Slope Aspect

Slope Aspect

Assessing the slope aspect is a means by which you can locate yourself in poor visibility

(©Crown copyright and database rights Ordnance Survey licence number 100054073 2013)


Slope Aspect 

Slope Aspect


In situation where there is a lack of clearly identifiable land features against which to take a bearing, you may need to analyse what’s called the ‘slope aspect’ and take a bearing on that. Compared to sighting a bearing on a prominent land feature, taking a bearing on slope aspect may seem a little wishy-washy. However, the technique is perfect for times of poor visibility when nothing else is visible.

So, picture the scene: you’re standing close to a slope but you’re unsure about your precise location. Imagine you had a heavy beach ball that would not be impeded by any rocks or blown off course by the wind. If you put that ball in front of you and gave it a push, in which direction would it roll down the slope? This is called the 'fall line'.

       
         ©Crown copyright and database rights Ordnance Survey licence number 100054073 2013

  • sight your compass on the route that the ball would take, and that is a bearing on the slope aspect. (Tip: the ball will go straight down hill and not diagonally!)
  • once you’ve sighted the bearing, subtract magnetic variation - you now have a grid bearing
  • place the compass on your map
  • align an orienting line on the compass housing with a vertical grid line, ensuring that the orienting arrow is pointing to the top of the map (north)
  • what you’re now looking for on the map is to identify a slope that matches the bearing
  • a matching slope will be one where the compass base plate is at 90° to the contour lines of the map, i.e. straight down the slope and not diagonal
  • it may take some trial and error to find the slope
  • scrutinise the map to ensure that there are not two slopes with the same bearing 
  • if you've followed the feature tick list recommendations, you should know your location minutes since, thus narrowing down the possible slope with the signed bearning


This Process has limited use if you’re walking in poor visibility along a long hill with the same slope aspect. However, there will come a point where features or angles of slope change, giving you an opportunity to take a bearing on such newly emerged features.

Like all other navigation techniques, it take practice to make perfect, so learn to do it when you know where you are and have good visibility.


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