Vertical Datum Coordinate Reference System

Introduction

SOS, as well as other OGC specifications, requires that a reference system be provided along with the coordinates when conveying geospatial information. Reference systems are encoded in the OGC services by using a code that uniquely identifies that reference system. OGC recommends using the EPSG (European Petroleum Survey Group) codeset, which holds more than 11,000 codes and has a online registry available at http://www.epsg-registry.org/.

Describing the vertical position of marine platforms and sensors is complicated by the lack of fixed and geolocatable reference systems in many marine environments. Marine platforms and sensors can float freely on the surface of the ocean, sit underneath a layer of water with uncertain density, or be mounted on the seafloor. A common vertical referencing problem occurs when the time-varying surface of the sea is ignored. This could happen when a floating platform is referenced to an ellipsoid or geoid, or when determining the depth of the sea for a bottom-mounted sensor.

To simplify, three base marine systems could be depicted: Sea level based systems, geoid based systems and bottom based systems. These are explained further in this section.

Common sitings for marine data

Sensors for marine systems can be located on several types of platforms:

1. Fixed sensor, geolocatable in three dimensions (e.g.: NOAA Tide Gauge with NAVD88
   
2. Fixed sensor, geolocatable in two dimensions (e.g., NOAA Tide Gauge with no NAVD88)
3. Fixed sensor sitting on the bottom (e.g. Sediment tripod)
   
4. Floating sensor suspended from the surface (e.g. the temperature sensor at 1m below the waterline of NDBC 44004 )
   
5. Floating sensor suspended from the bottom (e.g.: Polar Ice Thickness Sensor
   

Sea Level-based systems

Sea level-based systems refer to platforms floating on the surface of the water. The waterline of the platform may move up and down with the surface of the water. Its exact vertical location may not be known to adequate precision; however, the measurements with respect to the waterline would be well known. The EPSG code for vertical reference system referring to free surface of the water is EPSG:5113. The EPSG states this is “not recommend for use” because there is no consistent transformation from the free surface of the water to a geodetic datum.  However, if the data being served is best represented as measured from a floating platform, EPSG:5113 is the best fit.  As an OGC URN the combined CRS is “urn:ogc:def:crs,crs:EPSG:6.17:4326,crs:EPSG:6.17:5113”.

GeoID

The National Geodetic Survey (NGS) has adopted the following definition for geoid: “The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level”

A common geoid vertical reference system is the North American Vertical Datum of 1988 height (NAVD88), EPSG code 5703.   A combined OGC URN for WGS-84 latitude and longitude with a NAVD88 vertical height is: “urn:ogc:def:crs,crs:EPSG::4326,crs:EPSG:6.17:5703”. 

Combined CRSs using geoids in other locations would replace the EPSG:6.17:5703 with the appropriate EPSG code.

Bottom-based systems

Bottom-based systems refer to systems on the sea floor or tethered to the sea floor. An important issue about bottom-based measurements is that you can't survey them well because of several reasons.  1) GPS signals don't penetrate the water so you can't use a GPS directly 2) depth sensors depend on the density of the water, and 3) stationing high grade GPS over a deployment long enough to get high-accuracy bottom measurement is expensive.  Also, there is no EPSG code for using sea floor as a vertical reference system. 

Data referenced to bottom mounted sensors may be best represented within the OGC URN system with a 2-dimensional WGS-84 CRS using OCG URN code “urn:ogc:def:crs:EPSG::4326”. 

OGC Namespaces for Combined CRSs

• urn:ogc:def:crs,crs:EPSG:6.15:4326,crs:EPSG:6.15:5113 -- WGS84 with sea level
• urn:ogc:def:crs,crs:EPSG:6.15:4326,crs:EPSG:6.15:5703 -- WGS84 with NAVD88

Revision History

TODOs