SPOTL: Some Programs for Ocean-Tide Loading

Current Version (September 2013): see at end for bugfixes.
Portability and Usage
Tidal Models Included
Package History

Duncan Carr Agnew

Current Version (September 2013): see at end for bugfixes.


This package of programs is provided to allow computation of the load tides produced on the solid Earth by the ocean tides. The aim is to provide this capability in in a form that will (I hope) be straightforward to use, while at the same time allowing flexibility in combining different tidal and loading models – of which there are now a great many. A particular strength of this package is the ability to combine local and global models without a lot of effort in defining the boundaries between them. The package also includes programs to allow the computed loads to be converted into harmonic constants for any type of tide (including the ocean tide), and to compute the tide in the time domain from these harmomic constants. For completeness a program for direct computation of the solid-Earth body tides is also included.


The complete package (a gzipped tar file) is available here; the total size is just over 200 Mbytes. However, after source code has been compiled, and the compressed ASCII files have been converted to local binary versions and removed, the total size will be about 1.3 Gb.

If you are interested in learning more but do not want to download the entire package, here is a PDF of the manual and the man pages.

Portability and Usage

The programs are written in standard Fortran 77 (I hope). All the files are read with Fortran reads and writes, either binary (for the ocean-model and land-sea files) or ASCII (for the others). One C routine is used to do bitwise AND’s for reading the bitmapped part of the land-sea database. A few UNIX routines are used to provide the local date and to read command-line arguments. The package should compile and execute successfully on any system that supports the GNU Fortran and C compilers. The user interface is via the command line. All data files are exported as ASCII and converted to binary locally, so there should be no issues with byte-ordering. I have successfully run the code on HP-UX, Sun, and Mac OSX.

Instructions for installation are included in the package. I would appreciate being informed of any difficulties, partocularly with new Fortran compilers.

All the code is freely available (in both the monetary and open-source usages of "free"). Additional discussion is included in the distribution.

If you use this code, I would appreciate a reference to one of:

Agnew, D. C. (2012). SPOTL: Some Programs for Ocean-Tide Loading, SIO Technical Report, Scripps Institution of Oceanography; soon to also be available at the SIO Reports website.

Agnew, D. C. (1997). NLOADF: a program for computing ocean-tide loading, J. Geophys. Res.,102, 5109-5110.

Agnew, D. C. (1996), SPOTL: Some programs for ocean-tide loading, SIO Ref. Ser. 96-8, 35 pp., Scripps Institution of Oceanography, La Jolla, CA.

Tidal Models Included

Note that this list will change with time. I have not included many older models, though these are available on request.

Global Models

NAO99b, (1999), which is included to allow comparisons with GOTIC2.

FES 2004: the most recent version of the ‘‘Grenoble’’ tide models.

GOT4.p7: Version 4.7 (Year 2004) of the Goddard Ocean Tide Model.

TPXO7.2 (2010): Oregon State University solution, Version 7.2

TPXO7.2 ATLAS (2011): the same, combined with local models.

Equilibrium models for the long-period tides, from Richard Ray.

The DTU10 altimetric model, trimmed to include ocean areas only.

EOT11A, based on multimission altimettry.

HAMTIDE11A, which assimilates the altimetry data used in the EOT models into an inverse model that also includes hydrodynamics.

Local Models

Gulf of California, from a three-dimensional hydrodynamic model developed by Stock (1976) in a UCSD PhD thesis.

San Francisco Bay: interpolation of tide-gauge data.

The NAO99jB model of the Sea of Japan area (used in the GOTIC2 package),

Local models from the Oregon State University solutions, including the Bering Sea, the Pacific Ocean around Hawaii, the west coast of United States and British Columbia, the Gulf of Mexico, the Hudson Bay and surrounding waters, the East coast of North America, Maryland to Labrador, the Patagonian shelf, off the mouth of the Amazon, the NW European shelf, the Mediterranean and Black Seas, the Red Sea, the Arabian Sea and Persian Gulf, the Bay of Bengal, the East China Sea and South China Sea, North of Australia, Indian Ocean to Tasman Sea, Bass Strait and parts of the Tasman Sea and Great Australian Bight, and the Seas of Okhotsk and Japan.

Polar tidal models (Arctic and Antarctic) from L. Padman and collaborators.

Package History

1981 Verson 1 for internal use only (on a PDP 11-34), using the Schwiderski models (and some local models).

1987 Verson 2, developed for the National Geodetic Survey, combining into a single program specialized for gravity tides.

June 1996 Version 3.0, the first public distribution, and the first to use the Topex/Poseidon models.

1999 Verson 3.1, added the induced potential to the quantities computed.

June 2005 Verson 3.2.1: (A) Added two new global models (GOT00.2 and TPXO6.2), and one local model (Hawaii OSU model), (B) revised the local models for Canadian waters using an improved land-sea database, and (C) included an improved Antarctic coastline.

March 2012 Version 3.3.0: (A) older tide models replaced by models from 2004 on, including many local models, (B) Green’s functions available for both CE and CM reference frames. and models for the polar tides; (C) a higher-accuracy version of the harmonic prediction program, (D) inclusion of variable water density; (E) modifications to make it easier to use the programs to compute the effects of nontidal loads on land, and (F) some minor bug fixes and small modifications.

June 2012 Version (A) error in oclook fixed (was giving surface density rather than height.

September 2013 Version (A) error in handling density that caused different values for longitudes over 180.