1. MAPPING TECHNOLOGY NOTES

2. WEIGHTED LINE
Oldest method in which a weighted line is lowered into the ocean until it touches the ocean floor.

3. ECHO SOUNDER
Older method in which scientists bounce a beam of sound waves off the ocean bottom to measure water depth.

4. SONAR MULTI-BEAM
Ships send out many beams to collect data on a wide swath of the seafloor, rather than a single line.

5. SEISMIC PROFILING
Uses powerful shock waves to map subsurface rock structures.

6. SEA-SURFACE ALTIMETRY
Sends a radar beam from a satellite to measure the height of the sea surface, which closely reflects the shape of the sea floor.
Higher elevations, like mountains, have more mass so their gravity attracts a water bulge up to 30 m high. 
Troughs & trenches can form depressions up to 60 m deep.

7. SEA-SURFACE ALTIMETRY