Tuesday, 30 June 2015

We should drop the use of the word tetrad or at least learn what its area is!

The word tetrad is used to describe a square area of land with sides of 2km it has an area of 4km2. The term tetrad is widely used in biological recording in the UK, but it often misunderstood both in the UK and abroad. Logically it is derived from the ancient Greek word for four. Likewise a monad has 1km sides and has an area of 1km2 . A pentad and a hectad have sides of 5km and 10km respectively, but they have areas of 25km2 and 100kmso their name is based on the length of the side not their area. Outside the UK many people assume a tetrad has sides of 4km, because this follows logically, but lack of a logical series is perhaps why there are so many mistakes.

Mistakes are not hard to find, below are some examples published in official publications where the area of a tetrad is wrong...
Perhaps it is time to drop the use of this ambiguous term!


  1. This comment has been removed by the author.

  2. PS: I also think the terms are misleading, because monad recording has 100 times the spacial resolution of hectad recording, but the words give us the impression it is only 10 times better.

  3. Whilst accepting that the terms are misleading, isn't it "pentad" which is the odd one out: with hectad surely referring to its area.

    Whilst we are at it perhaps it would be nice for tetrad names to use the same arrangement of letters as the 100km grid squares (called myriads by Geograph). I find it a complete pain that the OSGB arrangement not only uses different letters ("I" is dropped by OSGB, "O" in DINTY) but that the letters run from west to east, north to south and from south to north, west to east in the two systems. I maintain look-up tables to simplify the conversion, but it's tedious in computer programs: something I generally avoid by having a complete table of square of various sizes.