Ground Versus Grid: A Pipeline Staked from a Stale Base Map
Your firm is setting control for a 2,000-ft water pipeline in San Bernardino County, within CCS83 Zone 5. Two temporary points, CP-1 and CP-2, are observed by 45-minute static GNSS sessions and processed against three NGS CORS in the National Spatial Reference System, producing NAD83(2011) epoch 2010.00 coordinates. The grid inverse between them is 1,999.756 ft. A total station at CP-1, backsighting CP-2, measures CP-2 at 2,000.000 ft, reduced to horizontal at the mean site elevation of 1,180 ft. The resident engineer holds a 1998 topographic base map carrying CCS83 Zone 5 coordinates computed in NAD83(1991), and wants the pipeline staked so the new work overlays that base map exactly.
1. Reconcile the 0.244-ft difference and state the combined factor.
Read the worked answer
The 2,000.000 ft is a ground distance; 1,999.756 ft is the same line expressed on the Lambert grid.
Combined factor = 1,999.756 / 2,000.000 = 0.9998780.
Decompose it to check the work:
- Elevation factor = R / (R + h) with R ~ 20,906,000 ft and h = 1,180 ft = 0.9999436.
- Scale factor = combined / elevation factor = 0.9998780 / 0.9999436 = 0.9999344, a reasonable value at this latitude in Zone 5.
Meaning: distances between monuments must be measured and set on the ground. Grid distances are for coordinate computation only. Staking the pipeline at grid distances without correction would put every station 0.244 ft short over 2,000 ft - small per setup, but cumulative over a pipeline and enough to miss the specified 0.10-ft tying tolerance.
2. How do you handle the 1998 base map produced in NAD83(1991)?
Read the worked answer
Do not fit the field work to the old map. NAD83(1991) is a different realization from NAD83(2011); in southern California the horizontal shift between them is on the order of a meter, and it is not uniform across a project.
Steps:
- Transform the 1998 coordinates into NAD83(2011) epoch 2010.00 (NGS HTDP or an equivalent published transformation) before making any comparison.
- Check the transformed base map against found monuments and against your GNSS-derived control; the residuals tell you how good the 1998 work really was.
- If the base map still disagrees materially, use the monuments and the new control as the basis of the project and report the base-map discrepancy in writing to the engineer.
- Note on all deliverables which realization and epoch the coordinates are in. A coordinate pair without the frame and epoch is not a position.
3. You must give the field crew an azimuth and distance to set a stake 2,000.000 ft from CP-1 along the line to CP-2. What corrections apply?
Read the worked answer
Two separate questions: what the instrument measures, and what the map azimuth means.
- Distance: the horizontal ground distance at site elevation is 2,000.000 ft. If the total station reduces slope to horizontal at the station elevation rather than the mean line elevation, apply the difference in elevation between CP-1 and CP-2 before comparing; and confirm that the scale factor, prism constant, and atmospheric corrections are set correctly in the instrument.
- Azimuth: the GNSS-derived azimuth is a geodetic azimuth. A CCS83 grid azimuth differs from it by the mapping angle (line convergence), which is small but not zero in Zone 5. If you direct the crew by grid azimuth while measuring ground distances, you must apply the arc-to-chord (t - T) correction to reconcile the two.
Cleanest practice: give the crew a geodetic azimuth and a ground distance, and require them to record the raw slope distance, instrument and target heights, and temperature and pressure.