GNSS Topography, Grid-to-Ground, and Drainage Grades
A solar developer hires your firm to map 55 acres of rolling pasture for a fixed-tilt array. The design engineer needs ground elevations to design drainage swales with a minimum 0.5% slope over an 800-ft run. Your control is on the State Plane Coordinate System (combined factor 0.99987), and the design will be built in ground coordinates. You collect RTK GNSS data on a 50-ft grid; the receiver reports horizontal precision 0.03 ft and vertical precision 0.08 ft. Two control points established by a previous survey are 1,000.00 ft apart by total station on the ground; their State Plane inverse is 999.87 ft. A check into a third point (a brass cap set in 1998) shows del = +0.12 ft. The engineer wants the deliverable in three days and asks whether GNSS alone is 'good enough' for the swales.
1. Is RTK GNSS alone adequate for this deliverable, and how do you handle the combined factor?
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RTK alone can be adequate for the horizontal mapping, but not without a coordinate decision and not blindly for the design. The observed State Plane inverse of 999.87 ft against a 1,000.00-ft ground measurement confirms a combined factor of about 0.99987. If the design is built in ground coordinates, grid coordinates must be converted to ground: divide grid distances by the combined factor, equivalently multiply by about 1.00013. If you deliver grid coordinates and the engineer designs in ground, every distance is short by about 0.013 ft per 100 ft - small per shot, but a systematic error across an 800-ft swale and the array layout. State the basis of bearings and whether the file is grid or ground.
The 0.03-ft horizontal precision is more than adequate for a 50-ft topographic grid. The risk is not random precision; it is a wrong projection, a wrong geoid, a bad base coordinate, or a scale blunder. Check the third control point, verify the projection and units, and run a total-station check between two control points after the adjustment.
2. What must you do to verify vertical control for the 0.5% swales?
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Vertical is the weak link. A reported 0.08-ft RTK vertical precision is a precision estimate, not a guarantee of accuracy, and the 0.5% grade over 800 ft gives only 4.0 ft of fall. An error of 0.10-0.12 ft is 2.5-3% of that fall and can create a flat or reverse segment, ponding, or a swale that will not drain. The +0.12-ft check into the 1998 brass cap also suggests a datum or geoid issue that must be explained, not averaged away.
Verify the vertical control with a level loop or a total-station trigonometric level run between at least three benchmarks, closing within a defensible tolerance for the design, often 0.05 ft or tighter for drainage. Confirm the geoid model used, antenna heights, base station coordinates, and whether the design datum is NAVD 88. Compare GNSS-derived elevations to the level run at several checkpoints. If the check point is genuinely 0.12 ft off, determine whether it is the cap that moved or the GNSS-derived datum; do not simply shift the whole site without evidence.
3. What should the deliverable and metadata include?
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Deliver a metadata package with the survey: coordinate system and zone, units, combined factor and whether the file is grid or ground, basis of bearings and convergence, vertical datum and geoid model, control point descriptions and sources, observation dates and equipment, and the level-loop closure. Include a point file, breaklines, spot shots, and a surface that reflects the swales and drainageways, not just a random grid.
Note the accuracy statement: horizontal positions meet the mapping requirement; vertical control was verified by level loop to a stated closure; elevations are suitable for drainage design with the stated tolerance. Show the two control points, the 1998 brass cap, and the check results. If the engineer wants GNSS-only vertical, put the limitation in writing and recommend spot field verification of swale inverts before construction. The deliverable should let the next user reproduce the coordinate and elevation decisions without calling you.