To recover an existing control point: locate it using the plan coordinates and a GPS rover or tape, confirm the monument description matches the plan (type, stamping, material), and verify the published coordinates by checking angle and distance to at least one other known control point. If the check fails by more than your project tolerance, do not use the monument without notifying the engineer — the point may have been disturbed.
Why Control Point Verification Matters
A disturbed control point that appears intact looks identical to an accurate one until you try to use it. On large projects, a concrete monument can be struck by equipment and settle by 0.5 ft without obvious visible damage. Using an unverified, disturbed control point propagates a systematic error through every layout and as-built shot on the project.
Locating Existing Control Points
Control points are described on the survey or project control sheet with: monument type (brass disk, iron pipe, rebar, nail in asphalt), location description (“SW corner of intersection, 2 ft back from curb”), and coordinates. To locate:
- Enter the published coordinates in a GPS rover and navigate to the approximate location.
- Search the described location for the monument. If the description says “brass disk in concrete,” look for concrete with a disk, not a rebar stub.
- If the monument is not found at the expected location, expand the search in a 5-ft radius. Then notify the surveyor — do not assume a nearby rebar is the correct point.
Verifying the Monument Is Undisturbed
After locating the monument, verify it by checking its position relative to at least one other known control point:
- Set up the total station on the monument.
- Orient with a backsight to another known control point.
- Shoot the original monument’s coordinates (from the control sheet) by staking out the point — the instrument should show “0.00” residual if the monument is in its published location.
- Alternatively, shoot a third control point and compare the measured coordinates to the published ones. If the agreement is within your project tolerance, the monument is verified.
When to Flag a Potential Problem
- Horizontal error > 0.10 ft at a check point suggests a disturbed monument or wrong monument ID. Do not use it.
- Monument freshly disturbed, cracked concrete around it, or evidence of recent equipment impact — flag it.
- Monument not at the expected depth for its type (a brass disk should be flush or near-flush; a deep offset from the described location suggests movement).
When in doubt, contact the project surveyor before doing any layout from suspect control. The cost of waiting is nothing compared to the cost of laying out an entire building or road alignment from a disturbed monument.