The inner surface of an underground tunnel defining structure, such as a culvert or pipe, is measured by apparatus including a portable hand held battery powered laser unit releasably attached to a rotor disk supported within the tunnel by an adjustable stand or tripod for rotation on a generally horizontal axis. The disk has peripherally spaced notches which receive a spring-biased latch member for positioning the disk and laser unit at predetermined angular positions about the axis. The laser unit emits a laser beam to a target on the inner surface of the structure and displays on the unit a precision distance reading from the laser unit to the target. The distance reading for each target is entered in a chart for comparison with a prior measurement reading to indicate changes in the shape or profile of the inner surface.