The Texas Coastal Ocean Observation Network and coastal elevation datums

Abstract

The Division of Nearshore Research (DNR) at Texas A&M University-Corpus Christi operates the Texas Coastal Ocean Observation Network (TCOON.) The network collects near real-time data, including water level, wind speed & direction, barometric pressure, water temperature, and air temperature from stations placed in bays and estuaries along the Texas coast. TCOON provides this critical data to many users, including those in the commercial shipping industry, marine construction, water-land boundaries, recreational boaters, and those responsible for marine safety and emergency evacuation in the event of a hurricane. Water level observations are carried out to national standards set by the US National Ocean Service (NOS) and are used to compute tidal datums including Mean Sea Level. For coastal inundation modeling, the relationship between tidal datums and terrestrial datums used for onshore topographic mapping needs to be well understood to assist planning in coastal areas. NOAA's National Ocean Service is currently addressing the issue of coastal elevation datums with VDatum software. VDatum is described on NOAA's web site: The coastal land-water interface depends on how water levels change and the land moves vertically in both space and time. To combine or compare coastal elevations (land heights and water depths) from diverse sources, they must be referenced to the same vertical datum in a common framework. Using inconsistent datums can cause artificial discontinuities that become acutely problematic when producing maps at the accuracy that is critically needed by federal, state, and local authorities to make informed decisions. The flow of water is also greatly influenced by gravity, not only the astronomic gravitational force from the moon and sun, but local gravity anomalies contained within the earth's crust. A better understanding to the gravity field is also under study by NOAA's National Geodetic Survey with the Grav-D program. NOAA's description of Grav-D includes: Accurate gravity data is the foundation for the Federal government's determination of heights, and whereas this document is not the appropriate place for a thorough dissertation on the definition of “height” and its inseparable connection to gravity- significant information on the subject of height and gravity has been documented in literature over the last century-a brief primer on the connection between gravity and height is provided below. One critical point to be made is that in the past 20 years, the use of GPS technology for determining fast and accurate ellipsoid heights has created a pressing desire for a similarly fast and accurate determination of orthometric heights. Ellipsoid heights cannot be used to determine where water will flow, and therefore are not used in topographic/floodplain mapping. Orthometric heights are related to water flow and are more useful (and are colloquially-although not quite appropriately- referred to by the more common term “height above sea level”). In order to transform from ellipsoid heights to orthometric heights, a model of the geoid must be computed, and geoid modeling can only be done with measurements of the acceleration of gravity near the Earth's surface. This presentation describes how tide data, tidal datums, terrestrial datums, VDatum, and Grav-D will assist in understanding the changes taking place along the coast.