distance measurement at the model 1891 batteries was based on horizontal triangulation from two different points, a primary and secondary, on separate islands. the rangefinders used were general von der launitz's models 1903 and 1905-07. the average distance between the two points at krepost sveaborg was . target information was sent from the primary point to the battery command point. if the primary point was located away from the battery or directing a different battery a rangefinder converter designed by colonel kovanko was used to calculate the firing solution. a weakness in using two measurement points was that any malfunctions in either of the rangefinders or in the telephone system would render the system inoperable. target indication between the primary and secondary points could also pose problems, as there would often be several similar target ships. the system was also not capable of measuring distances to targets in the rear of the battery; a maximum measurement sector was 140°.