50% or 45 degree hill ?   If not what would a vertical climb be.

[BillyBB]

1 in 1 would be 50%

1 in 0 is 100%

Personally I think the old way was much clearer and this thread illustrates that too!

[morg_monster]

I disagree with Billy...
The ratio method is the ratio of the vertical distance to the horizontal distance.
The percentage is just the same ratio expressed as a percentage.
1:4 = 25% because you are going UP 1/4 = 25% of the horizontal distance
1:3 = 33% because you are going UP 1/3 = 33% of the horizontal distance
1:2 = 50%
1:1 = 100% because you are going UP the same amount as the horizontal distance. This is a 45 degree slope.

You can carry on: say for every 1 unit horizontal you went 2 units vertically then the gradient would be
2:1 = 200% because you're going up twice as far as across.

A 90 degree slope (ie vertical cliff face) would thus be a gradient of infinity:1 or infinity%

just like a perfectly flat road has a gradient of 0:1 or 0%

[Sequin]

Morg_monster is correct - a vertical slope has an infinite gradient = you can climb as far as you like, up to infinity, before you travel a metre horizontally