Zmarties

Whilst considering producing a mashup with a database containing locations in the Netherlands, I was dealing with coordinates in the Dutch nation grid system known as RD coordinates (Rijksdriehoeksmeting).

For one off conversions there is an online converter which will convert to latitude and longitude.

As is very often the case, the wikipedia article on the Netherlands is a very good general reference.. It explains

There is a west-east coordinate between 0 and 280 km, and a south-north coordinate between 300 and 620 km. The reference point is the Onze-Lieve-Vrouwentoren (Our Lady's tower) in Amersfoort, with RD coordinates (155.000, 463.000) and geographic coordinates ca. 52°9′ N 5°23′ E