Offsets by Eye – Maximum Practical Length for Judging Perpendicularity Without Instruments In basic chain surveying, up to what offset length can a right angle from the chain line generally be judged by eye with acceptable accuracy before a cross-staff or optical square is preferred?

Introduction:
Setting out perpendicular offsets from a baseline is routine in chain surveying. While instruments like cross-staffs and optical squares ensure right angles, short offsets are sometimes judged by eye to save time. Knowing the practical limit avoids unacceptable plan errors.

Given Data / Assumptions:

• Flat, unobstructed ground.
• Competent observer with normal visual acuity.
• Acceptable plotting accuracy for small-scale plans.

Concept / Approach:

Angular error in visual judgment increases the lateral error at the feature by approximately offset_length * sin(error_angle). For larger offsets, even small angular misjudgments create significant displacement. Field practice commonly limits by-eye perpendiculars to about 5 m; beyond this, a cross-staff or optical square is recommended to control angular error.

Step-by-Step Solution:

Verification / Alternative check:

Surveying manuals cite ~5 m as the practical upper limit for judging right angles by eye, aligning with common site experience.

Why Other Options Are Wrong:

10–20 m offsets magnify angular errors; 2 m is overly conservative and unnecessarily slows work.

Common Pitfalls:

Relying on visual judgment for long offsets; not rechecking with an instrument in built-up or high-accuracy work.

Final Answer:

5 m