Abstract: An instrument and method is provided for accurately determining the geographic coordinates of a specific location on a map. The instrument comprises a flat transparent plastic sheet having a variable length scale printed thereon which is defined by a plurality of straight lines radiating from a common point. Indicia are also printed on the sheet for designating the radiating lines as graduations of a scale comprising either 2.5 or 5 minutes, and with each minute being further broken down into sixty seconds. To determine a coordinate, the location in question on the map is projected to the border along a direction parallel to one set of the geographic coordinates. The transparent sheet is then placed over the map, and the scale is aligned with respect to the border such that the outermost of the radiating lines directly overlie the tick marks on the map between which the projected location falls. The scale is then read above the point at which the projected location intersects the border of the map.