Land/sea/lake definition in the NAVOCEANO SST Processing chain
The NAVOCEANO SST processing software only generates SST retrievals for pixels determined to be over water. A global land/sea tag file is utilized within the processing software to make the determination before a SST retrieval is generated. Prior to April 2002, NAVOCEANO used a global 7km land/sea tag file to make the decision. Since April 2002, NAVOCEANO has utilized a 1km land/sea tag file. The 1km file was derived from a 1km land sea mask from USGS and from the GTOPO30 land mask from USGS. The land mask covers latitudes 80.3N to 80.3S and all longitudes and includes all coastal regions and lakes. Each 1km land mask cell also contains the distance that the cell is from land. This value is zero over land and up to a maximum of 50km for cells over water.
The SST processing software checks the land mask cell applicable to the pixel location being processed and will only make an SST retrieval if the location is determined to be more than a specified distance from land. The specified distance from land differs by satellite sensor and scan angle. Since the geolocation uncertainty of a satellite pixel is at least one pixel or more, NAVOCEANO does not make SST retrievals for pixels determined to be within one pixel of land. For nadir AVHRR LAC/HRPT this equates to 1km, nadir AVHRR GAC 4km, and nadir GOES 4km. The specified GAC distance from land increases with scan angle according to the formula: distance = 4km / cos(satzen)**2. Likewise for the other sensors, by substituting the sensor pixel spatial resolution into the numerator.
The land mask does a remarkable job of defining lakes throughout the world. We use commercial GMT software to make many of our graphical products and often its land mask does not show lakes that exist in the real world. When investigating whether these SSTs are incorrect, we type the coordinates into Google Earth and find lakes.
Access the land mask data netCDF file in Bzip2 compression here
(Last Updated: 04-07-2007)