Understanding Contours on a Map
Contours form the backbone of your map. They provide context to the features on the page and enable you to visualise the shape of the landscape in 3D. In an ever changing world, contours are a reliable friend and remain consistent. Woodlands will grow and fall and fences and walls will be endlessly built and broken down. Paths will be formed and eroded and streams will come and go with the seasons but the shape of the land will remain the same, constant and unchanged - unless you're talking geololgical time - but none of us will last that long anyway.
Contours can be a little hard to understand at first and will occasionally throw you off track but when you figure them out, you can rely on them during the worst of conditions in all seasons, rain, snow or cloud. Occasionally, they might be all you have to work with and when they are, you'll be glad to have put the work in before hand. Hopefully this blog will give you a basic understanding of what is going on.
It takes time to learn to read contours. To read them, it helps to understand a little bit about them.
• A contour is a linear representation of the shape of the ground at a specific height above sea level, either in metres or feet. In Britain, we use metres (m).
• Each contour line is called an interval.
• Each interval usually represents an increase of 10 or 15m of height, depending on which map you are using.
• The interval represents the shape of the ground only at the height of each contour and any detail in between won't necessarily show on the map.
• Height will be indicated within the contours as well as on summits and significant peaks.
The most basic rule when reading contours is that the closer they are, the steeper the ground is.
The more more spread out they are, the flatter the ground.
Understanding the contour detail a route passes through helps us visualise what kind of terrain we are entering and this in turn can help us avoid danger.
If your intended route intersects lots of closely grouped contours at a right angle, you know you're in for a steep climb directly up or down a slope.
If it crosses lots of closely grouped contours at a gentler angle, you'll be taking a less direct route through steep ground. (Think zig zags)
'Contouring' is when a route maintains roughly the same height, keeping level with a contour line across or around a landscape feature.
Another key piece of contour recognition is whether they are showing you a hill/slope/ridge type feature or a gully/valley/stream type feature. It can help to think of the contours as flowing downhill from the summit, and a simple way to visualise these contour features could be as 'innies' or 'outies'. A hill/slope type contour will curve outwards from the summit, while a valley/gully type contour will curve inwards.
It is really important to remember that contours only represent the ground at a set height. On OS maps it is 10m and Harvey maps, 15m. What this means is that the space in between two contours can have a lot of detail that won't necessarily show up on your map. To put that into context, a two floor house in the UK is probably around 10m high. If there was a hill/slope about the height of a house that was on a level at 100m height, that hill might not show up on your map! In reality, you will be able look at a hill, slope or feature of about that size and take it for granted that a contour line would run somewhere on that feature. It might be near the bottom, middle or top and the contour would only represent that particular section of the feature.
Another really key piece of information to look for within contours is rocky terrain. Ordnance Survey and Harvey show this information quite differently. Steep contours + rocky terrain = potentially dangerous craggy ground.
It can be difficult to make sense of contours and for me, helping people learn to visualise them is one of the harder aspects of teaching navigation.
When you are next out on a mountain walk and stop for lunch, get out the map and try to match it to features on the ground and vice versa. It's also a great way to just sit and experience your surroundings. It is interesting how much you will learn about an area just through looking at the map.
As an exercise, try to imagine pouring a giant bucket of paint over a mountain and visualise how the paint would flow downhill from the top. It would run down the outside of the slopes and into the gullies. That's how I see the contours 'flow'. If the line of paint stopped perfectly level right around the mountain, and the line was traced, that is how the contour for that height would look on a map.
Another visualisation is to try and imagine draping a giant table cloth over a mountain so that it was perfectly centred about half way up. Think about how the cloth would cling the shape of the ground right around the mountain. It would hug into valley features and around the hills and slopes. Again, the rim of the cloth would be how the contour line is shown on the map. If you had another one little higher up, that would represent the next contour and show the shape of the land at that height.
Something you can do at home is to use a mapping app such as Viewranger and Google maps 3D imaging together. Look over the same mountain both in 3D and on a topographic map and compare the two. I do this regularly when planning to visit new areas to walk in.
I think this is enough to get your head around for one lesson! Let me know if you need any help or would like to come out for some learning on a course sometime. Feel free to send an email.