NAVIGATION MADE EASY 2: RECOGNISING FEATURES ON A MAP
The last blog in this series dealt with grid references as an effective and safe way of navigating from point to point. It stressed the importance of being able to read a map, identify potential hazards, and plot a safe route accordingly. This blog looks at features as an essential part of route planning.
The first part –Identifying Features looks at how features are represented on a map. The second part – Navigating using features –looks at we use features to plan a route and navigate “on the run.”
The blog will look at features in and around Loch a’ Duin, an area that is used regularly by members of Tralee Mountaineering Club (TMC) to practice navigation skills.
WHAT DO WE MEAN BY A “FEATURE”?
A feature is a distinctive detail in a landscape that can be easily identified on a map and given a grid reference (plotted). Some features are easy to spot on the the Ordnance Survey Ireland (OSI ) 1:50,000 maps because they are represented by a combination of symbols and colours – a trail (—), a wall (––), a river (––), elevated ground or spot height (•73), a summit (brown circle, spot height, and name where relevant), Special features like archaeological sites (•) are also marked. Some maps (Harvey) have symbols for crags, cliffs, boulders, scree, and other features.
The full list of symbols is given at the bottom of the 1:50,000 map and it is important to be familiar with the key features that are used for navigation in the mountain – tracks and trails, walls, rivers/streams, lakes, spot heights, and summits and so on.
This is the easy bit.
Most features in the mountains are described by contour lines on a map. Contour lines are the continuous lines that represent changes in height in the landscape. Being able to read and interpret contour lines is an essential part of safe navigation.
The section of map above shows contours as they appear on OSI Map 70. The diagram below it shows how contours represent changes in the height and profile of mountains. The third image is a useful representation of the relation between contours and the landscape. Map reading involves using changes in height and profile on the map to identify features in the mountains and vice versa.
This video by Steve Blackhall of the Ordnance Survey office in Great Britain is a good introduction to the contours, just be aware that the maps used in the video are British maps.
NAVIGATING WITH CONTOURS PART 1: IDENTIFYING FEATURES
Navigating with contours involves thinking about the relation between contours and characteristic features in mountain landscapes. There are two aspects to this, knowing where you are and avoiding hazards. The first has to do with being able to read a map and the second has to do with planning and following a safe route (navigating).
Lets look at the first part – using contours to identify features. To begin, how do you know whether you are
- at sea level or on top of a mountain?
- on level ground or a steep slope?
- on the top of a hill or in the bottom of a hollow?
- in a valley or on a spur?
The answers are fairly obvious: Just look around you. What if
- you are planning a recce and need to plot a route through unfamiliar terrain?
- you are in the mountains and the weather has changed, leaving you with little or no visibility?
Take the following scenario. You are heading into to Loch An Duin from Kilmore Cross (Q52294 08919), intending to climb to An Starraicín (Q52769 06407) at a height of 456 metres (see map below). Visibility is less than 20m and you have to navigate by map. You have reached the edge of a lake but the map shows two lakes, one on either side of An Starraicín.
How do you know whether you have reached Loch an Dúin or Loch Chom Calláin?
Loch an Dúin is approached along ground that lies between the the 100m and 120m contours, rising 20 metres over a distance of one kilometre approx. The contour formed by the edge of the lake is given as 116 metres; the height of every lake is indicated by the blue coloured number on the “surface” of the lake. To reach the lake you will climb 1m in height for every 50m in horizontal distance travelled, a gradient of 1:50.
Loch Chom Calláin is approached along ground that rises from 100 metres to 230 metres over a distance of 1.3 kilometres approx. The contour formed by the edge of the lake is given as 231 metres. The key feature here is middle section of the route. The contours between 130 metres and 230 metres are closer and more or less evenly spaced over a distance of 600m approximately. This means that you will be climbing 1 metre in height for every 6 metres in distance travelled, a gradient of 1:6. This is a very different profile to the approach to Loch An Duin.
The question now is this:
Did you approach the lake by travelling along relatively flat ground or by climbing a moderately steep slope?
This scenario demonstrates how you can use information provided by contours to identify features and confirm your location. This is a fundamental part of navigation. Certain features have characteristic contour profiles that make them easy to identify, incorporate into route planning, or use when navigating “on the run.”
Here are some examples:
Here are some standard descriptions of each of the features shown above. All the features are on routes used regularly by members of Tralee Mountaineering Club (TMC).
Summit / Peak
This is the top of the mountain. It is a point on a surface that is higher in elevation than all points immediately adjacent to it. It is usually marked with a brown circle, a spot height, and sometimes, a name – not all spot heights are summits and not all summits have names, Unnamed summits are usually referred to by the number (elevation) given as the spot height.
a continuous elevated crest running for some distance between summits. The sides of the ridge drop away from a narrow top.
Ground that runs down from a hill to lower ground. A spur is an erosional feature that usually forms the side of a valley. Spurs provide access to and escape from the high ground for mountaineers.
An aréte is a narrow rocky spur which separates two valleys. It is typically formed when two glaciers erode parallel U-shaped valleys.
A vertical, or nearly vertical, rock exposure. Cliffs are a real hazard but can be very useful as a “handrail,” a feature that you can follow, at a safe distance from the edge of course.
The lowest point of a ridge or saddle between two peaks, typically providing a pass from one side of a mountain range to another.
This is the side of the mountain, running from the summit to the foot. Slopes can be convex or concave.
A convex slope is rounded like the outside of an upturned bowl, i.e. it goes from less steep at the top to more steep at the bottom; the space between contours gets narrower as the elevation reduces.
A concave slope is rounded inward like the inside of a bowl, i.e. it goes from more steep at the top to less steep at the bottom; the space between contours gets wider as as the elevation reduces.
a small valley like formation formed by parallel spurs. The low ground between the spurs is sometimes called a draw and is usually associated with drainage or a rivers/stream.
A gully is a long narrow valley with steep sides.
There are two types of features that can be used to navigate in the mountains. The first type is marked on the map using a combination of symbols, colours, and numbers and includes tracks/trails, rivers/streams, spot heights etc. The second type consist of formations that are characteristic to mountain landscapes and include summits, ridges, spurs, cols and so on. These features are described by changes in height and profile (shape) that are represented on the map by contour lines.
THE NEXT BLOG IN THIS SERIES LOOKS AT NAVIGATION USING FEATURES (JUNE)
PREVIOUS BLOGS IN THE SERIES:
NAVIGATION MADE EASY : WEB RESOURCES
NAVIGATION MADE EASY: GRID REFERENCES