What is a Slope Aspect and how can I use it in Navigation?
AKA Slope Angle
The aspect of a slope is the direction it takes. Imagine rolling a ball down the slope. The route it would take down is the fall line.
The direction of that fall line is the slope aspect or slope’s direction.
To work out the slope aspect, keep your compass horizontal and point the direction arrow in the same direction as the fall line. You have to imagine the line your ball would have taken. Twist the bezel until the North Needle is in the North indicator lines in the bezel, or to remember it ‘Put Red Fred is in his Bed’.
THE PART EVERYONE FORGETS…
Now read off the bearing from the index line. This is the slope aspect in degrees magnetic.
Subtract the current Grid Magnetic Angle for the area you are in. e.g. For the Peak District in 2019, it is approx. 1.5 degrees, or MINUS ¾ of a notch. Now adjusted, you have a grid bearing for use on the map.
To calculate your position on the hill, lay the compass on the map with the direction arrow facing downhill. (Contour line numbers going down). Now don’t touch the bezel, leave the bearing set.
NOW USE THE BEARING TO FIND OUT WHICH PART OF THE SLOPE YOU ARE ON…
Move the whole compass over the map in the area you are in, until the index lines in the bezel are parallel with the grid lines on the map. (Look through the bezel to the map, and twist the whole compass about to line them up, don’t’ move the bezel).
Keeping the lines parallel, slide the compass through the area on the map where you think you may be.
Fixing your position
Where the contour lines perfectly cut across the baseplate at 90 degrees (perpendicular) is where you could be. That particular slope has an aspect matching the bearing you took.
On a curving slop this could give you a fix as only one section is likely to have that exact slope aspect.
Curving slopes work best as there is only likely to be one part with that unique aspect.
Use an altimeter to determine which contour you are on and when combined with the aspect can give you an exact fix.
Remember to rest your altimeter every time you reach a spot height or trig as pressure changes throughout the day; or use a GPS/Sat Nav altimeter with at least 4 satellite fixes to determine your altitude exactly.
Why not book a navigation course now?
The Ultimate Navigation School is a charity providing navigation training to hill walkers, with all our net profits supporting the following charities:
Mend Our Mountains, Fix the Fells, John Muir Trust and Mountain Bothy Assoc.