1. Zero error calibration of horizontal position
After the bubble is stable, read a at one end of the bubble in the horizontal position, then turn the level 180 degrees and place it in the original position of the plate, record the reading b at the other end of the bubble according to the first reading, and take two readings. Half of the difference is zero bit error.
2. Zero error calibration of vertical position
Adjust the indexing head so that one end of the bubble in the horizontal position is aligned with the long scale line, turn the indexing head through 90 degrees, read c at the corresponding end of the bubble in the vertical position, add c to the zero error of the horizontal position, which is the lead Zero error in vertical position.
3. Zero error calibration at 45 degree position
Adjust the indexing head so that one end of the bubble in the horizontal position is aligned with the long scale line, turn the indexing head through 45 degrees, read d at the corresponding end of the bubble at the 45-degree position, add d to the zero error of the horizontal position, and it is 45 Zero error in degrees position.
