Abstract

Achieving great heights with NOAA tools: VDatum and LOCUS
Track: Training
Authors: Michael Dennis

Heights ("elevations") are complicated. Some are reckoned as straight lines perpendicular to a reference surface (ellipsoid heights), some are curved lines parallel to gravity at every point (orthometric heights), and some "heights" have no geometric meaning at all yet tell you where water will go (dynamic heights). The "datum" to which heights refer can be a mathematical ellipsoid surface, which in turn is referenced to one of several different global or regional frames. It can be a vertical datum defined by a geodetic leveling network, or by a "geoid" - a gravitational equipotential surface more-or-less representing global mean sea level. It can be defined by a particular tide gage as local mean sea level, or mean high water, or mean lower low water, etc. Adding to the complexity, heights are measured by a wide variety of equipment, each with corrections, models, and methodologies that yield particular types of heights referenced to various datums over a broad range of accuracies.

NOAA has created two tools, VDatum and LOCUS, to help geospatial professionals make better use of height data and measurements. VDatum is free software developed jointly by NOAA's National Geodetic Survey (NGS), Office of Coast Survey (OCS), and Center for Operational Oceanographic Products and Services (CO-OPS). It vertically transforms geospatial data among a variety of tidal, orthometric, dynamic, and ellipsoidal height systems. This allows users to convert their data from different vertical (and horizontal) references into a common system and enables the fusion of diverse geospatial data into a uniform reference system. For example, VDatum can be used to combine a bathymetric survey referenced to a local tidal datum with a digital elevation model based on the North American Vertical Datum of 1988 (NAVD 88) into a single seamless surface model.

LOCUS (Leveling Online Calculation User Service) is a free Internet-based NGS service that checks, corrects, and adjusts leveling data submitted by users and provides heights with respect to published NGS vertical control. Philosophically, it is similar to the popular NGS Online Positioning User Service (OPUS) used for GPS data. As with OPUS, the intent is to make it as simple as possible for users to get correctly adjusted heights by uploading their leveling data via the Internet and immediately receiving results. For example, if a user wants to level with respect to NGS NAVD 88 benchmarks, LOCUS will apply the appropriate corrections (including the gravity model) to convert the observed leveled heights differences to NAVD 88 orthometric heights.

Whether combining existing vertical datasets from a variety of sources (VDatum) or correcting and adjusting precise vertical measurements (LOCUS), NOAA has developed tools that will assist users in achieving great heights.