How is 52% of the data for Irish hydrographic charts categorized?

The categorization of 52% of the data for Irish hydrographic charts as pre-1900 indicates that a significant portion of the existing information stems from historical surveys conducted in the 19th century or earlier. This reflects the long history of hydrographic surveying in Ireland, particularly during a time when navigation and maritime activities became crucial for commerce, exploration, and defense.

Historical charts provide foundational data that are often critical for understanding the evolution of coastal features, navigational hazards, and changes in bathymetry over time. They also serve as an important reference for modern surveys and developments. Recognizing that a considerable amount of hydrographic data originates from this early period emphasizes the importance of archival records and traditional surveying methods that were employed.

The other options suggest more recent data categories, which might include contemporary advancements in hydrographic surveying but do not account for the foundational legacy of the earlier data that governs a significant portion of the charts.