A datum is any reference system against which measurements are made, and forms the basis of all hydrographic survey work. Vertical datums have traditionally come in two categories: those based on a gravitationally level surface, called orthometric datums, and those based on tidally-derived surfaces, called tidal datums. A third category of datum, which has come into widespread use since the advent of space-based navigation such as the Global Positioning System (GPS), are ellipsoidal datums which are based on a simplified mathematical model of the shape of the earth.
Different geospatial applications may utilize different vertical datums. All GPS positioning data are referenced to an ellipsoidal datum. Since NOAA’s nautical charts utilize tidal datums with depths referenced to mean lower low water (MLLW), hydrographic survey data must be referenced or “reduced” to MLLW for chart compilation.
Enabling Multidisciplinary Use of Coast Survey’s Data
Since 2016, NOAA hydrographic field units have been acquiring bathymetry using an ellipsoid reference in order to improve the efficiency, accuracy, and flexibility of survey operations. Bathymetry referenced to the ellipsoid can subsequently be transformed to any one of many ellipsoidal, orthometric, or tidal datums supported by NOAA’s Vertical Datum Transformation (VDatum) tool, freeing the hydrographer from reliance on concurrently operating shore-based water level gauges. NOAA hydrographers utilize VDatum to transform bathymetric survey data to MLLW for compilation to NOAA’s nautical charting products. Other users may utilize VDatum to transform NOAA bathymetry to an orthometric datum – such as NAVD88 for USGS topographic maps – for the production of seamless raster datasets.
Benefits of Ellipsoidally Referenced SurveysEfficiency - When real-time positioning methods are used, data collection and processing (final datum reduction) can be performed concurrently.
Accuracy - Ellipsoid-based approaches can be significantly more accurate than traditional survey methods and will be critical for enabling precision navigation applications in the future.
Flexibility - This method eliminates the need for concurrent water-level gauges for survey support, increasing operational flexibility, particularly in disaster response areas.