Chapter 2 recap
Ground truth
The key takeaways from this chapter’s 2 lessons.
Utility records and quality levels
- Utility records show where networks were believed to be; their accuracy varies widely.
- Survey standards grade each utility by quality level: from records only (D) to exposed and measured (A).
- In the UK the reference is PAS 128; in the US, ASCE 38. Both use levels D, C, B and A.
- The quality level should travel with every utility in the model and decide safety zones and trial holes.
Coordinates over long corridors
- Projected grids distort distances slightly; the grid scale factor says by how much at each place.
- Height above the datum adds an elevation factor; together they make the combined scale factor.
- Ground distance = grid distance ÷ combined scale factor (for factors close to 1).
- Long projects often use a low-distortion project grid so that grid and ground almost match.