hydrographic surveys of water areas

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804 CURRENT TOPICS. [J. F. I. Hydrographic Surveys of Water Areas. (U. S. Coast and Geo- detic Survey, Iooth Annual Report.) The report states that the postwar period has witnessed revolutionary changes in the method of making a hydrographic survey of water areas--the principal step in the processes of chart production. The application of recent innovations based on the utilization of the velocity of sound in sea water permits a surveying vessel while traveling at full speed to gather a continuous profile of the bottom. Visibility of land or weather conditions are non-essentials. The rate of progress is multiplied and permits the definite comple- tion of projects formerly expected to extend into the indefinite future. The two principal operations in the survey of a water area com- bine the measurement of thousands of water depths and the deter- mination of their positions on the earth's surface. Depths are measured by accurately ascertaining the time for a sound at the ship's hull to travel to the bottom and reflect back. The location at which the depth is measured is learned in a similar manner. The sound of a bomb exploded in the water near the ship, radiating in the water in all directions, is picked up by micro- phones at two shore stations of known positions. The minute im- pulse is automatically magnified and radioed back to the ship. There its receipt is recorded by the same delicate instrument which previously recorded the time of the explosion. The travel of the radio wave is practically instantaneous. Con- sequently, the elapsed times are those required for the sound to travel from the bomb to the respective microphones. The results obtained are two distances from the ship to two known points at the shore, and in consequence the position of the vessel. R.

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804 CURRENT TOPICS. [J. F. I.

Hydrographic Surveys of Water Areas. (U. S. Coast and Geo- detic Survey, Iooth Annual Report.) The report states that the postwar period has witnessed revolutionary changes in the method of making a hydrographic survey of water areas--the principal step in the processes of chart production.

The application of recent innovations based on the utilization of the velocity of sound in sea water permits a surveying vessel while traveling at full speed to gather a continuous profile of the bottom. Visibility of land or weather conditions are non-essentials. The rate of progress is multiplied and permits the definite comple- tion of projects formerly expected to extend into the indefinite future.

The two principal operations in the survey of a water area com- bine the measurement of thousands of water depths and the deter- mination of their positions on the earth's surface.

Depths are measured by accurately ascertaining the time for a sound at the ship's hull to travel to the bottom and reflect back. The location at which the depth is measured is learned in a similar manner. The sound of a bomb exploded in the water near the ship, radiating in the water in all directions, is picked up by micro- phones at two shore stations of known positions. The minute im- pulse is automatically magnified and radioed back to the ship. There its receipt is recorded by the same delicate instrument which previously recorded the time of the explosion.

The travel of the radio wave is practically instantaneous. Con- sequently, the elapsed times are those required for the sound to travel from the bomb to the respective microphones. The results obtained are two distances from the ship to two known points at the shore, and in consequence the position of the vessel.

R.