A new, high speed instrument for mapping the magnitude of a variable, such as temperature, vibrational amplitude, radiation, height, etc., at different positions on a surface is described. This instrument records the values of the variable as broken line segments on a two‐dimensional map. The position and length of each line segment represent the position and associated average value of the variable during the recording of that particular line segment. Visible density of the map is greater within regions of greater line length so that a qualitative picture is obtained at a glance. A quantitative measurement can be obtained from the map with a ruler. The present instrument was designed for application to heat transfer testing of nuclear fuel elements. However, the method may have applications in any field where it is desirable to superimpose a quantitative record on a qualitative picture.