Giving locations

[Lutonman]

Thanks Martin, I know its now common to have equipment installed underground e.g. Gas Valve and location devices are placed over them to record the details of the item and the GPS location recorded automatically.

[MartinR]

GPS uses the WGS84 model, as amended 1997, 2004 and 2014.  That's the point with old charts, if the chart uses WGS72 or OSGB36 (as Admiralty charts used to do) and you use GPS = WGS84 then you could be 300 feet off.  Not much of a problem in navigation, but a major issue in pilotage.  The old charts work fine by themselves; it's just the combination that could be, quite literally, lethal.

[John Walker]

Excellent Martin - I found your info very interesting - thank you

[Lutonman]

Thank you MartinR took me back to my A level Geography and first two years in the drawing office at SEGB (Segas) at Maidstone.
Where does GPS come in I wonder?

[MartinR]

I thought I'd write a brief note about giving locations and sensible precision.  I'll try and cover most of the field, so some of this will be teaching my various grandmothers (you lot!) to suck eggs.  However they may be some readers to whom the detail is new.  The fundamental problem with all mapping and referencing is that the earth is not a perfect sphere (still less a flat plane).  All mapping must therefore be an approximation using a number of conventions.

Latitude and Longitude
Put simply, this is the angle North of South of the equator and East or West of the Prime Meridian.  However, due to the earth's shape and since the start of the space age, this model is a bit too simple.  Different attempts have been made to develop a mathematical model of the earth, known as the World Geodetic System.  The current model is WGS84, previous models were WGS60, WGS66 and WGS72.  One change is that the Grenwich meridian is no longer the prime meridian, the current prime meridian passes 5.3 seconds east of the Grenwich Meridian, or about 335 feet (102m).  The difference is due to two factors, WGS84 accurately allows for local gravitation differences, and the effect of plate techtonics moving Grenwich West.  All marine charts must show the basis of the mapping.  With the arrival of GPS, which is based on WGS84, reading a local chart or a WGS72 chart and steering by GPS could easily put you on the beech outside the breakwater rather than safely within a harbour!  I believe that aeronautical charts follow the same conventions, the difference could put you on the grass instead of the runway.Co-ordinates may be expressing in several ways: 51°37'18"N, 000°25'37"E or 51.3603,0.1925 (North and east are positive, South and West negative) or for nautical purposes: 51°37'.59N, 001°12'.48E (since 1' is 1 nautical mile, this gives miles, tenths and hundreths and is what is shown on charts).

National Grid
The National Grid is based on an approximation that holds good over the area of Great Britain.  The grid uses a model of the earth called OSGB36 which became the official definition in 1936, but is based on earlier work By Airy in 1830.  It is empathically NOT aligned to any of the WGS systems which were developed decades later.  The grid itself extends from somewhere near Iceland across to western Sweden, down into Germany and across to the Bay of Biscay.  It is 25 squares, each 500kmx500km which are labelled A-Z (excluding I).  In practice, N, S and T cover most of the mainland are the ones usually met with.  Each large square is divided up into 25 smaller 100kmx100km squares, again labelled A-Z.  For instance square T extends from Yorkshire down past the South coast and Eastwards including the Netherlands.  Square TQ covers Kent from around the Medway west to central London, and TR covers Easter Kent and Calais.
Whew, that's the easy bit!  Within the small square numbers are used.  First are the "eastings", that is the distance east from the edge of the square.  The first digit of the easting defines the distance east in 10s of km.  The second digit in km.  Third, fourth and fifth digits are in 100s of metres, 10s of metres and metres respectively.  Next are the "northings" which work in exactly the same way, but traveling North.  Putting this all together, The Isle of Sheppey can be located by TQ 9 7 which is a 10kmx10km square, quite sufficient.  Canterbury city could be located by TR 14 57 which gives a 1km square.  The Cathedral is at TR 151 579 which locates it to a 100m square.  The gateway is at TR 1496 5791 which places it within a 10m square.  Finally, spaces may be provided to help the user, but are often omitted, the references above could be TQ97, TR1457, TR151579 and TR14965791.  Very occasionally you may see all-figure grid references but they can be ambiguous and should be avoided.


Postcodes
Post codes have their origin in the division of London into districts in 1857 (yes, over 150 years ago) such as EC, numbered division of the districts came in 1917 (N4) and other cities adopted the practice from 1937 onwards (Sheffield 7).  during the late 1960s the codes we know today were introduced.  The last part of a postcode may not be quite what you expect though, and thereby lies a problem using them for location.  The number is a "sector" which is usually, but not always, geographically based and the last two letters are the "unit".  This may be a group of houses, or a single address, or a sub-section of a property, or an organsiation or part of an organisation.  It is the quantity of mail that is the deciding factor.
If you see a postcode with the digits in the first part in the range 90-99 (for instance NE99), then all you know is that it is dealt with at the office (in this case Newcastle).  There are dozens of special postcodes such as GIR 0AA (Santander, formerly Girobank), BX1 1LT (Lloyds Bank) or BX2 1LB (Loyds Bank Scotland).  The last illustrates another point, postcodes do not align with local government or even national boundaries.

What3wordsThis is a purely commercial enterprise.  The whole world is divided into 57 trillion 3mx3m squares.  Each square has a set of three words which should uniquely define it, but similar sounding words, particularly when pronounced in an accent, can confuse the system.  One analysis has reported areas where there is a 1 in 500 chance of ambiguous words.  The database that relates words to locations is protected by copyright and fairly fiercely defended.  Whilst you can publish the words for a specific location, converting back needs a licence and payment of a fee.  There are also occasional insensitivities, "tribes.hurt.stumpy" apparently refers to a temple, and "mile.crazy.shade" is a war memorial (examples taken from Wikipedia).


Precision
One bugbear that regularly occurs is the issue of precision.  Co-ordinates and the National Grid can be extended indefinitely.  Perhaps there is a reason to identify a 10cm square (drain cover perhaps) but rarely.  Long lists of numbers (the grid is the worst offender) can lead to errors.  Lets start with the Grid:

• XX 9 9 - identifies a 10km square, appropriate for a major feature, but rarely seen.
• XX 99 99 - identifies a 1 km square, great for cities and districts, common.
• XX 999 999 - identifies a 100m square, large building, parks, common.
• XX 9999 9999 - identifies a 10m square, virtually everything else, common.
• XX 99999 99999 - identifies a 1m square - why?

The following table is one I drew up a few years ago.  The distances are accurate for latitude, but logitude varies across the face of the earth.

1°        60 nm    69 mi    111 km
0.1°       6 nm     7 mi     11 km
0.01°            1232 yd      1 km
0.001°            123 yd    110 m
0.0001°            12 yd     11 m
0.00001°            1 yd      1 m

10'       10 nm    11 mi     19 km
1'         1 nm     1 mi      2 km
0.1'       1 ca   608 yd    556 m
0.01'              61 yd     56 m

10"               350 yd    300 m
1"                 35 yd     30 m
0.1"               10 ft     3 m

• nm = nautical miles
• ca = cable (1/10 nm)
• mi = statute (ie land) miles
• yd = yards
• ft = feet
• °  '  " = degrees, minutes and seconds