A headset for use in validation of a construction task or setting out a construction site which comprises an article of headwear having a plurality of sensors (602a, 602b, 602c... 602n) mounted thereon which are responsive to one or more signals received from a position tracking system at the construction site , augmented reality glasses incorporating one or more displays for displaying a virtual image of a building information model (BIM) when viewed by a user and an electronic control system comprising: a model positioning engine for receiving and processing model data representing the building information model defined in an extrinsic, real-world coordinate system and using a transformation between the extrinsic coordinate system and an intrinsic coordinate system which is defined by the position tracking system for positioning and orienting the model in the intrinsic coordinate system; and an image rendering engine for receiving tracking data representing the position and orientation of the article of headwear on the construction site in the intrinsic coordinate system, rendering a virtual image of the model relative to the position and orientation of the article of headwear and transmitting the image to the one or more displays where it is viewable by a user as a virtual image of the model; wherein the tracking data is derived from sensor data representing the responses of the sensors (602a, 602b, 602c... 602n) to the one or more signals emitted by the position tracking system ; and the transformation is derived by relating the coordinates of one or more control points of known location in the extrinsic coordinate system to their corresponding coordinates in the intrinsic coordinate system, the coordinates of the one or more control points in the intrinsic coordinate system being derived from sensor data representing the responses of one or more sensors positioned at the one or more control points to the one or more signals. a model positioning engine for receiving and processing model data representing the building information model defined in an extrinsic, real-world coordinate system and using a transformation between the extrinsic coordinate system and an intrinsic coordinate system which is defined by the position tracking system for positioning and orienting the model in the intrinsic coordinate system; and an image rendering engine for receiving tracking data representing the position and orientation of the article of headwear on the construction site in the intrinsic coordinate system, rendering a virtual image of the model relative to the position and orientation of the article of headwear and transmitting the image to the one or more displays where it is viewable by a user as a virtual image of the model; wherein the tracking data is derived from sensor data representing the responses of the sensors (602a, 602b, 602c... 602n) to the one or more signals emitted by the position tracking system ; and the transformation is derived by relating the coordinates of one or more control points of known location in the extrinsic coordinate system to their corresponding coordinates in the intrinsic coordinate system, the coordinates of the one or more control points in the intrinsic coordinate system being derived from sensor data representing the responses of one or more sensors positioned at the one or more control points to the one or more signals. <EOS>