The UTM/UPS, also known as UTM, geographic location system has the desirable characteristics of a uniform grid, the units are metre, angles are ground correct and areas are correct.  To the user the area on the map is effectively a flat plane.  Inevitably there is a cost for this convenience.  The UTM projection introduces a third north, grid north, to join true north and magnetic north.– Grid north can differ from true north by as much as -3° to +3°.  The difference is known as grid convergence.  In addition to the grid convergence issue, the flat plane (a zone) has a limited east-west extent.  There are neighbouring zones (60 in all to cover the earth), however there are discontinuities in the coordinates at the zone-boundaries.

The 60 zones are designated 1 — 60, bounded by meridians spaced at 6 degree intervals. (60 x 6 = 360)  For example, Castlemaine lies in zone 55; Maryborough lies in zone 54  The zone boundary between zones 54 and 55 runs (north-south) between Baringup and Maldon.  (People not familiar with Victorian geography perhaps could find their neighbouring zone-boundaries at Distribution Mapping Software.) 

Eastings are the east-west component of a coordinate; northings are the north-south component of a coordinate.  The units of eastings and northings are metre.  Each UTM zone is divided into 20 lateral zones each of which is assigned an alphabetical zone character from C through to X excluding I and O. 

The diagram to the right shows the anatomy of a full UTM reference, 55H 260215 5902114 (the Mount Alexander Cairn).
The reference is formed from these components.

• [zone number] A number in the range 01 – 60.
• [zone character] A single letter.
• [6 digit easting]
• [7 digit northing]  Just north of the equator the northing may have leading zeros.

The full UTM reference above has a resolution of 1 metre and covers the entire world except for regions near the poles.

A 6 digit grid reference, 602 021 is formed from selected digits in the full reference.  ('Rounding' may required in the determination of the easting and northing values.)  A 6 digit grid reference has a resolution of 100 m and limited geographical coverage. 

A grid reference system needs reference data, termed a datum.  There are a large number of datums.  The World Geodetic System 1984 datum, WGS84, is dominant internationally, however the Australian datum, GDA94, differs from WGS84 by less than a metre, so in normal practice the two datums are interchangeable.

The image to the right is a snip from a CCM topographical map. It contains;

• 2 eastings notated 251, 252
• 2 northings notated 5878, 5879
• map name
• a comment on the map datum
• zone information, grid spacing and scale
• an example UTM reference (on map, not on snip)
• scale bar

The notation on the eastings and northings is abbreviated– 3 trailing zeros are omitted.  The complete specification of the western easting is 251000 (6 digits) and of the southern northing 5878000 (7 digits). 

The zone information applies for the whole map.  It is common for medium and large scale maps to be constructed within a single zone.  The grid spacing of 1000 m is consistent with the grid notations. CCM maps often have awkward scales, this is a consequence of trying optimise the coverage and the resolution together with the mathematical reality that 10 has only two factors.

The sample UTM reference is provided so the user can check the coordinate format.  A sample reference for this snip might be –
384 peak 55H 251134 5878724.

We will use the junction at the east end of Morgan Trk as the point of interest, POI.

1. Select the easting to the west of the POI. >>> 251
2. Retain the last two digits. >>> 51
3. Estimate west-east distance of POI between eastings in tenths. >>> 8
4. Append result of step 3 to step 2 >>> 518
5. Select the northing to the south of the POI. >>> 5878
6. Retain the last two digits. >>> 78
7. Estimate south-north distance of POI between northings in tenths. >>> 6
8. Append result of step 7 to step 6 >>> 786
9. Combine step 4 and step 8 to form the 6 digit grid reference >>> 518 786

The 6 digit grid reference has an resolution of 100 m (the distance between adjacent references) and an uncertainty of 50 m (for border line cases).  Dividing the distance between eastings (say) into half (0.5) and then, if need be quarters. (0.25, 0.75) can assist the estimation.  A method that requires no estimation is to pick up the distance in question with pencil marks on the edge of a piece of paper and carry it down to the scale bar.