The first requests for sea level information arose from the needs of inshore navigation and access to ports with substantial tidal ranges. There was a need for real-time or only slightly time-delayed measurements.
To meet this need, the information was obtained by direct vision:
- Either from the tide staff available at the entrance to ports or along channels in the maritime portion of rivers;
- Or, in the major seaports, daytime signals using cones and cylinders, and night-time signals using green, red and white lights.
More modern systems were developed in some estuaries