romer
/ˈrəʊmə
/noun
A small piece of plastic or card bearing perpendicularly aligned scales or (if transparent) a grid, used to determine the precise reference of a point within the grid printed on a map.They can place a car on the surface plate (an accurately measured flat plane) and use what is called a romer arm to calculate the car's dimensions to exact specifications.
Origin
1930s: named after Carrol Romer (1883–1951), its British inventor.