Even for an armchair sailor such as myself this is a really interesting book. The first, large, section deals with ‘the limits of accuracy’ of nautical charts. Maps and charts are always a projection of a curved surface onto a flat representation. The projection necessarily introduces distortions. If the curved surface is a sphere the distortions can be fairly easily accounted for; if an ellipsoid the problem becomes harder but still tractable. But if, as is the case with the earth, the curve is a complicated shape, the distortions can be significant. This problem was partially solved in the past by mapping modest sized sections of the planet using an ellipsoid model that was a best fit for the mapped region. Such a section is called a ‘datum’, and there are many of them that together cover most of the earth’s surface. It was not until the early 80s with the advent of satellite photography and laser metrology that a global best-fit datum was able to be produced. The global datum is an ellipsoid that is globally a best fit for the not quite ellipsoidal earth. Use of this datum is becoming the standard for new nautical charts, though not universally yet. Much chart data is still derived from much earlier surveys, and even recent surveys will suffer from the distortions caused by the difference between the global datum and the actual shape of the earth.
None of this caused a really acute problem in the time before GPS. Navigators knew that their own ability to determine their position was worse than any inaccuracies in their charts, and so steered very well clear of any charted hazards. But with differential GPS you can determine your position to within a very few meters of your ‘actual’ position: ‘actual’ relative to the actual earth. This means that you now have to worry much more about the datum distortions, as well as inaccuracies caused by the sometimes very old surveys - some dating from the early 19th century.
Calder deals clearly with these issues, as well as many others regarding electronic charts and the dangers of zooming in further than the underlying data can justify, the question of timely updates, differences in charting standards and notation around the world, and many other topics.
The bulk of the book is a very detailed explanation of the standard symbols used on nautical charts, their interpretation and meaning, and the differences to be found depending on the publisher of the chart. This section is an excellent reference, I’m sure, but not really the sort of thing you would use for bedtime reading.