Resection Setup: Setting Up Over an Unknown Point

Resection is the process of computing the coordinates of an unknown instrument point by measuring angles and/or distances to two or more control points of known coordinates. It allows you to set up the total station anywhere with a good view of existing control, without needing to occupy a pre-positioned benchmark. Most modern total stations have a built-in resection function — use at least 3 control points for a reliable solution.

When to Use Resection

Resection is useful when:

• All available control monuments are occupied (other crews on site using them)
• You need a setup location with a better line of sight to the work area than any existing control point offers
• Control points were disturbed and you need to check their validity
• You want to set up on a convenient location (like a building corner) without pre-positioning a control point there

Resection Procedure (Modern Total Station)

1. Set up the total station on the unknown point. Level carefully. The instrument point coordinates are initially unknown.
2. In the instrument’s survey menu, select “Resection” or “Free Station.”
3. Aim at the first control prism. Enter the control point ID or coordinates. Measure the angle and distance. The instrument stores the observation.
4. Repeat for at least two more control prisms (three minimum for a reliable solution).
5. When enough observations are entered, the instrument computes the unknown station coordinates using least-squares adjustment. It displays the solution and the residuals on each observation.
6. Review the residuals. Residuals should be small (typically < 0.05 ft on distance, < 5″ on angle). Large residuals indicate either an error in a control point coordinate or a measurement error.
7. Accept the solution. The instrument can now be used as if it were set up on a known control point.

Minimum Control Points and Geometry

Two control points is the mathematical minimum for resection, but the solution can fail or be unreliable if the instrument, CP1, and CP2 are nearly collinear (lie nearly on the same line). Use three control points whenever possible. Aim for a well-distributed geometry — control points spread around the instrument at different azimuths, not all in one direction.

Verify Before Working

After computing the resection, stake a fourth known point and compare the measured position to the known position. The check should pass before doing any layout or as-built work from the resection station. A failed check means one of the control observations had an error — re-observe all control points with fresh measurements.

See also: Total Station Setup and Backsight Procedure and Traverse Closure and Adjustment.