For those that do bathymetry survey using RTK GPS, what vertical datum do you use or set on both base/rover units?
Ellipsoidal Height or Orthometric Height (EGM96 or other geoid)?
Are there any significant reasons choosing one over the other?
I would survey in the datum that clients wants. Most likely, it would be orthometric heights. You could "elevate" your base in setting up the system using ellipsoidal for convenience. Unless the geoidal undulations are to be considered.
Example, your survey is one square mile. Geoid contours run east-west at an 0.5 inch/mile. Then keep it simple, straight orthometric heights elevated to ellipsoid, don't worry about geoid model. Clear as mud?
Step by step
1. You have a UTM base point with an orthometric elevation.
2. Set your base on that point, geodetic parameters UTM zone whatever.
3. Key in that point in your job.
4. Start pumping.
It is a bathymetric survey. Unless your accuracy is extraordinary, this simple method should work for you and your client.
I have tried to research orthometric values from GPS but can't seem to find any definitive answer.
When you get an orthometric value from a Rover unit, what are you really getting from the 2 scenarios:
1. Rover Ortho Ht = Base Ortho Ht + Base HI - delta Z - Rover HI
or
2. Rover Ortho Ht = Lat/Long of Rover projected onto EGM86 Geoid model (or any Ortho Geoid model)?
Scenario no. 1 is like a trigonometric vertical distance/elevation computation from a total station
Scenario no. 2 is like getting the ortho value by pin pricking a certain lat/long on the geoid model.
Look up Hypack hydrographic software. They have good information on geodesy.
Don't worry about geoid model at current time. First order of business is to get a clear understanding on how GPS/GNSS works, mathematically speaking.
Google the words "geodesy and hypack" together, lots of helpful information on line.
I would take a look at EGM2008
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html
1. Get a GOOD igs08 coordinate on your base
2. Run your survey in igs08 LLH (igs08 Ellipsoid Height)
3. Observe several "local" Bench marks that you wish to constrain to (local orthometric height).
4. Solve EGM2008 'N' values using the igs08 data and the NGA software.
Analyze all of the data, and I suspect that you can arrive at a linear shift in the 'N' values that will satisfy your needs (agree with the local ortho heights within reasonable limits).
Just a SWAG on my part...but that's where I would start.
Loyal
That link has more terms that I do not understand. 🙂
The readme is a little hard to read...
Loyal has the right idea, but instead of using ellipsoidal height, key the orthometric height and include a geoid model file if you have one handy. Unless your survey goes for miles, that should do the trick. It is bathymetry job, don't get overboard. Keep it simple.