These contours are derived from computer tapes of the Digital Relief of the Surface of the Earth from the US National Oceanographic and Atmospheric Administration. These are organised on a uniformly gridded database with a grid spacing of 5′ latitude by 5′ longitude. This means that, approximately, one degree of latitude is 111 kilometres throughout (there are slight differences by latitude due to the shape of the earth). One degree of longitude varies as follows:

Longitude 0 degrees, 111.3 km; 10 degrees, 109.6 km; 20 degrees 104 km; 30 degrees 96.4km; 40 degrees 85.4km; 50 degrees 71.7km; 60 degrees 55.8km; 70 degrees 38.2km; 80 degrees 19.4km.

Oceanic bathymetry was carried out by the US Naval Oceanographic Office as at 1988. The tape data are sonar soundings corrected by echo-sounding correction tables to account for variable sea conditions (chiefly water temperature), combined with Admiralty Charts and national soundings together with land topography measured from satellites. This grid system, despite its coarseness, presents far more detail in important areas th′an is available on the Admiralty Charts, which show large gaps in soundings in both the Java Sea and the Gulf of Thailand. Nevertheless, the NOAA tapes should be read as only pointing to phenomena that seem worth much closer investigation. Taken on that basis, they are an excellent beginning. The maps presented relate to key points in the ice age.