What datum was in use in the UK, along with the frequency of data collection from 1915 to 1921?

The correct choice identifies the Ordnance Datum Newlyn as the datum in use in the UK during the specified period from 1915 to 1921, with the data collection frequency of every 15 minutes. This datum is significant as it is the official reference point for measuring elevations in the UK, established based on mean sea level at Newlyn, Cornwall. The regular collection of tidal data every 15 minutes allowed for precise monitoring and analysis of tide behaviors, which was crucial for nautical navigation, coastal development, and understanding hydrodynamic processes in the region.

The choice emphasizes both the importance of the Ordnance Datum Newlyn in providing a consistent vertical reference for diverse applications and the critical frequency of observations that enabled the collection of reliable tidal data over time. This frequency strikes a balance between gathering enough data to reflect tidal changes without overwhelming the processing capabilities or resources available at the time.

Understanding this context is essential for those studying hydrographic surveying, as it illustrates how accurate datums and systematic data collection were foundational to effective maritime navigation and coastal management.