The instrument described automatically maps two-dimensional contours of spatial intensity distributions (i.e.,iso-dose, iso-potential, etc.). A fast response detector is driven by two reversible motors, one a scan drive, theother an error compensation drive. Sequential alteration of the roles of the two motors provides complete mappingof continuous or discontinuous contours. Provisions are made to assure 1% recording accuracy. A remote readoutrecorder allows isolation of the pickup device. Maximum plotting speed is 36 in./min or about 5 min for a familyof ten contours in a 10×25-cm area.