HP's Ocean Function Page

View the ocean function
Edits of the ocean function
Ocean Function 5 minutes resolution
Ocean Function 2.5 degrees resolution
Fortran 90 Ocean Function


Copyright: Hans-Peter Plag.

Here, I give access to data sets and tools related to the ocean function (OF). The OF is defined to be equal to 1 on ocean areas and 0 on land areas. The digital OFs provided here are all derived from the ETOPO5 data set. The OF is provided with three different resolutions, namely 5 X 5 minutes, 0.25 x 0.25 degrees, and 2.5 x 2.5 degrees (OF_5m5m_v1_0.dat - note this file has 9 Mb -, OF_0_25d0_25d_v1_0.dat, and OF_2_5d2_5d_v1_0.dat).

A version of the ocean function which does not include any semi-enclosed seas will be made available soon. The different ocean functions will also be made available as a spherical harmonic expansion.

In addition, we provide the Fortran 90 routine ocean_function(long,lat,longr,latr) (note the source code has 11 Mb!), which returns the value of the OF for any point, where long and lat are the geographical position in degrees, and longr and latr the requested resolution in longitude and latitude, respectively. For a given point, the routine averages the ocean function of a grid element depending on the requested resolution and returns one if the average is larger than or equal to 0.5. Note that the routine was created by a program and is rather large. However, once it it compiled, it is very fast. For a simple test of the routine, we also provide the program test_ofc.f90, which illustrates the use and creates a file with a 2.5 degree resolution ocean function. Except for the header, the file should be identical to the file OF_2_5d2_5d_v1_0.dat available above.

The ocean function is derived from the ETOPO5 data set by first replacing all heights above and below zero by 0 and 1, respectively. This leads to the function shown here. This function has a number of problems due to some land areas being below sea level. In a careful manual editing, these areas are identified (see the global plot, or sectorial plots for North-East, South-East, North-West, South-West or all plots together here), leading to the final ocean function shown here. Note that this ocean function defines an ocean that covers 70.98% of the Earth's surface. For better comparison, the three global plots can be viewed together here.

The high-resolution ocean function is then filtered to a resolution of 2.5 degrees. The filter simply considers grid cells with more than half the area in ocean as ocean cells, and all other cells as land. This function does not approximate the coast line very well, as can be seen here. This ocean function covers 71.37% of the Earth surface.