vertical based rangefinders, where the known base length is the height of the rangefinder from sea level, were first used at viapori in the 1880s. vertical based rangefinders required only one instrument, but they were susceptible to interference from vibration caused by firing of the guns. vertical based rangefinders also required a substantially high measurement site that was a problem on the low islands. to measure distance with reasonable accuracy to a target away the rangefinder had to be at least 10 fathoms () and preferably 20 fathoms () high - often impossible to achieve. measurement sites had usually the rangefinder on a stone or masonry column high lying on a granite foundation. the measurement pavilions had simple wooden walls and sheet metal roof protecting them from the elements and were in size. vertical rangefinders were no longer used on the new forts built on the outer islands.