|Topo map scale block|
All topo maps, and most orienteering maps, will have a scale block. The scale block shows you the relationship of the map distances to actual distances.
Depending on the origin of the map, many topo maps will have a standard scale. (1:25000, 1:10000, etc). However, increasingly, course designers are using custom scales so that they can fit their map to a certain printing size.
|Compass scale tool|
Many compasses have built in scale measurements on their edges. If you have one of these compasses, and it matches the scale in use on the map, this is the easiest way to measure distances.
Simply use the appropriate scale on the side of the compass (check this twice to avoid mistakes!), lay the compass on the map and directly measure the distance.
Another feature of topo maps is often a grid pattern (marked in blue on the map to the right. These grids can also be used to estimate distances since they are uniform sized. Often the grid will be either 1km or 10km along each edge. Simple count the grid squares to know approximately how far you are from your destination.
|Topo map grid|
A problem, of course, with either of these measuring methods is that you are almost never traveling in a straight line along a map (unless you are following something like a power line or a nice straight logging road). In most cases, the easiest way to measure a distance on a winding trail is to use a piece of string to trace the path you intend to follow and then measure that string along your scale to find the distance.
Knowing your distance is one thing, actually predicting how far you have walked or, worse, snowshoed or bushwhacked is much harder. Much of the time this will come down to experience and estimation. Keep a keen eye on your map and the features that you are passing as you walk, thumb along on the map (details to follow in another post) and do your best to understand how quickly you are moving over the terrain. If you are the type of runner or walker who normally tracks distances over time you will find this easier. For the rest of us, this will be a constant challenge that only experience will help.
As you get closer to your destination (within a couple hundred meters), switch to counting steps. This is a pretty accurate way to travel a known distance. This will be helpful in situations such as "75m north of trail junction".
Also, remember that, in my experience, people tend to over estimate the distance they have traveled walking, snowshoeing or especially bushwhacking.