Techniques > Navigational Questions & Answers

Getting a fix using the angle of the slope

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Hugh Westacott:
Surely, the most important principle for someone new to navigation to learn is that the closer the contours, the steeper the slope. It doesn't take very long to learn the pattern of contour lines that indicate that a slope is too steep to be attempted. This will, of course, vary according to age and fitness of the walker. I don't see the point of translating this into a slope angle expressed either in degrees or as a percentage or fraction.

Nobody has yet come up with the answer to the conundrum I posed. If you do not have an altimeter, Lyle suggests on page 154 of his book [that], by measuring the slope angle that you are on and relating this to the contour spacing on your map, you can determine your elevation.. Frankly, if this is all the information that is available, I cannot see how this can be relied upon to give an accurate position fix.



I think one key point is that, assuming its the same card, its for 10m intervals as used in the more hilly areas. I live near the West Pennines where the interval is 5m (on the 1:25k) and therefore different to the norm.....not sure if its because its classed as Urban or its the smaller hills.

The description on the web site was updated to note this.

Does one need to know the Angle, if he knows the Slope to be, say H= 50, D= 120, therefor Slope, or Incline, is 5 in 12?  The Angle,in this case, is arctan 5/12,which is between 22 & 23 degrees. Railways & Roads use this method, the Gradient. Just another way of expressing the same information. 5/12 in the Tangent Tables is 0.41667. Finding 12 out & 5 up on a Protractor, & extending a line to the rim, will give a fair result, which is how Quadrants were put to use, in centuries past. If you have other tools, put in the time to master them, because nothing as as simple as it first seems, & the more subtle aspects only show themselves to the diligent student.

Pete McK:
I agree with CP and measure the slope to confirm other techniques, such as slope aspect and usually checking the slope in more than one area.

The area I live in the Lake District is area prone to inclement weather and sudden poor visibility and I have found using the slope angle card in this way works well for me.

Hugh Westacott:
John wrote:

<I live near the West Pennines where the interval is 5m (on the 1:25k) and therefore different to the norm.....not sure if its because its classed as Urban or its the smaller hills.>

The Ordnance Survey has strict rules on whether the contour interval should be 10m or 5m on 1:25k maps. The fundamental basis is clarity and ease of use. In areas where a 5m interval would make the map too cluttered, the 10m interval is employed. Thus, there is no such thing as a 'normal' contour interval; every sheet uses either a 10m or a 5m contour interval. Great Britain is covered by 470 Explorer sheets. I don't know how many there are in each category but I do know that below the 350į national grid  easting just south of Nottingham, virtually all of the approximately 90 Explorer maps covering that part of England have a contour interval of 5m. The exceptions are Dartmoor, Exmoor, the Welsh Marches and few small, isolated areas such as the Shropshire Hills. North of Nottingham there must be many more on both the east and west coasts of England. I suspect, too, that there will be at least some sheets with a 5m interval along the east coast of Scotland and also in the northeast.

I have number of books on maps and navigation and am astonished how many of them seem to assume that navigation in lowland areas is so easy that it is not worth covering the special techniques that are used. Navigation in the south of England is often more difficult than it is in good conditions in upland areas. This is because you are normally required to follow rights of way which, in little-walked areas, may not be waymarked or even visible on the ground. The most important feature is the field boundary, not contours (it would take a genius to navigate across the East Anglian fens or the Somerset Levels relying entirely on contours!), so a 50k map is not much use. It's most unlikely that you would come to any harm if you got lost because you will not be far from a road or habitation, but it can be a tricky exercise especially in popular waking areas where there is often a maze of unofficial paths. When I was writing footpath guides, I used to survey and navigate my chosen routes using the Outline Edition of the Pathfinder map (the 1:25k predecessor of the Explorer). It was monochrome and omitted contours which made it easy to follow field boundaries and to annotate in the field.

The correction on Lyle's website now states:

Alternatively, by measuring the slope angle that you are on and relating this to the contour interval on your map, you can determine your elevation.

Sorry, I must be dense because I cannot see how, if all you have is the map, the angle of the slope and the slope angle tool, you can determine your elevation. Surely, you must have something such as a linear feature that is marked on the map that will establish the exact line of the route that you are following. I donít believe that walking on a bearing would be work because of the inherent inaccuracy of this technique, especial when walking on a steep incline.

Incidentally, nobody has mentioned that the contour interval on Harvey maps is 15m with a supplementary contour where considered necessary.



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