In the first part of this series we took a brief look at what the different scales of map mean. In Part 2 we’ll try to understand what contour lines can tell us and how they can help us to find better, safer skiing.
Maps are a bird’s-eye view of the world, turned into symbols and printed on paper or a screen. They have to show a 3-D world on something that is just 2-D. All mapmakers have to choose somewhere, or a datum, to measure their altitudes from. They can then draw lines to join places that are the same height above this datum. These are contour lines. If you’re walking along a contour line, you’re neither climbing nor descending.
In order to unlock a map, we need to have a look at its key. One of the problems of using online maps is that it can be hard, or even impossible, to find the key. The symbols that mapmakers use are fairly standard around the world. But there can be differences or something unusual that we haven’t come across before. The key will also tell us what the vertical interval (height difference step) is between the contour lines. This can vary from country to country, scale to scale and map to map. Each of the two maps required for the BASI Level 4 EMS assessment in La Grave have the same scale but they have different vertical intervals for their contours.
One of the best avalanche safety tools that we have is to choose to ski on the right terrain, at the right time. Maps can tell us the aspect of the slope, whether it faces north or south and also how steep the slope is. If the contour lines are bunched together, the slope is steep. If the contour lines are spread out, the slope is flatter. Exactly how steep depends on the spacing of the contour lines, their vertical interval and the scale of the map. As we mentioned in Part 1 some maps that are designed for winter use, shade slopes over 30° in pink. It’s very hard to estimate by eye but there are cheap plastic cards available that help with this task.
Good places to use the map to locate yourself when skiing are where the slope quickly becomes flatter or steeper. The area where the spacing of the contour lines changes abruptly should be the area that you are in.
In the picture on the left, the widest (blue and yellow) stripes are the flattest places. The narrowest, pink stripe is where the slope is steepest. The join between each colour is where the contour lines would be. The model also shows a slope which starts as a convex roll and becomes concave at the bottom. This is a shape that we generally want to avoid when we’re skiing. The convexity creates extra stresses in the snowpack (which might release as an avalanche) and also makes it impossible to see what terrain we’re committing ourselves to ski down.
A good way of avoid slopes like these is to ski down ridges until we can see into bowls, valleys or couloirs properly. Smaller ridges are called spurs and smaller valleys are sometimes known as re-entrants. In the left-hand image below there could be a ridge, going downhill from left to right. Once some extra information is added, we can see that there is actually a valley, sloping downhill from right to left.
Information that tells us which way is uphill or downhill can include rivers & streams, black lines showing cliffs and the labels on index contours. Index contours are thicker than ordinary contour lines. Depending on the map, you might get an index contour every 50m or 100m. Where there’s space, mapmakers will label the height of the index contour. In most countries (not Switzerland), it’s the convention that if you’re reading the index contour label the right way up, you’re looking uphill.
Interpreting and visualising the shape of the land from contour lines is an acquired skill; just like a good short turn or skiing bumps. A great way to practice your skills is to get a map of the area where you ski regularly. Ski the pistes and stop at obvious changes in slope steepness or shape and try to work out where you are. If you’d like to learn more about navigation in the mountains, Mountain Training UK’s “Navigation in the Mountains” by Carlo Forte is the best book to get. In the next article, we’ll take a look at how to use your compass.
“Regular maps have few surprises: their contour lines reveal where the Andes are, and are reasonably clear. More precious, though, are the unpublished maps we make ourselves…”
Alexander McCall-Smith, “Love Over Scotland”
This is part 2 of a four part series by Dave Roberts. Find Part 1 here, and check back next week for Part 3!