nav+-+identifying+features+and+position

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grid references
**Grid references** are the most commonly used way of describing a position (e.g. yours, the destination or of a point feature) on a map.

Grid references are different from co-ordinates; a grid reference describes a square (the more digits in the grid reference the smaller the square) while a pair of coordinates describe a specific point.

In the UK the Ordnance Survey maps at 1:50000 & 1:25000 are marked with blue lines running north-south and east-west 1km apart, creating squares with sides 1000m long These lines are numbered consecutively (increasing to the east - "eastings" and to the north - "northings") from 00 to 99. A two letter code is used to identify each of these 100km sided squares; that way to give a grid reference that is unique the six numbers must be prefixed by two letters, e.g. NY 123 456

Similar systems are used in other countries: **Longitude and latitude** are widely used when covering large distances; be aware that at big distances these do not form a grid with squares as lines of longitude converge at the poles and are not strictly parallel (though this is unlikely to be an issue when walking unless undertaking a massive trek). **Universal transverse mercator** excludes the polar regions but is widely used elsewhere. **Military grid refence system** is derived from UTM and UPS: as the name suggests used by NATO forces. The US national grid system is based on MGRS.

**four figure** GR
 describes a square 1km by 1km. The first two figures give the number of the north-south line on the **west** of the square and the second two figures give the number of the west-east line on the **south** side of the square.  **The four figures describe the southwest corner of the 1km x 1km square.** For large features, such as a village, a 4 figure GR may be accurate enough. For smaller features a more accurate way of describing the position of a feature is needed.

<span style="font-family: Verdana,Geneva,sans-serif;">example
<span style="font-family: Georgia,serif;"> <span style="font-family: Georgia,serif;">The 4 figure GR of the trig point indicated with the red arrow above is 91 41. The easting is given before the northing.

<span style="font-family: Verdana,Geneva,sans-serif;">**six figure** GR
<span style="font-family: Georgia,serif;">takes the square (which has sides 1km long) described above but divides each side into 10, creating 100 smaller squares each with sides 100m long. <span style="font-family: Georgia,serif;">the area indicated by a 6 figure GR is bigger than a soccer or rugby pitch. <span style="font-family: Georgia,serif;">The 3rd figure of the 6 figure GR is the 100m division to the west and the 6th figure is the 100m division to the south. <span style="font-family: Georgia,serif;">**The six figures give the southwest corner of the 100m x 100m square.** <span style="font-family: Georgia,serif;">In most circumstances a 6 figure GR is sufficiently accurate.

<span style="font-family: Georgia,serif;">In the example above the 6 figure GR of the trig point is 917 412; it lies just between 700m and 800m east of easting 91 and between 200m and 300m north of northing 41.

<span style="font-family: Georgia,serif;">When generating a 6 figure GR from a map it is more accurate to use the romer of a compass.

<span style="font-family: Verdana,Geneva,sans-serif;">video explaining grid references as used in the UK
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<span style="font-family: Georgia,serif;">A quiz to check understanding of 6 figure GRs: <span style="font-family: Georgia,serif;">An [|introduction] to the grid reference system used by the OS <span style="font-family: Georgia,serif;">[|Interactive guide] to use of the British National Grid <span style="font-family: Georgia,serif;">A short description of use of a [|romer] <span style="font-family: Georgia,serif;">[|Using a romer to determine grid reference] <span style="font-family: Georgia,serif;">[|video]on use of a romer to help generate a 6 figure grid reference <span style="font-family: Georgia,serif;">[|longitude and latitude] explained <span style="font-family: Georgia,serif;">[|using UTM] <span style="font-family: Georgia,serif;">[|UTM, MGRS & USNG]

[|Catching features] are those (ideally linear such as stream, wall) that are beyond the objective; if reached you know you have gone beyond the planned objective. Also called limitng features. Funnel features may be regarded as catching features to either side; the route proceeds between them.
 * Collecting features** are those you plan to encounter //en route//. Also called tick features. Best combined with "thumbing" to ensure stay on planned route.

Video showing how to take bearing from map and apply to compass:

relocation
<span style="font-family: Georgia,serif;">see this page if you need to refresh your understanding of bearings - intersection and resection will not work if you cannot take or apply bearings accurately. Presentation "where am I?"

"Practice getting lost" (Glenmore Lodge article) [|getting lost] - how to avoid it and what to do if it happens

intersection
If you want to know where you are on a linear feature (such as a ridge or a path) then a bearing can be taken from a feature on the ground, applied to the map and position identified with "intersection". **It is better to avoid needing to use this method by monitoring time, distance covered and features passed.**


 * 1) You need to be sure you are on the linear feature before you go any further.
 * 2) Identify a feature that you can see on the ground and on the map - ideally the line to this will be approximately perpendicular to the linear feature.
 * 3) Take the bearing of the feature from where you are, then correct for magnetic deviation (subtract 2 degrees in UK).
 * 4) Set the compass housing with this grid bearing.
 * 5) Put the edge of the compass so it passes through the feature on the map.
 * 6) Rotate the whole compass (pivoting about the feature) until the north south lines in the compass housing are parallel to the north south grid lines on the map.
 * 7) **Because you are applying a back bearing from the feature to your position you need to have the //south e//nd of the orienting lines pointing //north// on the map.**
 * 8) Where the compass edge (or a straight edge applied to the compass edge) crosses the linear feature is your position.

resection
If you are not on a linear feature then a single bearing (as above) will not be sufficient to find your location. A bearing to another feature that is present on the ground and on the map is needed: ideally this will be as close as possible to perpendicular to the other bearing. Where the two back bearings cross will be your position. Accuracy can be increased by using a third feature - it is most unusual for the three lines to meet at a single point; they usually produce a small triangle which contains your position.

Presentations on bearings, resection etc: [rh Navigation bearings.ppt]
 * If you have adequate visibility and can see three known features that are also on your map, you will probably be able to find your position by setting the map and comparing contours and other features with what you can see.**

[|video] The ability to use this can help to stay on course and to relocate when off course: • Face downhill and aim direction of travel arrow downhill • Turn housing until orienting arrows line with north arrow then deduct 3 degress (mag to grid get rid) • Place compass on map with orienting lines parallel to NS grid lines then move until edge of compass is perpendicular to contour lines (ensuring height lost in direction of compass); this is a possible location
 * Aspect of slope**

Alternatively if you know from you map which slope you want to travel on and want to use aspect ot help confirm you are there: • Place compass edge on slope with edge perpendicular to the contour lines and the direction of travel arrow pointing the right way (downhill if the that is the way you will be travelling, uphill if you will be ascending the slope) • Turn housing until orienting lines parallel with NS grid lines (check north at north of map) then add 3 degress (grid to mag, add) • When you reach the slope, when the compass is orientated so north arrow pointing to N on the houing, the direection of travel arrow will point direclt up or down the slope (depending on how set)

"Practice getting lost" (Glenmore Lodge article)

estimating distance from a feature
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. 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) 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) 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) Your distance from the features = multiple x known distance x 10

Link with further explanation

Powerpoint:

<span style="font-family: Verdana,Geneva,sans-serif;">double bearing technique
This can be used to estimate your position from a feature that is visible. 1) Measure its magnetic bearing from your initial position (needs to be between 10 and 45 degrees) 2) Walk in any direction (but in a straight line) until the bearing of the feature is double the original value - **keep track of the distance covered from start point to this point.** 3) The distance you travelled as the bearing double will be the distance from the end point to the feature 4) As you can apply a back bearing from the feature on the map, and because you know an approxiamte distance from the feature you can now estimate your position

more on this technique [|here]