Squiggly Brown Lines Andy Callan continues his series on navigation skills with the ups and downs of contours
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irst things first, “Contours are imaginary lines connecting points of equal height above sea level”. Quite a few years back I was running part of a Mountain Skills Course on Scarr, above Lough Dan in Wicklow. In the early part of the training a lot of time is spent asking people to describe how the contours are shown on the map and relating that to what’s happening on the ground. Things were going very well, or so I thought until one of my clients fell in alongside me and an eerie silence followed – the sort of quiet when you just know that somebody’s about to come out with a real beaut . “Andy, you know these contour lines we’ve been talking about?” “Yes” says I, half dreading what was coming. “Well, next time we cross one, can you point it out?” I didn’t have the heart to crush his enthusiastic innocence; any of you who are familiar with Scarr will know there’s an old field boundary running around the top of the hill. As luck would have it we were within 100m of it and it was on our line of travel. So, as we crossed it I pointed it out – at this point it actually follows the line of a contour – he was delighted and it helped get my point across. Only problem is there’s still someone out there who thinks some contours actually exist... Contours are a by-product of an experiment by Nevil Maskelyne to determine the mass of the earth which took place on Schiehallion in the Central Scottish Highlands in 1774. This involved taking hundreds of measurements of the deviation of a plumbline caused by the mass of the mountain, at differing heights all around the mountain. In order to compile these findings, the measurements were plotted on a map and lines connecting points of equal height were found to give a “bird’s eye” view of the mountain. (See figure explaining Perspective/Elevation Plan). The closer the gap between the contours, the greater the detail shown. On all Irish OS maps, contour lines are spaced at intervals of 10m with a heavier index contour every 50m. Other privately produced
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maps i.e. EastWest or Harveys may use a different contour interval (usually 15 or 20m), so always check the marginal information beforehand. These maps may also include form lines – a bit like “half contours”- to show significant details that fall between contours and would not otherwise be shown. Almost all maps use interval shading where the background colour changes as you gain height, this starts with green, changing to beige and becoming progressively darker to brown as you reach the highest tops. Obviously, well spaced contours show relatively flat or gently sloping areas, whereas densely packed contours indicate steeper slopes. Irish OS maps do not indicate cliffs with a separate symbol unlike their northern counterparts, so recognising contours that touch, or almost touch each other as areas to be avoided is vitally important. The great thing about contours is that they don’t change. Almost every other feature on your map rivers, roads, buildings, and especially forestry may shift or get knocked down, but barring meteor strikes or thermonuclear war, contours remain constant and ultimately define your location. Get into the habit of building a mental image of the ground between you and your intended target - are you going up or downhill, cutting straight across the contours or traversing them at an angle? Should the high ground be on your left or right? If you decide that you’ve reached point X on the map, then the contour detail should confirm it by matching the shape of the ground. You can’t pick and choose the things that back your choice of location and ignore those that just don’t fit. Either it all matches and you’re in the right spot, or it doesn’t. This leaves you two options (a) you misinterpreted the information in the first place or (b) you’re in the wrong spot! To help develop a 3D picture of the common contour patterns, I like to think of a feature having 4 slopes – front, rear, right and left. In all cases think of what they would look like if you were looking uphill.