NAVIGATION MADE EASY 1: GRID REFERENCES

 

Untitled
Catherine McMullin, Dáithí Ó Conaill, and Monica Dillane of TMC investigating a megalithic tomb in Loch a’Duin.

 

Q 52763 08134

 

The above photo shows members of Tralee Mountaineering Club (TMC) examining a megalithic tomb in Loch a’Duin on the Dingle Peninsula. The tomb was placed in the valley using an ancient navigational system that was, apparently, centred on the Spring Equinox.

It is located on a small plateau east of An Scoraid river, about 900 metres southeast of the Kilmore junction on the Conor Pass road. The location is marked by a red dot on the 1:50,000 Ordnance Survey map; red dots signify the location of archaeological features in the landscape.

 

IMG_8472

 

There is a more precise way of describing the location of the tomb. View Ranger gives its position as Q 52763 08134. This is called a grid reference, which is  an alpha-numerical set of geographic co-ordinates that is used to locate a feature on a map. Grid references are the basic unit of a navigational system that divides the country into twenty 100 X 100 kilometre zones. This is called the National Grid

Loch a’Duin is located in Q, one of two squares in the National Grid that cover the Southwest. This is the first element of every grid reference. It is followed by two set of numbers. The first is called the Easting (52763). The second is called the Northing (08134). These allow the navigator to zoom in on each zone, pinpoint the location of a feature on the map, and navigate towards it with confidence.

 

THE GRID

 

UTM and National Grid

 

This diagram shows the national grids of Ireland and Great Britain superimposed on a global grid, which is known as the Universal Transverse Mercator or UTM for short. It is named after Gerardus Mercator, a Flemish map maker who invented a system of world wide navigation in 1569.   The UTM was adapted to provide an agreed system of  geographical co-ordinates that could be used in global positioning systems ( GPS) like View Ranger.

The Ordnance Survey offices of Ireland and Northern Ireland adopted a modified version of the UTM in 2001. This was done to increase the accuracy of GPS measurements by minimising distortions in mapping across the island as a whole.  This is called the Irish Transverse Mercator (ITM) or, simply, the Irish Grid

The Irish Grid provides the organisational and geographical basis for navigation in Ireland but mountaineers usually use 1:50,000 maps. The ground covered by these maps is not the same as the area in each zone of the grid; each zone contains more than one map and many maps extend beyond the boundaries of individual zones.

 

Grid100km & Q
The Irish Grid (left) and a section of the grid superimposed on the corresponding 1:50,000 maps issued by Ordnance Survey Ireland (right).

 

Loch A’duin is positioned on Map 70 in zone Q of the national grid. Iveragh is located in zone V, the dividing line running along the southern shore of the Dingle Peninsula. Grid references for Iveragh – maps 78 and 83 – begin with the letter V. The zone is clearly identified on each map and the boundary between zones is clearly shown.

 

 

THE MAP

 

IMG_8447

 

Each map in the 1:50,000 series uses a grid made up of 30 by 40 squares measuring 2cm and representing one kilometer on the ground. Each square is defined by a vertical and horizontal blue line. The vertical lines are called Eastings and the horizontal lines are called Northings.

Eastings measure distance from west to east. The line dividing East and West is called the prime meridian.  This was  agreed in 1884 as the line of longitude running through the Greenwich Observatory, near London.  Anyone who has visited the Observatory will recall a metal strip in the ground that marks 0º Longitude; the line dividing East and West and the first of the Eastings. The prime meridian used in GPS is approximately 102 metres East of this line. This will read as 0º on satellite based systems.

 

Meridian
An aerial view of the Greenwich Observatory. The dotted white line represents 0º Longitude and the solid white line represents the line that divides East and West in modern global positioning systems. Source Why the Greenwich meridian moved.

 

Northings measure travel northwards from the equator.

