The tall consultant pores over a huge printout that looks like a nautical chart, checking information and making notes.

Slack water can be estimated using a tidal atlas or the tidal diamond information on a nautical chart.

It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart.

The pilot must either be familiar with those visual references or be able to discover them from a map, aeronautical chart or nautical chart.

The portolan was a realistic, detailed nautical chart, gridded by compass lines, that could be used to deduce exact sailing directions between any two points.