ce120_exam2

• Topography is the art or practice of graphic delineation in detail usually on maps or charts of natural and man-made features of a place or region especially in a way to show their relative positions and elevations
• Mean sea level is The average height of the sea for all stages of the tide
• Topography is The art or practice of graphic delineation in detail usually on maps or charts od natural and man-made features of a place or region especially in a way to show their relative positions and elevations
• Topographic map is A two-dimensional representation of a three-dimensional land surface. It shows both horizontal and vertical positions of terrain by using contours or variations in colors and hue
• Contents of a topographic map

  Contour

  Spot Elevation

  Map Scale

  Contour interval

  Topographic symbols
• Ridge is If higher values are inside the bend
• Valley is If lower values are inside the bend
• Hydrographic survey Observations and measurements which are undertaken to determine and subsequently portray the topography above and beneath the water surface, as well as to locate important marine features
• Basic operations of hydrographic survey

  1. Reconnaissance

  2. Establishment of horizontal control

  3. Establishment of vertical control

  4. Topographic survey

  5. Hydrography

  6. Preparation of hydrographic map
• Equipment for hydrography

  • Sounding pole
  • Leadline
  • Sounding machine
  • Fathometer
  • Signals
  • Tide gage
  • Sextant
  • Sounding craft
• Importance of tides Navigation

  • Diving and salvaging operations
  • Fishing industry
  • Recreation
  • Launching of ships
• Cause of tides

  Principal cause: difference in gravitational attraction exerted by the moon upon different parts of the earth

  Secondary cause: similar difference in the attraction of the sun

  The portion of the earth's surface nearest the moon is attracted more powerfully
• Tide is The half cycle of rhytmic rise ad fall of the surface of bodies of water due to the gravitational attraction of the sun and moon acting upon the rotating earth
• Tidal current is The horizontal movement of the water accompanying tides
• Flood tide is The period within which the water f=surface is rising and moving in toward the shore
• Ebb tide is The period within which the water surface is falling and moving seaward
• Slack water is The instant at which the tidal current is changing direction and flows neither in nor out
• Set is Direction of current flow
• Drift is the Speed of current flow
• High tide is the Maximum height to which the water surface rises above the standard datum plane during a given period
• low tide

  The greatest depression of the water surface below the standard datum plane during a given period
• tide range is the range of the tides in the difference between the high and low tide at a certain location on a given date
• Tidal day Consists of an idealized average period of 24 hours and 50 minutes between the occurrence of two successive high tides of the same type at the same location
• Stand of the tide The period around which the greatest peak or depression of the tides is reached during which any change in height of the tide is scarcely noticeable
• Tidal waves Waves generated at the surface of the ocean by the gravitational forces of the sun or moon that cause changes in the level of water bodies
• Tide gaging Measurement of the height of the tide (important in hydrographic surveys because water level varies with time)
• Tide gage is an Instrument for tide gaging
• Tide station is A site where the tide gage is located
• Sounding is The process of measuring the depth of a body of water at a particular point
• Sounding party

  • Instrument man
  • Recorder
  • Leadsman
  • Coxswain
  • Lookout
  • Signalman
  • Fathometer attendant
  • Chief of party
• Range lines are Well-defined lines on courses whose position are know and along which soundings are taken
• Position fixing is The process of determining the horizontal position of a measured depth with respect to an establishment system of horizontal control
• Methods of locating soundings

  • Range line and an angle from shore
  • Intersecting range lines
  • One angle and stadia distance from shore
  • Two angles from shore
  • Two angles from boat
  • Distances along a cross rope wire
  • Direction and vertical angle
  • Time intervals along a range line
• Time intervals along a range line is Simple and economical, used when required accuracy in not high, principle employed: distance = velocity x time, main constraint: velocity of the boat must be constant
• Range line and an angle from shore is Commonly used when range lines extend only to relatively short distances, the boat is steered along a number of parallel ranges, a transit of theodolite at one end of the baseline on shore is used to measure the angle between the base line and the reference sight of the boat
• Intersecting range lines is The boat proceeds to the intersection of any two range lines and soundings are taken, best adapted to small bodies of water where range signals can be easily erected and seen
• One angle and stadia distance from shore is Similar to stadia surveying, convenient for surveys pf small bodies of water, the theodolite or transit is set up on shore and the stadia rod is carried in a heavy flat-bottom boat, not suitable where soundings are taken far from shore and water is rough
• Two angles from shore


  Adapted when it is difficult to established range lines on shore, disadvantage: requires two shore observers; requires synchronized observations by the instrument at both stations
• Two angles from boat - Advantage: Chief of party can exercise direct control and supervision over his team, all hydrographic operations are performed on board the boat, observed soundings may be plotted directly while the survey is being executed
• Distances along a cross rope or wire is Suitable for surveying detailed cross-sections of rivers and channels, very accurate but is more expensive to undertake compared to locating sounding along range lines