A display apparatus for displaying an operational area to an operative of a host platform , said operational area being defined within an external real-world environment relative to said host platform, the apparatus comprising: a viewing device configured to provide to said operative, in use, a three-dimensional view of said external real-world environment; a display generating device for creating images at the viewing device; a user input configured to receive user input data representative of a specified target or region in respect of which an operation is to be performed and thereby defining an initial geometric volume for the operational area, said user input data including data representative of the location within said external real-world environment of said specified target or region and data representative of said operation to be performed in respect thereof; and a processor configured to: use said user input data to generate or obtain three-dimensional image data representative of an adjusted geometric volume based, at least, on said initial geometric volume and on said operation to be performed, and display one or more images depicting said adjusted geometric volume and created using said three-dimensional image data, on said display generating device, the apparatus being configured to project or blend said one or more images within said view of said external real-world environment at the relative location therein of the specified target or region, the apparatus being characterised in that : the processor is further configured to use said user input data and data from other sources to determine a likelihood of success of the specified operation to be performed, and generate an output representative thereof. a viewing device configured to provide to said operative, in use, a three-dimensional view of said external real-world environment; a display generating device for creating images at the viewing device; a user input configured to receive user input data representative of a specified target or region in respect of which an operation is to be performed and thereby defining an initial geometric volume for the operational area, said user input data including data representative of the location within said external real-world environment of said specified target or region and data representative of said operation to be performed in respect thereof; and a processor configured to: use said user input data to generate or obtain three-dimensional image data representative of an adjusted geometric volume based, at least, on said initial geometric volume and on said operation to be performed, and display one or more images depicting said adjusted geometric volume and created using said three-dimensional image data, on said display generating device, the apparatus being configured to project or blend said one or more images within said view of said external real-world environment at the relative location therein of the specified target or region, the apparatus being characterised in that : the processor is further configured to use said user input data and data from other sources to determine a likelihood of success of the specified operation to be performed, and generate an output representative thereof. <EOS>