This is the first of a series of caches placed exactly on the Greenwich Meridian. Standing on the Meridian line at the Greenwich observatory visitors are sometimes surprised to note that their GPS does not read longitude 000 00.000. The reason is down to the fact that the worlds first GPS satellite system was calibrated form Maryland USA. When their calibrations were eventually mapped back to Greenwich, the alternate model they had used to describe the shape of the globe gave a zero value 100m east of the Greenwich meridian.
All GPS devices read 000 00.000 when standing on the International Reference Meridian. Google earth also uses the IRM as zero longitude. The co-ordinate system is sometimes referred to as WGS84 for World Geodetic System defined in 1984. In SE England the Greenwich Meridian reads between W000 00.089 and W000 00.090 on WGS84 co-ordinates. Interestingly because the Eurasian continental plate is drifting NE at 2.5cm per year the Greenwich obsevatory will be on the IRM in about 5000 years time.
There is a more detailed explanation on this topic on this link
This park in Peacehaven is named after the Meridian but there is no marker anywhere in it that indicates where it lies. This cache is placed exactly on the Greenwich Meridian at W000 00.089.