Levelling instrument care – permanent adjustments of a dumpy/automatic level: How many permanent adjustments are classically recognized for a level instrument used in surveying?

Introduction / Context:
Permanent adjustments ensure that a level reads true horizontals when the bubble is centered. Recognizing how many such adjustments exist helps technicians maintain instruments and diagnose errors quickly in the field.

Given Data / Assumptions:

• Instrument is a dumpy or automatic level used with a levelling staff.
• “Permanent adjustments” are those maintained at all times, not temporary set-up steps.

Concept / Approach:
Classically, a level has two permanent adjustments: (1) The axis of the bubble tube (or the line of the compensator in an automatic level) must be perpendicular to the vertical axis. (2) The line of collimation (line of sight) must be parallel to the axis of the bubble tube. When both are satisfied, centering the bubble makes the line of sight horizontal. Other checks (like focussing to eliminate parallax) are routine but not permanent “adjustments.”

Step-by-Step Solution:

Verification / Alternative check:
Two-peg test detects collimation error; symmetrical readings before and after leveling confirm correct permanent adjustments.

Why Other Options Are Wrong:

• 3, 4, or 6 adjustments overstate the number for a level; such counts might apply to other instruments (e.g., theodolite).

Common Pitfalls:
Confusing temporary levelling (foot-screw operations) with permanent adjustments; assuming automatic compensator removes the need for collimation checks.

Final Answer:
2 in number