I put an SQM on a 1 km grid across Lisbon and made a lovely contour map of something that cannot honestly be contoured. Four months, one admission.
Chart Datum at Avonmouth sits 6.50 m below Ordnance Datum Newlyn. Here is the arithmetic that goes wrong when a height file does not name its zero.
0,0 is three kilometres of open water in the Gulf of Guinea and the busiest coordinate on Earth. Why zero and nothing keep collapsing into each other.
Gauss closed this in 1827 and Tissot gave us a way to measure the damage. Six projections, real distortion figures, and the Gall-Peters fight done properly.
You've read everything.