If this is the first time you are joining the Map Reading series, you should start from the first post.
Last time we covered symbols on a topographic map and I mentioned contour lines as a symbols on the map, but they are much much more than that. Contour lines are the heart of a topographic, they should be the main feature to use for navigation and with enough practice can be very powerful.
What are contour lines?
A contour line is a line on the map that connects all the dots that are at the same height in their immediate surrounding. To simplify it – if you will follow a contour line in real life you will not ascend or descend, but will stay at the same elevation the whole length of the line.
In a map you will see many contour lines and they represent the change in elevation between lines and create natural features. Contour lines are printed in regular intervals on a map (will be specified in the map legend) and every few lines there will be a thicker line – an index line. Index lines represent another regular interval and help identify big changes. For example: if a map has contour intervals of 25 meters (or feet), usually an index line will be every fifth line and will occur every 100 meters (or feet). Having index lines helps find the main elevation “jumps”.
Each contour line is a height line, and the next line will show the next change in elevation; it has a number that shows the measured height of that line. The change in numbers is a change of height and not distance!
In general, the closer the lines are, the steeper the terrain is, and the further away they are the gentler the land is (“flatter”). Curving of contour lines help us see a shape in the landscape and identify the specific feature. Let’s see how we can actually understand and use contour lines.
How can we use contour lines?
Because contour lines show change of elevation, they actually allow us to visualize a three dimensional view of the the land features we will see. Lets look at the example below:
The picture above shows a section of a map with contour lines: in the picture there are index lines, contour lines and a few features that can be easily recognizable. North in this section of the map is directly up, heading to the top of the image. To make this section “come alive,” below is a 3 dimensional representation of this section of the map:
The image above shows the same map section in 3D, indicating where north is for easy orientation. You can identify the saddles, the trig point and the V shaped valley very easily.
Being able to “visualize” the shape that the contour lines create makes a big part of the ability to use them properly for navigation, and it is a good skill to practice.
Wrap up and extra points
First, a few extra things to know about contour lines:
1. On the map lines are always ascending, so if the numbers go up – it is an increase of elevation and if the go down, it is a decrease of elevation. (Another way is to look for the closest “high point” find how your area relates to it – lower/higher?)
2. High points are marked with triangular points (“trig points”) and are useful reference points for understanding features.
Contour lines are probably the most important and the hardest part of reading a map to really get your navigation in order. I will explore a few more aspects of the contour lines, such as frequent contour patterns for easy navigation, and we will focus on contour lines when delving into actual navigation using a map.
Grab a topo map of your area and see if you can start recognize any landscape patterns compared with the map and we will see if that matches on the next Map Reading series post.