Pentax PLP-601 Instruction Manual page 28

[Selecting the reference level detecting accuracy]
Push the detection accuracy selection switch to select the needed
detection accuracy.
High accuracy detection:
To be selected for detecting the reference level with accuracy.
Low accuracy detection:
To be selected if high accuracy is not needed or when stable reference
level cannot be obtained due to slight vibration at job site.
NOTE:
• For the detection accuracy to be selected, check with the mark in the
liquid crystal display (see the figure on the above).
• The detection accuracy alternates each time the detection accuracy
selection switch is pressed.
• High detection accuracy is initiated when the power turns on.
• If the point to be measured is distant, the reference level may not be
displayed stably by the influence of heat waves shimming or the
instrument slightly vibrating. Under such conditions,
select 'Low detection accuracy' .
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High accuracy
Low accuracy