What was the method used before the adoption of LAT as Chart Datum?

The method used before the adoption of Lowest Astronomical Tide (LAT) as Chart Datum was Low Water Ordinary Spring Tide. This reference point was utilized for hydrographic surveys and nautical charts prior to the widespread acceptance of LAT.

Low Water Ordinary Spring Tide represents a specific tidal condition, occurring during periods of normal spring tides, which are characterized by the highest and lowest tidal ranges. As such, it provided a reliable and consistent baseline for navigable waters during these predictable tidal events.

Using Low Water Ordinary Spring Tide allowed surveyors to account for varying conditions of water levels which are crucial for safe navigation and for determining clearances, depths, and other critical factors in hydrographic surveying. This datum was favored historically because it offered a measurable standard for safe passage, although it does not account for extreme low water conditions that might occur during exceptionally low tide events, as represented by LAT. The transition to LAT was a significant step as it allows charts to represent the extreme low water that can be expected, thereby providing more precise and safer navigation information.