Advances in Seamless DTM Integration for Nearshore Surveys
by Joe Betit
The integration of the near shore interface of horizontal and vertical datums has been a persistent source of problems for engineering design of major infrastructure and their construction dimensional survey layout. Compounding the inherent datum integration problems is the different approaches to survey used by offshore and onshore surveyors despite their often using the same equipment.
Offshore surveyors work in a dynamic water environment without fixed monuments so they normally use GNSS survey equipment, GNSS continuously operating reference stations and software operating in full geodetic survey mode. In contrast, onshore surveyors laying out large plant infrastructure are in a static ground based, very high precision dimensional control environment. They use GNSS to establish accurate geodetic positions on fixed monuments and for real time machine control of earthwork machines. Overlaying the geodetic survey control is a local horizon plane coordinate system of Scale Factor 1 established using robotic total stations that is used for engineering design and dimensional survey layout. In addition, benchmark monuments with published national vertical datum information are used as the basis to extend high precision elevation information into the project.
Enter the dawn of Seamless Datums that provide vertical offshore reference frames integrated with the onshore vertical reference frames. Projects such as the European Union BLAST (Bringing Land and Sea Together), United Kingdom VORF (Vertical Offshore Reference Frame) and the USA VDatum (Vertical Datum Transformation software tool) are making great strides in providing effective integration of the land and sea vertical datums. The GNSS global reference ellipsoid is used as the horizontal and vertical datum integrator in these systems. The GNSS ellipsoid coordinates and heights are the common reference frame used for most of onshore and offshore surveys today. It is also the most stable, repeatable and realizable of the ellipsoids in use now.
The most advanced projects are the VDATUM and the VORF systems. The VDatum, VORF and Blast projects have all brought effective regional or national systems on line in the last three years that are already being used in production surveys. Systems such as the US VDATUM are continental and multi-ocean in scope so it has been proceeding in phases region by region guided by the priorities of environmental and economic importance. Now, other nations such as Australia with their Demonstration Project VDatum are working on implementing similar systems that are continental and multi-ocean in scope. There is extensive ongoing collaboration between the national agencies, prominent private sector hydrographic survey companies and universities.
These seamless datum systems are continually evolving models that are based on integration of an astonishing array data sources: tide gauge, tidal modelling, a multitude of chart datums, GNSS, satellite altimetry, GNSS derived ellipsoidal heights, on-shore benchmark information, satellite gravimetric geoid, coastline polygons, river polygons, bathymetric models, ellipsoid models and geodetic projection systems. A recent addition to the available data sources is the combined use of red (ground) and green (shallow water) LiDAR to map the near shore interface of beach and bathymetry DTM surfaces.
In addition, the underlying software of the models are accessible and can have more fine-grained survey data input into them to enhance the accuracy of the immediate project areas of interest. The reference surface used for transformations in these models are similar in concept to gridded geoid models.
Beyond the needs of the designers and surveyors of the industrial infrastructure at the near shore, there are a myriad of other exceptionally valuable uses for these new seamless vertical datum models. Some examples are storm surge modeling, coastal management, shore erosion management, disaster planning and response, sea level/land subsidence studies, nearshore ecosystem zone studies, maintenance of channel depth, maintenance of harbor depth, beach replenishment, Continental Shelf exploration and greatly improved efficiency of offshore hydrographic surveys.
For the geospatial communities there is an economic opportunity benefit to be derived from these highly integrated vertical datum reference frame multi-data source modeling systems. It is a similar situation to be found in the rapidly expanding drone industries. A drone is an integration of a very wide array of flight, navigation, communication and sensor technologies with associated impacts on data processing and integration into civil air space control. This has resulted in the leveraging of the basic flight machine into an ever-expanding array of services and products far beyond what any one element of the drone could accomplish on its own.
All the collaborating governmental geomatics and offshore survey agencies, universities and private sector participants that have pressed ahead with these projects deserve our applause and thanks. This is truly a case of unsung government and private sector technical heroes, their work little understood or appreciated by the ordinary civilian world, pushing ahead into a future that will provide huge benefits in health, safety, welfare and productivity for the world.