This page is about the cartographic mechanism called "Romer", for the person, see Ole Rømer.

A romer is a simple device for accurately plotting a Grid Reference on a map. Essentially, it is a specially marked out ruler which matches the scale of the map in use. The scales are laid out in reverse such that by lining up the numbers given in the grid reference with the gridlines for the square in question, the corner of the romer lies on the referenced spot. A small hole at this point allows the location to be marked with a pencil. Romers are made from transparent plastic so the map can be viewed through them. They are an essential piece of equipment for Rallying, and any other use of maps where accuracy is needed.

The illustration below shows how the romer is used. Here, we are plotting the reference 696018. The marks corresponding to (6, 8) on the romer are lined up along the gridlines (69, 01). The hole in the corner yields the exact point, the church at Little Plumpton.