Johnson Level & Tool Mfg. 40-6902 - Manual (Page 7)

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5.2 Instrument accuracy check
Set up the instrument in an area that is as level as possible and
which is about 220 ft. long. Place two matching level rods about
200 ft. apart with the faces toward each other. Position and level
the instrument so that the distance from the instrument to each rod
is the same. (Fig. 1)
Take a reading on each rod with the instrument. Note the difference
and record them. Next, move the instrument to another point in line
with the two level rods as shown in Fig. 2. Level the instrument and
take readings on the two level rods. The difference should be the
same (A-A should equal B-B). The difference between A-A and B-B is
the instrument error at 200 feet.
Fig. 1
Fig. 2
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