USGS topographic map extract showing Yosemite Creek, a campground, contours and coordinate markings.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Local 100 km square. eastings first, then northings. 100 m precision.

Along first: 24.6 km east. Then up: 51.3 km north. Each slider step is 100 m; grid lines are 10 km apart. Contours, river and road provide fictional landmarks; the planar grid retains equal scale. A British National Grid reference also needs its 100 km letter pair.

Grid references

Read and plot a six-figure grid reference by locating the easting before the northing.

Drag to rotate. Hold Ctrl and scroll to zoom, or pinch on a touch screen.

Why it happens

A grid reference locates a cell using eastings followed by northings. More digits identify a smaller cell, while the letter pair identifies its wider British National Grid square.

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A real USGS topographic sheet of Yosemite Creek. Read the sheet's coordinate system and scale before giving a position; it is not an OS National Grid map.

Try a comparison

Set Easting before Northing and read the six-figure reference. Add the letter pair when explaining how a full British National Grid reference identifies the wider square.

Reading the result

The board is fictional, and a more precise reference does not by itself make a measurement more accurate.

A common misunderstanding

A six-figure British National Grid reference identifies an exact point.

A six-figure reference identifies a 100-metre square within the relevant lettered grid square. More digits specify a smaller area, but extra precision does not guarantee an accurate observation. Read eastings before northings and include the letter pair when the wider location is needed.

Cases

Exam language

Easting

A grid coordinate measuring distance east from a defined origin. Read eastings before northings in a grid reference.

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A real USGS topographic sheet of Yosemite Creek. Read the sheet's coordinate system and scale before giving a position; it is not an OS National Grid map.

Northing

A map-grid coordinate measuring distance north from the grid’s defined origin. Read it after the easting when giving a grid reference. The coordinate locates a position within a projected grid; it is not the same quantity as latitude.

Four-figure grid reference

A reference identifying a 1 km square on a standard British National Grid map by its southwest corner. Read the easting first and then the northing. It identifies a square, not an exact point within it.

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Ordnance Survey sheet NO, northeast Scotland. British National Grid eastings and northings are measured from a false origin southwest of Great Britain. Four figures identify a 1 km square; six identify a 100 m square. This reduced 1:250,000 sheet illustrates the system; use a detailed map for precise references.

Six-figure grid reference

A reference locating a 100 m square within a standard British National Grid kilometre square. Read three easting figures before three northing figures. Adding figures improves reference precision, but the map and observation still limit accuracy.

OSGB

Ordnance Survey Great Britain. OSGB36 is a geodetic datum used with the British National Grid; it differs from the global WGS84 reference system.

False origin

An offset origin used to keep map-grid coordinates convenient and usually positive. The British National Grid uses one southwest of Great Britain.

Grid square

An area identified by a map grid. In British National Grid notation, four, six and eight figures identify 1 km, 100 m and 10 m squares respectively.

WGS84

A global geodetic reference system used by GPS, including a reference ellipsoid and coordinate frame. Converting to the British National Grid requires a projection and datum transformation.

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The historic Greenwich Meridian makes a reference longitude tangible. Modern WGS84 zero longitude lies about 102 metres east of this line.

Adjust parameters

Easting246Northing513