A tool or attachment jig for mounting an optical triangulation sensor on a turbine blade in a position to measure the gap between the distal tip of the turbine blade and the adjacent shroud. The tool includes a device for indicating the orientation of the sensor. This means that the tool can match a measurement from the optical triangulation sensor with a position on the circumference of the shroud. By taking a plurality of measurements as the turbine blade is rotated with respect to the annular casing, an angular profile for the gap between the turbine blade and the casing can be obtained in a much quicker and more efficient way than a conventional system.