measuring+distance+on+map+&+ground

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=measuring distance on a map = It is vital to know the scale of a map before the distance between two points on the map can be determined. One of the key aids to measuring distance on an OS map or Harvey map (used in UK) are the **grid lines** that form squares with 1 km sides. (the distance diagonally across the square is 1.4 km).

The easiest way to assess the distance in a straight line between two points is to use the romer on the compass. **Making sure the correct scale romer is used** (compasses made for use in the UK usually have 1:50000 and 1:25000 romers) then distances of up to 1 km can be measured accurately: if the 0 is put at one point the number on the scale at the other point will indicate how many hundred metres apart the two features are. For example the “10” on the romer scale indicates 1000m from the origin and “4” indicates 400m.)



For longer distances measured in a straight line the scale on the side of the compass may be needed. To use this the distance measured needs to be multiplied by the representative fraction to determine the distance on the ground, e.g. 6cm on a 1:50000 map represents 6cm x 50000 = 3000m.

Most walks do not follow straight lines, though, and the ability to measure a meandering route is needed. The easiest way to do this is to use a measuring wheel.  An alternative way is to try to line up a piece of string along the route, then straighten out the string and measure this straight distance as above. Make sure the string used does not stretch! The edge of a piece of paper and a sharp pencil can be used. One corner of the paper is put at the start point and the edge placed along the route. A mark is made on the paper with the pencil where the route bends. The paper is reorientated with the edge following the route, with the mark kept at the bend and the process repeated (each mark will be at a different place). This is repeated until the destination is reached where a final pencil mark is made. The distance of the route = distance from the corner of the paper to the final mark on the paper x representative fraction. = = =measuring distance on the ground =

pacing
With practice the method of measuring short (ideally < 500m) distances by counting paces can be surprsingly accurate. Like many other navigational techniques it needs to be prepared and practiced //before// it is needed for real. 1) Find two points that are 100m or 200m apart. 2) Pace them (wearing usual footwear and carrying usual equipment) at least twice. 3) Each time count the number of times the same foot strikes the ground (ideally if stepping off //from// the left foot count the number of times the left foot strikes the ground). 4) Calculate the average number of double paces required to cover 100m - for most people this will be in the range 55 to 65.

If you are luck and you have exaclty 60 paces / 100 meters, it is relatively easy to work out how many paces are needed to cover, say, 20 metres (12). If the number of paces you require to cover 100 metres does not lend itself to easy mental arithmetic it is useful to calculate the number of paces for specific distances beforehand and write them in a convenient location (see spreadsheet below). To cover 50 metres count the same number, but apply them to single paces (i.e. each time either foot strikes the ground).

<span style="font-family: Georgia,serif;">Remember than fatigue, heavy loads, bad weather, inclines and difficult terrain will all affect the accuracy of pacing. If you anticipate having to rely on pacing you should repeat the steps above in different conditions (e.g. through thick heather, uphill, in snow as well as on the flat) and record the numbers and take them with you when you go out.

[|Pacing] (video)

<span style="font-family: Georgia,serif;">When walking with a new group, with a heavier pack or in different terrain, one should assess how the accuracy of pacing and timing early on in the walk - before they are needed for navigation - so that adjustments can be made. For example if the group is moving more slowly than expected it might be inaccurate to apply the timings for 5 kph in the table above.

timing
<span style="font-family: Georgia,serif;">If you are using timing to estimate distance travelled when navigating, a stop watch will make this much easier (and more accurate). It may be worth designating a person other than the navigator to monitor time (in the same way as asking someone else to pace - assuming they know their pace count for 100m).

<span style="font-family: Georgia,serif;">A nomogram for estimating time a walk will take (click on the tab for instructions on its use)

<span style="font-family: Georgia,serif;">Table shows time taken to cover specific distance at known speed (mm:ss):
 * || 4km/h || 5km/h || 6km/h ||
 * 100m || 1:30 || 1:12 || 1:00 ||
 * 500m || 7:30 || 6:00 || 5:00 ||
 * 1000m || 15:00 || 12:00 || 10:00 ||

estimating time on the hill: media type="youtube" key="5fMDwfex1mA&rel=1" height="355" width="425"

Although many walks aim to reach one or more summits, when covering ground quickly is the imperative detouring round - rather than going over - a hill may be faster. A direct route involving an ascent of 150m would be expected to take at least 15 minutes; in the same time one would often cover 1 km on the flat.

Spreadsheet to convert paces to distance (you need to enter the number of double paces per 100m then copy down the chart):

estimating distance from a feature
<span style="font-family: Georgia,serif;">If you know the size of a feature or the distance between two features (from a map) you can estimate your distance from it/them. <span style="font-family: Georgia,serif;">For example if you knew the distance between two points was 200m, and the thumb edge moved half way between them when the eyes were swapped the distance from the points is 200 x 0.5 x 10, i.e 1000m
 * 1) <span style="font-family: Georgia,serif;">Close one eye, hold a thumb up in front of you at arm's length and align one edge of the thumb with one edge of the feature.
 * 2) <span style="font-family: Georgia,serif;">Swap eyes and estiamte how many multiples of the known distance the edge of the thumb (be careful to use the same edge each time) has "moved".
 * 3) <span style="font-family: Georgia,serif;">Your distance from the features = multiple x known distance x 10

<span style="font-family: Georgia,serif;">Link with further explanation <span style="font-family: Georgia,serif;">Presentation with diagrams to explain:

a technique for converting miles and kilometres
<span style="font-family: Georgia,serif;">While it is possible to do the maths in your head most of the time (miles to kilometres multiply by 1.6, kilometres to miles multiply by 5/8) errors are likely to occur if you try to do this when tired or wet. One way round this is the use the Fibonacci sequence of numbers: this starts with two "1"s then each number is the sum of the two previous ones: <span style="font-family: Verdana,Geneva,sans-serif;">1 1 2 3 5 8 13 21 <span style="font-family: Georgia,serif;">Consecutive numbers can be used to convert between miles and kilometres - go to the next higher number to convert to kilometres and to the next lower one to convert to miles). <span style="font-family: Georgia,serif;"> <span style="font-family: Georgia,serif;">For example: <span style="font-family: Georgia,serif;">to convert 5 miles go to the next number up: 8km <span style="font-family: Georgia,serif;">to convert 13 km go to the previous number: 8 miles <span style="font-family: Georgia,serif;"> <span style="font-family: Georgia,serif;">what if the number you want to convert is not in the series? find numbers on it that add to the one you want: e.g. to convert 18 miles to kilometres, use the 5 and the 13 - this gives a conversion of 8 + 21 = 29 km