Estimation of distance travelled
This is one of the hardest things for a navigator to do, yet it is one of the most important. In many situations it will be a major guide as to where you are. For example, if you are following a ridge or track and have to pick a particular side spur or side track when they all look similar, you will have to work out how far you have come. To estimate the distance you have travelled requires:
- regular checks of the time
- experience in estimating your actual speed in kilometres per hour
- ability to calculate distances from your map
- simple mental arithmetic or use of rules of thumb for typical group speeds in different conditions.
For example, if you knew you had been walking for half an hour since leaving the last definite feature, and you estimated your speed during this time at 4 km/h, then you should be about 2 km away from that feature.
The elapsed time interval may be used in two ways to estimate distance. The first, and most useful for forward planning requires scaling from your map the distance to your next objective, calculating the approximate time required to reach it, and hence predicting your time of arrival. If you arrive at the objective too soon or too late, it is probably not the right one. The second approach requires noting the time taken to get from the last known feature to where you are now. From that you can estimate your distance from the last known feature which assists in locating yourself on the map.
Pace counting to determine distance travelled, while widely used in orienteering and rogaining, is seldom used in bushwalking except perhaps for short distances in conditions of very poor visibility.
Accuracy with estimating speed only comes with experience, although some guidelines are given under route planning in Chapter 3. For those who are not a whiz at mental arithmetic, it may be helpful to carry a copy of the chart shown in Figure 3.1 in your map case. It can be used to determine one of distance, time or speed, given the other two, simply by laying any straight edge (such as the edge of your map) across it.