Hydrographic Surveys and Oceanography

Hydrographic Surveys

• Hydrography is the process of gathering information about bodies of water and adjacent land areas.

Preliminary Steps

• Reconnaissance is a preliminary step in selecting the most suitable manner of executing a survey.

Tides and Water Levels

• A tide is the daily cycle of rhythmic rise and fall of the surface of oceans, seas, lakes, and other bodies of water due to gravitational attraction.
• Mean Sea Level is the average height of the sea for all stages of the tide.

Measuring Water Depth

• Soundings involve measuring the depth of a body of water at a particular point.
• Echo Sounding measures water depth by sending high-frequency sound waves to the water bed and timing their return.
• A Fathometer is an echo-sounding equipment that produces, transmits, and receives sound waves, calculating depth based on the time interval of the echo's return.

Positioning and Mapping

• An Electronic Positioning System uses unmodulated radio waves to determine distance between a ship and ground stations based on travel time of the waves.
• LIDAR (Light Detection and Ranging) is a remote sensing method that uses light to measure ranges/distances to the Earth.

Water Management

• Dredging is the process of widening, enlarging, cleaning, or deepening of channels in harbors, rivers, and canals.
• Dams are barriers built to hold back or impound water.

Surveying Methods

• Three Point Resection is a method to find the coordinates of a point in a body of water using three known control points.
• Stream Velocity is the speed of water flow in a channel.
• The Cross-section Method is used to determine the effective capacity of a body of water when a moderate degree of precision is required.
• The Contour Method is used to establish a reference base line or shore traverse to locate the water line and desired shore topography.

Formulas

• Stream discharge: Q = AV
• Area in each section using Velocity-Area Method: a = ½ (b1+b2) h

Locating Soundings

• Intersecting Range Lines is a method of locating soundings where the boat proceeds to the intersection of any two range lines and soundings are taken.

---

Astronomical Observations

Celestial Bodies

• Celestial Poles are points on the surface of the celestial sphere pierced by the extension of the earth's polar axis.
• Celestial Axis is the prolongation of the earth's polar axis.
• Zenith is the point where the plumb line at the place of observation projected above the horizon meets the celestial sphere.
• Nadir is the point on the celestial sphere directly beneath the observer, opposite to the zenith.

Observer's Reference Points

• Observer's Horizon is a great circle of the celestial sphere where a plane, perpendicular to the plumb line at the place of observation and passing through the center of the earth, cuts the celestial sphere.
• Observer's Vertical is a vertical line at the location of the observer, coinciding with the plumb line and normal to the observer's horizon.

Celestial References

• Celestial Equator is a great circle perpendicular to the polar axis of the celestial sphere, an extension of the plane of the earth's equator outward until it intersects the celestial sphere.
• Vertical Circle is a great circle passing through the observer's zenith and any celestial body, perpendicular to the horizon.