An upper stem diameter measurement (“USDM”) and basal area determination device for timber cruising operations incorporates a viewing window in which are projected variable, visual brackets for manual alignment by the user, or automatic setting under processor section control, with the left and right sides of a target tree stem or trunk. The device further includes a built-in inclinometer such that computations of height and stem diameter can be automatically adjusted depending on the user's line of sight with respect to a horizontal plane. In a preferred embodiment, a user actuatable keypad is provided for inputting data, such as a desired operational mode, a specified basal area factor and the like, a user viewable display as well as control buttons for adjusting the visual brackets and indicating an acceptance of various of the device parameters and operational characteristics.