Understanding Maps - Part One
Sunday, 07 January 2007 07:32 | 
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Understanding Maps – Part One
Most people go out and paddle sections of rivers that their friends have done before or that they’ve seen in a guide book. Perhaps this is adequate for your needs but there is so much more to be gained if you can find a new run on your own. Even if this is not your goal, being able to read a map correctly can go a long way towards safety. A map will be able to tell you how to get to the nearest road or some kind of escape route from a gorge. Map reading skills are something that every boater or outdoors person should have in his/her arsenal. Of course being able to find the correct map in the first place can be quite a mission on its own without some knowledge.
The position of something on a two dimensional plane (like the Cartesian plane) is usually indicated by reference to 2 perpendicular (at ninety degrees to each other) lines, called coordinate axes. Remember in school with your ‘graphs’, reading the ‘X’ (horizontal axis) position first and then the ‘Y’ (vertical axis) next, the same is for reading maps.
On a map the lines running horizontally across are called lines of latitude. These are basically circles of different sizes, the biggest circle being at the equator, zero latitude. At the poles the circles shrink to a point and the latitude is then 90º (north) or -90º (south).
The lines running vertically through the map are called lines of longitude or ‘meridians’. These run from pole to pole and every ‘line of longitude’ cuts through the equator. Since the equator is a circle, we can divide that into 360 degrees.
The number representing the latitude or longitude on map, are actually angles, measured in ‘degrees’, ‘minutes of arc’ and ‘seconds of arc’. These are represented as º,` and “ and have nothing to do with time. There are 60 minutes in a degree and 60 seconds in a minute. Again, this is not time! Think of it as a measurement in distance, because it is.
If one looks at the diagram below: The red lines are the lines of latitude and the green lines are the lines of longitude. With all these lines criss-crossing each other, they form hundreds of squares. The square below is a degree square. This square is divided into 16 smaller squares, these smaller squares are 15’ by 15’ (15 X 15 minutes) and are the 1:50 000 maps you commonly know.
Now to add on to this diagram we shall examine it with more information. As we know already, there are 60 minutes in a degree. The minutes can be seen below and are depicted as the yellow writing. Most people have seen the code at the top of many topographical maps, for example, 2344DA. Now what does this mean? Let’s look at the diagram below and examine the block with the stars. This block or 1:50 000 map, which is what it is in on paper, is 2930CC. All sixteen maps inside the larger degree square will start off the same –> 2930. This can be seen in the
To make sure you know what I mean, let’s examine a few more examples. In the diagram below look at the red, yellow and blue blocks. Remember these blocks represent a 1:50 000 scale map each.
Red –> 3030AD
Yellow –> 3029DA
Blue –> 3029BB
The A, B, C and D in blue being the first letter and the smaller purple letters the second one. Simple enough. Just be careful when moving from one so called degree square to the next. With the blue and yellow blocks, they are both under the degree square 3029. I hope this is fairly obvious at this stage. The red block in the degree square of 3030.
Now that you know how to find the map you’re looking for, let’s examine how to interpret contour lines. Maps are 2 dimensional and therefore the height of mountains and hills cannot easily be reproduced on paper. Altitude is represented by contour lines. Contour lines link a series of points at the same distance above sea level. On a map they may look a little confusing but with practice one can quickly develop a 3 dimensional image in ones mind of how the terrain will look. Take a look at the contour lines below.
The smallest loop, or contour line, in the centre is near the highest point of this hill. That altitude is 1440m in this example. The highest point of the hill in reality would be somewhere inside that small loop, unless of course the hill was totally flat on top. The peak of hills and mountains are usually indicated with a dot and an accompanying altitude. The next contour interval is 1420m, the interval being 20 meters on most 1:50 000 maps. The following 1400m and so the altitude decreases until the last contour line shown on this example as 1340m.
To gain an insight as to how the terrain in the map above would look like in real life, the following 3 dimensional picture is provided.
Remember that in reality the transition is altitude would be more gradual and not stepped like the 3D model shows.
Sometimes it may a little tricky to find what the altitude is on a particular contour line as the altitude may be marked somewhere else. In this instance it is possible to count the contour lines, provided of course you know whether or not you are gaining or losing altitude. Every fifth line is darker than the lines in between and this would represent a number such as 1400 meters or 1700 meters or 200 meters. The lines in between, 1420, 1440, 1460 would be lighter.
Looking at any map, one will almost always notice rivers. The contours make a sort of an ‘arrow’ on the river and this arrow shape will always point upstream. Take a look at the example below.
This river will flow from the top to the bottom of the screen. Also marked on this river is a waterfall. Rapids and waterfalls are occasionally marked with 2 small parallel lines that are perpendicular to the line indicating the river. This is not that common though and the majority of rapids and waterfalls are not marked. On the right hand side of the diagram is a ridge that juts out. In 3D this situation would look roughly as follows.
And a final example to see if you can visualise what is happening.
The importance of reading contour lines cannot be overemphasized. They are very useful in finding out how far a river drops in altitude over a given a distance. This can give you a good idea as to how wild or placid the river may be. A river that drops 5 meters per kilometer will be much more calm and gentle than a river that drops 20 m/km. It is useful to know the gradient of rivers that you have paddled so as to compare to rivers yet to be explored.
That’s all in part one. Part two will follow at a later stage, hopefully.
Article and diagrams by: Adrian T.
The above was written with the intention of being fairly simple to understand. If there is something that is horribly incorrect then please do not hesitate to contact me so I can rectify the problem. Hope it has been useful.