QDGC (quarter-dregree-grid-cell) or QDS (quarter-degree-square) corresponds to the area shown on a 1:50 000 map (15' x 15') and is approximately 27 km long (north-south) and 23 km wide (east-west).

These grid-cells are obtained by drawing lines of latitude and longitude at 15 minutes (15') intervals across a map. This creates a grid of 1/4^o squares. It bears mentioning that "quarter dregree square" is a misnomer in that there are 16 in a degree square, not four.

If you are working with maps 1:250 000, draw lines with a long ruler and pencil along the 15' intervals, which are clearly marked on the margins. Your map is now divided into QDGCs.

Having found your position in a QDGC, you need to know the code of that particular square. The code consists of four numbers and two letters (ie 2624BD). The code can be easily worked out using the guide below:

1. Each degree square is designated by a four-digit number made-up of the values of latitude and longitude at its top left corner, e.g. 3218 (the large square in the diagram below.
2. Each degree square is divided into sixteen "quarter dregree squares", each 15' x 15'. These are given two additional letters as indicated.
3. In the diagram, the hatched area indicates the QDGC 3218CB.