I have been using GPS for bathymetry survey for several years. I use 3 GPS units - 1 rover on boat, 1 base on a Govt GPS Control Point and 1 base on a Govt Benchmark with MSL elevation.
I use the difference between ellipsoidal ht derived and MSL elevation value at the Govt benchmark. I apply this difference to get MSL values at the rover.
My concern is how far can I use this method? My project area keeps on getting farther and farther away from the nearest shoreline.
Anyone can refer to me a "maximum" distance to transfer MSL values to the rover? tried to read IHO Standards but most refers to manual methods.
That value is only good in a small area around the benchmark. The geoid slopes usually perpendicular to a shoreline, for the reason that there is often a deep trench offshore (deficiency of mass, the opposite of a mountain). This worldwide map shows the phenomena well because it has contours:
If you do not have decent geoid model in the area you are working in you can at least use the EGM08 model in a relative sense (i.e. use difference in geoid heights to mode the slope). It does not model small localized features in the geoid, but it will give you the general trend.
When I started using GPS there was no geoid model at all, we had to occupy benchmarks surrounding the project. Of course, offshore you have a problem because all of the benchmarks are onshore, and there can be a change in slope at or near the shore.
On the east coast of Florida, where the land is quite flat and featureless, there is a relatively large east-west deflection of the vertical and a much smaller north-south deflection. These values are simply the tilt of the geoid.
What sizes of areas are you usually surveying?
10 years or so ago, areas are just 1-2 has next to a client's property. Recently however, clients are requiring a strip bathymetry survey of 100 m x 10 km perpendicular to shoreline. I set my limit to 10km because I tell my client this is the limit of GPS technology. I know I can get good positions up to ~20km but I want to be on the safe side of my data.
For areas that are far from the shoreline, I usually find 2-3 more base points along the shoreline that would put it nearer the farthest point of the survey. Eventually you just run out of dry areas for your bases.
