An optical measurement system for determining outer contours of one or more contact wires (2a, 2b) of an overhead line, comprising an elongated first instrument base and an elongated second instrument base, wherein the first instrument base comprises a first mounting member configured for releasable connection to a first side of a vehicle, and wherein the second instrument base comprises a second mounting member configured for releasable connection to an opposing second side of the vehicle, wherein the vehicle is arranged for movement underneath the one or more contact wires (2a, 2b) of the overhead line; wherein the first instrument base comprises a first laser projector and a first camera assembly longitudinally spaced apart along the first instrument base, and wherein the second instrument base comprises a second laser projector and a second camera assembly longitudinally spaced apart along the second instrument base; wherein the first camera assembly comprises a first overview camera, a first detail camera and a first mirror moveably arranged on the first instrument base in front of the first detail camera, and wherein the second camera assembly comprises a second detail camera and a second mirror moveably arranged on the second instrument base in front of the second detail camera; and further comprising a processing system configured for communication with the first overview camera and the first and second mirror (13, 15); wherein, in a mounted state of the first and second instrument base (3, 4) on the first side and second side of the vehicle, respectively, the first and second laser projector (7, 9) are configured to project a first and second transverse laser line (TL1, TL2), respectively, on the one or more contact wires (2a, 2b); wherein the first overview camera is arranged to capture an overview image of the one or more contact wires; wherein the processing system is arranged to process the overview image for locating the one or more contact wires (2a, 2b), and wherein the processing system is further arranged to control a first position of the first mirror and a second position of the second mirror based on the located one or more contact wires (2a, 2b), and wherein the first detail camera is arranged to capture a first detail image from the first mirror of the projected first and second transverse laser lines (TL1, TL2) on the one or more contact wires (2a, 2b), and wherein the second detail camera is arranged to capture a second detail image from the second mirror of the projected first and second transverse laser lines (TL1, TL2) on the one of more contact wires (2a, 2b). an elongated first instrument base and an elongated second instrument base, wherein the first instrument base comprises a first mounting member configured for releasable connection to a first side of a vehicle, and wherein the second instrument base comprises a second mounting member configured for releasable connection to an opposing second side of the vehicle, wherein the vehicle is arranged for movement underneath the one or more contact wires (2a, 2b) of the overhead line; wherein the first instrument base comprises a first laser projector and a first camera assembly longitudinally spaced apart along the first instrument base, and wherein the second instrument base comprises a second laser projector and a second camera assembly longitudinally spaced apart along the second instrument base; wherein the first camera assembly comprises a first overview camera, a first detail camera and a first mirror moveably arranged on the first instrument base in front of the first detail camera, and wherein the second camera assembly comprises a second detail camera and a second mirror moveably arranged on the second instrument base in front of the second detail camera; and further comprising a processing system configured for communication with the first overview camera and the first and second mirror (13, 15); wherein, in a mounted state of the first and second instrument base (3, 4) on the first side and second side of the vehicle, respectively, the first and second laser projector (7, 9) are configured to project a first and second transverse laser line (TL1, TL2), respectively, on the one or more contact wires (2a, 2b); wherein the first overview camera is arranged to capture an overview image of the one or more contact wires; wherein the processing system is arranged to process the overview image for locating the one or more contact wires (2a, 2b), and wherein the processing system is further arranged to control a first position of the first mirror and a second position of the second mirror based on the located one or more contact wires (2a, 2b), and wherein the first detail camera is arranged to capture a first detail image from the first mirror of the projected first and second transverse laser lines (TL1, TL2) on the one or more contact wires (2a, 2b), and wherein the second detail camera is arranged to capture a second detail image from the second mirror of the projected first and second transverse laser lines (TL1, TL2) on the one of more contact wires (2a, 2b).