If this is the first time you are joining the Map Reading series, you should start from the first post.
After covering most aspects of map reading, it is time to get into using a map outdoors. This time it is all about measuring distance on a map. This is a relatively simple subject and will give a little break after the last post about coordinates. So first:
Why we need to know the distance on a map?
- Route planning – gives us the tools to know what to expect and helps us pack better and be minimalist with our gear (and leave more room for some treats).
- Checking progress – knowing how far you have gone will help in navigation and so in assessing your speed.
- Evaluating finish time – if you have a destination when outdoors, you need to know if you can make it, especially if limited by sleeping facilities (or lack of) or if you need to “beat the darkness.”
- Great tool for kids – keeping kids interested in being outdoors can be a challenge, and giving them a chance to measure progress gets them more involved.
How do we measure distance?
1. Using the grid
We talked about the grid on the map here; and beyond giving all location coordinates, they are also a great tool for measuring distance. Since all maps have a scale, they have also a simple conversion from grid size to distance- usually 1 square side on the grid will be equal to 1 km or 1 mile in real life (that means it is 1.5 the unit in diagonal lines). Knowing what conversion your map uses prior to going out will save a lot of time in navigation.
Ordnance Survey maps (UK) 1:25000 – each square side equals 1 kilometer
USGS 7.5 minutes map (USA) 1:24000 – each square side equals 1 mile
1:50000 maps (most countries) – each square side is 1 km
These rules apply to most topographic maps, but make sure to check
2. Using a string and the scale
Using just the squares is usually not accurate enough, especially with larger scale maps, so using a piece of string (or paper, or stick etc) and relaying it on the distance scale will help you figure out the distance you have covered.
3. Using a specialty tool
Measuring the distance on a map prior to a long trip, or if you are going out a lot and do so often, can become a chore that can be solved by an amazingly simple tool: Map Measurer. These can be digital or analog and they are very simple to use.
The digital map measurer will require scrolling to the right map scale in the menu (i.e 1:25000) and just running the tip of the measurer on the route. It then displays a distance on the screen. The analog measurer uses a dial to set the right scale and measures the distance by using a wheel that is connected to display dial that shows the distance (in miles or kilometers). Both tools need calibration (or just to be checked) and using on the map’s distance scale to assure an accurate reading is recommended.
4. Compass romer
On a navigation compass, the housing (that holds the actual compass) has a ruler with odd numbers – this is a “romer” that measures distance on the map using the right scale (will be printed on). Essentially this is a portable map scale that can be placed on any part of the map to measure the distance.
Romers can also come as a stand alone item and not part of a compass.
As promised, measuring distance on a map is simple and should be used often. When we will talk about route planning, this skill will have an important part.