Racing Tips – Orienteering Part 3 – Measuring Distance on a Map
Over the coming weeks we will be posting information that we hope will help you improve your adventure racing skills. If you have topics or questions you would like us to cover, send us an email at flatfeats(at)gmail(dot)com and we’ll try to address them in the near future.
The topic for this post is measuring distance. But before we get too far, our previous posts talked about the basics of orienteering and taking a bearing. The following video gives a brief overview of these concepts with an orienteering twist. Notice the orienteering maps. In future posts we will give more detail on reading these types of maps and some of the techniques to improve your speed.
Understanding Distance and Scale on the Map
Before we discuss distance in the real world let’s first get to know more about how to read distance on a map. Maps are a scaled down representation of the real thing. A short distance on your map can represent a much longer distance in real life. “How much longer?” you ask. Well, that all depends on the ratio of your scale.
Scales are usually found on the bottom portion of maps and contain information on contour lines as well as a ratio (more on contour lines later in the post). If a map has a ratio of 1:50,000 this means 1 millimeter on the map equals 50,000 millimeters or 50 meters in reality. If something is 20mm on that map that means it’s 1000 meters (20×50) or 1km of distance in reality.
Note: We’ll use the metric system in our examples as it’s really easy to scale up and down.
1 mm on map = 50,000 mm in reality
1 mm on map = 5000 cm in reality
1 mm on map = 50 m in reality
1 mm on map = 0.05 km in reality
1:50,000 scale (1 mm on the map equals 50,000 mm in reality which is the same as 1 mm equaling 50 m)
1:10,000 scale (1 mm on the map equals 10,000 mm in reality which is the same as 1 mm equaling 10 m)
Using the Bar Scale
Most maps also have a graphical bar scale. These bar scales can come in handy for quickly checking a distance on the map. Think of the bar scale as a ruler on your map. All you need to do to get a distance between 2 points is to use something to measure the distance (a piece of paper, a twig, etc) and place that measurement along the bar scale to get that distance.
You may be asking yourself “What if the distance I want to calculate is a river or a winding trail? What do I do?”. The answer to that question is fairly simple. Take a piece of string and follow the trail/river on your map with it like this. Once you have the distance, straighten out your string and measure it with the bar scale.
Alternatively, many compasses have rulers on them that you can use to measure distances instead of using the bar scale.
Contour lines are used to show the change in elevation on a map. Lots of lines close together means a steep change in terrain. In orienteering these lines are represented on the map by brown lines. Contour lines can be represented by many other colours depending on the style of your map. If you are unsure what the contour colour is, check the map’s legend.
Above we see 2 maps of the same area. The first map has grey contour lines of 10 meters and the second has brown contour lines of 2.5 meters.
Contour lines not only important at showing the terrain, they are important in calculating distance as every elevation line adds to the overall distance. Need an example? Let’s use the map above with the 10m contour lines and plot a few points.
If you were to measure a straight line from point A to B on the following map it would be the same distance as a straight line from A to C (let’s say it’s 4 mm which turns out to be 40 m of distance in reality). The actual distance, in real life, from A to C is longer than A to B (about 10 m longer). How do we know it’s longer? If you look at point A to B there are no contour lines to cross. This means the area is relatively flat with not much elevation change. If you look at point A to C you will notice that a straight line would cross 3 contour lines at 10 meters a piece for a total of 30 meters of elevation change. 30 meters of elevation change doesn’t mean 30 meters of added distance because you’re moving on a slope. It’s the Pythagorean theorem to the rescue. Math nerds rejoice! You could do the math while in the outdoors if you wanted: a2 + b2 = c2 where “a” would be your distance (run) and “b” would be your elevation change (rise). In our example above we would travel 40 m with an elevation change of 30 m (402+ 302 = c2) which would equal to 50m. This article may explain it better than I can. I’m no math nerd.
In the flat prairies this may not make a huge difference to your overall distance but when terrain is varied or mountainous the difference can be quite significant. For our particular event, just know that changes in elevation add to the distance you need to travel.
I hope the math at the end didn’t hurt your brain too much. In our next post we’ll explain how to calculate your pace and compare it to the distance in your map.