 

Every Easting and Northing on a 1;50,000 map has a value between 00 and 99. On Map 70 the Eastings run from 20 to 60 while the Northings run from 88 to 99 and 00 to 18. Grid references contain two sets of numbers (co-ordinates) that define the position of a feature on a map in relation to Eastings and Northings, the Easting are always given first.

Q 52763  08134 is given as the grid reference for the tomb in Loch a’Duin.  52 refers to the Easting and 08 refers to the Northing.  These define a one kilometre square, which is read from the right (East) of the Easting and above (North) of the Northing. The point where these lines intersect –the bottom, left-hand corner – is called the origin and all travel eastwards and northwards within the square is measured from this point.

 

Easting Northing
The square formed by Easting 52 and Northing 08. Q 52000  08000 is the grid reference for the  origin, the point where the two lines intersect..

 

There is a problem with a grid reference that merely identifies a square on the map. It presents you with a square kilometre of ground to search for a feature that may only be three or four metres in size. The solution is simple. Each square is divided into a grid made up of 100m units. These are numbered 0-9 and are represented by the third number in the Easting and Northing component of a grid reference –  Q 52763  08134. This grid reference puts the position of the tomb in a 100m square that is 700m east of  line 52 and 100 metres north of line 08 on the map.

 

Grid 2&4

 

This still leaves us with a large area of featureless terrain to search but the last two digits of grid reference Q52763  08134 tell us that we have to walk 63m eastwards and 34m northwards from the origin of this square to locate the tomb. This coincides with the red dot that marks the location of an archaeological feature that is identified as a Tuama Meigiliteach.

 

CONCLUSION

Grid references are an essential part of your navigational skillset. They enable you to locate your position on a map, pinpoint features in the landscape, and navigate safely from feature to feature.

 

PRACTICE MAKES PERFECT

Grid 4

 

Use this system to generate grid references for the following features:

  1. The Gallán (Standing Stone) north of the tomb.
  2. The point at the edge of the forestry where a stream enters An Scoraid.
  3. Spot height 73.
  4. The start of the track leading into Loch a’Duin, from the Conor Pass road.
  5. The point where the track crosses the 100m contour.
  6. The point where the track meets An Scoraid.
  7. The point where An Scoraid crosses the 100m contour.

Handy hint:

You can use the 1:50,000 scale an your compass (Silva Expedition) to quickly and accurately establish a grid reference. Here’s how:

 

Romer IMG_8481

Q 52763 08134

Q (NATIONAL GRID) 52 (EASTING) 7.63 (100M GRID)  08 (NORTHING) 1.34 (100M GRID)

 

Here is another type of navigational problem.

There is another  way of navigating to the location of the tomb. Open View Ranger, place the red dot marking the position of the tomb at the centre of the crosshairs and mark with a Point of Interest (POI). Then activate the Navigate To function. View ranger will present you with an onscreen,  point-to-point route to your objective and will guide you along it in real time. Simple!

Here is the problem.

An Scoraid river lies between the track into the valley and the tomb. To reach the tomb by this route you will have to cross the river. River crossings are one of the most dangerous aspects of mountaineering.

Find the spot where the track is closest to the tomb and read the course of the river in relation to the contour lines. The river is flowing through a fairly narrow  re-entrant. Would you consider crossing the river at this point?

Have a look at the flat area north of this, where the river drops from the 100m contour to the 90m contour over a distance of approximately 200m. Does this look like a better option? Remember a 10m drop is equivalent to the combined height of 5 very tall people.

The river is a very dangerous feature between you and your objective, the tomb. You need to navigate to a safe river crossing first, then from the river to the tomb. If you can’t identify a potentially safe crossing, don’t go to the tomb by this route.

The point is this; grid references allow you to pinpoint the position of a feature but you still need to (1) understand the nature of the terrain in which that feature is located and (2) be capable of assessing the risks and the opportunities presented by it. That is where skilled map reading contributes to safe navigation.

 

Previous: Navigation Made Easy: WEB RESOURCES

Next: Navigation Made Easy: FEATURES (May 2018)

 

End.

 

 

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