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Projection Systems

GIS data around the world can be presented in several different ways including:

• Degrees, minutes and seconds, latitude, longitude and elevation.

• Decimal degrees, latitude, longitude and elevation.

• Cartesian coordinates, X, Y and Z (elevation).

Degrees latitude and longitude work all over the world, and GIS devices always work in these formats for this reason, but they then have options to transform the results into any of the other formats.

The disadvantage of degrees is the formatting and management of the special characters in computer systems can be problematic, and they are not so user-friendly to negotiate.

Cartesian coordinates are more straight forwards coordinates on a graph, just like we all studied in maths when we were in middle school:

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In this system, we can give and X and Y coordinate of any point in the country in metres which describes where it is, and all of the points are in the top-right quadrant (i.e. both are positive) of the graph.  The Z coordinate represents the height ‘Above Ordinary Datum’ which is usually taken to be sea level.

The problem with this system is that the world is so big (and it is not flat) that we would not be able to write down the coordinate values of locations on other continents of the zero-zero point was say at Greenwich in London, because the numbers would be too big.

So, we introduce projections into the mix which are rectangular sets of Cartesian coordinates all over the world (and there are a lot of them), where the coordinates inside each projection can efficiently and easily be handled in metres.

These projection systems are typically identified by a few varying numbers of digits, and in the UK, we are fortunate that the country is so small, that we only need to remember one value:

27700 is the projection system number for England, Scotland and Wales, and 29900 is the corresponding number for Ireland.  HADDMS work is only normally conducted in England, although the other nations often use the same standard for highways drainage investigations.

If we then take this system and apply it across the whole country and divide it up into 100km squares, we now see the Ordnance Survey grid tile index:

Where the zero-zero point is in the bottom left-hand corner and each square with 2 letters is 100km x 100km (or 100,000m x 100,000 m)

So, the bottom-left hand corner of the square NZ is at X coordinate 400,000 and Y coordinate 500,000 and every point all the squares cam be represented in metres.

This is basically the same as what you learned at school, except that the numbers are much larger, and they usually include maybe 3 decimal places, so we can plot coordinates to mm accuracy.  The Z value is always given as metres above sea level, and can in some cases be below sea level, so when considering the lowest point in England (Holme Fen near Peterborough) which is approximately 3m below sea level (i.e. Z = -3) and the highest motorway in England (the M62 near Saddleworth Moor) which is 372m (i.e. Z = 372) above sea level, we can get a good grip on the kind of values that might be considered appropriate in this scale.

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