Related Clues:

• Perhaps where the water is calm one may start to reckon how high it is
• It's used in reckoning the height of hills
• At this point, land appears
• Zero point for map heights
• Standard for measuring map heights
• From which the heights of hills etc are measured
• It rises every year
• High ground is above it
• Witness told has some neck in between the French and the Spanish indicating how the land lies!
• Elevation standard
• Ben's height is measured relative to this Cork daredevil biker
• Where heights are measured from
• Calm conditions, presumably, at this reference height?
• From which altitudes are measured
• Altitude reference point