UKH

Hi All,

Am determined this will be the winter where days climbing are actually remembered for climbing and not the fear of straying into a thirty five degree slope.

Am becoming a bit of a geek about it.

is there any kind soul out there that can see me dusting off my calculator and tell me how big the index contour gaps would be on a 40k Harvey map for a thirty degree slope?

forty five would be about 1.9mm, right? As the contours are 15m so the fats are 75m?

thanks in advance btw

I reckon 75m contours on 30deg slope will be 3.25mm spacing on plan at 1:40,000

How about:  https://shavenraspberry.com/shop/navigation-aids/navigators-slope-angle-too...

Let's take this from first principles (for the benefit of anyone reading...).

We can use the map scale to find out how many millimetres are used to represent a given distance on the map.

Let's start with 1000m, or 1000000mm.

With a map scale of 1:Sk (where S is, for instance, 25, or 40, or 50), we divide the given distance by the scale factor:

Representation = distance/scale
= 1000000/S.1000
= 1000/S

So, for a map scale of 1:25k, 1km is represented by 1000/25 = 40mm

Which we know is correct.

More generally, then, for a given distance, d (metres), and a given map scale 1:Sk, the representation in millimetres is easily found by

r = d/S mm

So, the 15m contour spacing with a 1:40k scale would give

r= 15/40 = 0.375mm

Of course, that's the vertical distance, and we want the horizontal distance, so we need to use some simple trigonometry:

tan(angle) = vertical/horizontal, so
horizontal = vertical/tan(angle)

Your 15m contour spacing, at 30 degree slope, on a 1:40k map would thus be represented by a contour spacing of:

r = 15/40.tan(30) = 0.65mm

The thick (75m) contours would be five times this, or 3.25mm

As you say, for 45 degree slope, it would be 75/40.tan(45) = 1.875mm

tl;dr;

Contour spacing for a given angle is separation/scale.tan(angle)

I wrote a bit of PostScript code to plot little tools to measure slope using contour spacing (and print to transparency). It is fully configurable to whatever map scale and contour spacing you want...

If anyone would like a copy, send me an email address.

Awesome, now can you teach me how a derivative works? I’m late on my homework