www.rambler-info.org.uk - UTM/UPS
This system assigns world-wide locations a decimal numeric position on rectangular two-dimensional grids.
Many will already be familiar with the OSGB "Grid Reference" system, which does the same for UK locations.
UTM/UPS is actually two systems: UTM is the main one in practice and UPS is used for the polar areas only.
These polar areas extend from 84°-90° around the North Pole and 80°-90° around the South Pole.
The names are UTM="Universal Transverse Mercator" and UPS="Universal Polar Stereographic"
30U 0614769 5852359 is a point in my front garden.
30U 0614969 5852459 is a point 200m further east and 100m further north.
The surface of the Earth, for the purposes of UTM, is mapped as 60 sectors running North/South. These are numbered 01-60 running Eastwards from the International Date Line.
For all bar the most precise work, these may be regarded as flat strips, since their 6° width is small.
These strips do not run from Pole to Pole: they terminate at 84° North and 80° South. In the polar regions the UPS system takes over!
Each of these long and narrow sectors is subdivided into 8° zones and given a letter, starting at C for the most southerly and ending at X for the most northerly. Aide-memoire.
If you are likely to be operating in Norway or at Svarlbard (to NE of Norway) then you should be aware of some anomalies in the labelling system. These two anomalies are the only ones.
There is an excellent map to be found within the Wikipedia reference.
These letters prevent duplicate references in the North and South hemispheres: we will later see that the southern hemisphere uses an offset zero to avoid negative coordinates.
To create a set of UTM coordinates for a location, we need three components: 1) a sector identity, 2) an "East-West" measurement and 3) a "North-South" measurement. This is done as follows...
1) Two numerals followed by a letter e.g. 07V if we are about to give the reference for the southern end of the division between Canada and Alaska.
2) A distance in metres, measured Eastwards from the centre-line of the sector, always with an extra 500,000m added to prevent negative coordinates.
3) A distance in metres, measured Northwards from the Equator of the sector, for Northern Hemisphere references.
Southern Hemisphere references have an extra 10,000,000m added to prevent negative coordinates. You will know if you are dealing with a southern hemisphere reference if its letter is before "N" in the alphabet.
An example, for my front garden in the Midlands of the UK would be
30U 0614769 5852359 using UTM and, for comparison,
SK 47465 23704 using OS Grid to the same precision.
Note the letter "U" - I live in the northern hemisphere.
The polar regions are handled by the UPS system, where the area is mapped to a flat circle and then divided into squares, each of which is given a two-letter designation. coordinates give the location within each of these squares.
Since there are few of us who will ever enter these regions, may I refer you to Peter H. Dana's website for more details.
1) The sectors are not, perhaps, centred where you might have expected. For example, Sector 30 lies to the W of the Greenwich Meridian and Sector 31 lies to the E. In other words, there is no sector centred on the Greenwich Meridian.
2) Where the sectors meet there will be two possible valid UTM/UPS coordinates. However, if you move a step to either side, then one of the two will cease to be valid.
3) The UTM system starts at the International Date Line, while longitude starts at the Greenwich Meridian. It will be worth remembering that these are different!
4) References containing C to M are in the Southern Hemisphere and those containing N to X are in the Northern Hemisphere. Aide-memoire.
5) Using coordinates in metres generates large numbers. At least one map manufacturer has found it convenient to label the map grid in kilometres, with correspondingly smaller numbers.
6) The system of UTM coordinates has many similarities to UK Ordnance Survey map references. (UK grid references)
For comparison, UK Grid References normally uses two letters and a 6-digit number (eg SK474237) because its units of measurement are 100's of metres, but it can equally well use a 10-digit number if you want 1m precision.
7) The UTM system is now being used on some maps. The way in which they are being used varies, but some information will be listed here....
8) There are some strange interpretations of the system. If you would like to see an example of non-standard use of UTM, then I have documented one from the website of the Austrian Alpine Club.
9) The Gartrip software, which I use with my GPS and otherwise like very much, insists on having the data entered in km rather than m, but with a decimal point included. It also makes a complete hash of entering the data unless both numbers and the letter are entered for the UTM sector.
10) The long narrow shape of the UTM sectors means that 6 digits are needed for the "Easting" and 7 digits for the "Northing". If you look again at my example reference above you will see I have added a leading zero to make them both 7 digits. This is the practice on some GPS displays, but is by no means universal.
11) Whilst UTM can reference any point on the Earth's surface, it can pose one awkward problem for map makers. If they are operating in a country with more then one UTM sector, then they have a big problem where the sectors join. Basically they would end up with a curving triangle of countryside which would appear twice on their map!
They can get around this by using a non-standard form of UTM with the origins of the sectors rotated to a more convenient position.
12) For the record, the Earth has a circumference of 40,075km, measured at the Equator.
Hence a UTM sector is roughly 700 000m wide and 20 000 000m long.