Joe Betit has been involved with cutting edge surveying and geospatial technology for his entire career. With a diverse background that includes academia, President of the California Land Surveyors Association and some of the most demanding construction survey control projects, I am honored to have Joe provide this article on the challenges of integrating datums 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.
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