Frequently Asked Questions about the free ocean tide loading provider

• Which ocean tide model is the best?
• How are the ocean loading values calculated?
• Are the values also accurate near the coast (less than 1km)?
• Why do I only get ocean tide loading parameters for 11 harmonics?
• Known problems and limitations?

Question: `Which ocean tide model is the best?'

As usual, there is no simple answer to this. Try using the latest 5 models: GOT4.7, FES2004, TPXO_Atlas, HAMTIDE and DTU10. These should give a good indication of the error in the ocean loading values at your stations.

Schwiderski is less accurate than the other models and should only be used for reference purposes.

Question: `How are the ocean loading values calculated?'

The ocean loading is calculated for the harmonics M2, S2, N2, K2, K1, O1, P1, Q1, Mf, Mm and Ssa using the program olfg. If the station is close to the coast, the post-processor olmpp is automatically run. This last program has been updated and has now a coastline resolution of about 0.6km. The coastline used is taken from the GMT package.

To force conservation of tidal water mass, a layer of water with a certain phase lag is subtracted from the models globally. Furthermore, the Green's functions are for a Gutenberg-Bullen Earth with a continental crust.

Question: `Are the values also accurate near the coast (less than 1km)'

1) For displacement ocean tide loading the value is as good as your ocean tide model in the neighbourhood of your station. As a rule of thumb: Is your station near a shallow water sea, you are in trouble. Stations on islands in the open oceans should be okay.

2) For gravity: You are in trouble here because the direct gravitational attraction of the tidal water mass is more than likely dominating the loading. You will need a higher resolution coastline and should replace better ocean tide values in the model near the station. (Been there, done that, and it's a pain)

Question: `Why do I only get ocean tide loading parameters for 11 harmonics?'

These 11 harmonics are the largest in amplitude and represent most of the total tidal signal. The other harmonics can be obtained by interpolation between these given values. Some ocean tide models provide the smaller harmonics and work is under way to include them on this ocean tide loading provider.

Question: `Known problems and limitations?'

Use this link to read about problems reported by the loading team or our users. We look forward to record also YOUR response. Please contact Hans-Georg Scherneck, hgs kanelbulle chalmers.se.