Bathymetry is the study of the depths of the bodies of waters which includes the seas, ocean, or lakes and study the land structure of the area under water. The study of underwater depth is a different thing compared to its land counterpart such as topography and the hypsometry since it is more difficult to measure things under water. An early method of bathymetry is the usage of calibrated or measure ropes with anchor (or any heavy object). The rope is lowered to measure the distance of the land under the sea to the sea level. In order to get information, they have to locate points in the sea and measure them approximately.
This method is so inefficient for several reasons. First, as the rope is lowered, it may land on an area not perpendicular to the ship; either the anchor is pushed by the waves or the ship is gradually moving. Second, they cannot measure the entire desired area since it would be exhausting. Instead they have to strategically locate points where they will measure the depth. Third, the ropes that they use may not be sufficient enough to measure deeper areas such as trenches. Lastly, it would be very hard to measure ocean floors in areas where there are no islands because there are no points of reference and ships that time do not have advanced navigating devices and therefore can be lost in the middle of the ocean.
The discovery of SONAR really helps since it is not time consuming and it can give more accurate results.