Wartime results indicate the average error for direct spotting was 50-80yds regardless of range. Radar retains a fairly significant advantage in range spotting at extended ranges as well. Radar not only has smaller average ranging error but provides ranges continuously typical time to receive ranges, plot then derive a mean range line is 15-30 seconds depending on the density of ranges. Using an error budget* derived from fleet experience an Iowa class BB using all 5 range finders might produce an error of: While optical ranges can indeed rival radar ranges at times, they typically exhibit a large variability from “cut to cut” which is why ranges are plotted and a mean range line is derived from this plot. The recommendation even with Mk13 was to use radar for range, and optics for bearing if possible. Near misses for deflection would show as hits on the radar. The optics were better at determining if the MPI was left or right of the target than even the MK13 radar. The same 1984 test also found that the optical range finder could not determine the pattern of the fall of shot of a salvo beyond 26,000 yards, but it could still determine if the MPI was long or short (but not by how much) all the way out to 40,000 + yards. The German large range finder was 10 meters wide. Compare to the German tests of rangefinders posted by Thorsten here: This means that the accuracy of the optical rangefinder was about 150 yards at 44,000 yards. So at 44,000 yards the accuracy of the radar was about 50 yards. The accuracy of MK13 was 0.1% of the range with a tolerance of 5 yards. It was within 100 yards deviation from the Mk13 radar range all the way out to 44,000 yards. From tests in 1984, the accuracy of the Iowa's foretop range finder was compared to indicated range on the Mk13 radar.
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