Gridsquares

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What is it?

Gridsquares - otherwise known as "Maidenhead QTH Locator System" is the system recognised by the International Amateur Radio Union (IARU) for QTH location.

Gridsquares are based on the internationally recognised World Geodetic System (WGS) which is used to provide precise location details of every point on the earth's surface. This information is critical in applicatons such as GPS and other satellite technologies. The term Maidenhead comse from the town in which the system was developed.

The current system is known as WGS84, 1984 being the year in which international agreement was last reached and gives location accuracies to within 1 metre. The next incarnation of WGS is due in 2010

How accurate is it?

Compared to the conventional degrees/minutes/seconds system, most gridsquare calculations are relatively inaccurate. each gridsquare is usually 5 minutes of longitude deep and 2.5 minutes of latitude wide. In terms of distance, one minute is equivalent to one Nautical Mile on the surface of the earth (approximately 1.6km). Hence each gridsquare had an area of approximately 12.5square nautical miles (32 square kilometres).

How are gridsquares calculated?

Gridsquares are usually expressed as a six character reference, eg QG62ll. This is sometimes extended to 8 characters giving increased accuracy of location.

Using the example of QG62ll:

  • The first two characters QG represent an area <math>20^0</math> wide and <math>10^0</math> high (about 720 000 square mi or 1 843 000 square km)
  • The second pair of characters 62 represents a rectangle <math>2^0</math> wide and <math>1^0,</math> deep (approximately 7000 square mi or 18000 square km)
  • The final pair (often written in lower case) ll represents a square approximately 5' by 2.5' (approximately 12.5 square mi or 32 square km)

Where do the letters and numbers in a gridsquare come from?

The surface of the earh is divided into "squares" or fields, labelled A - R along lines of latitude starting at <math> 180^0 </math>, and A - R along lines of longitude starting at the south pole, giving 324 fields.

World Gridsquare fields map from HamShack

Each of these fields is divided into 100 smaller parts by sectioning each of the sides into 10 equal lengths. These are labelled 0 - 9 from west to east and 0 - 9 south to north.

These are further divided into 324 smaller sections by sectioning the sides into 18 equal lengths labelled a - r from west to east and a - r from south to north.

Tim Makins, EI8IC has an excellent page with more detail.

How do I convert from gridsquares to degrees/minutes/seconds?

A number of very good online conversion programs are available including:

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What is my gridsquare

The most accurate gridsquare calculator to date (October 2009) is found here: Gridsquare calculator from QTH map