Vessel Positioning Techniques
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Methods of getting ship position
01. GPS (Latitude and Longitude)
02. Cross Bearings
03. Ranges of Two or More Points.
04. Combined Range and Bearing.
05. A Bearing and Sounding
06. Running Fix.
07. Doubling the Angle on the Bow.
08. The Four Point Bearing.
09. The Transit Bearing.
10. Danger or Clearing Bearings.
11. Vertical Sextant Angle.
12. By Astronomical object.
GPS (Latitude and Longitude)
This is the obvious one! Switch it on, and during a few seconds your position are going to be displayed as a Latitude and Longitude. Remember the default datum is WGS84, check the datum menu if your unsure .
Cross Bearings
Bearings taken may be:
01. Relative – like pelorus or radar in ship’s head display.
02. Compass – using the magnetic compass
03. Gyro – when a gyro repeater is used.
In any case the bearing must be converted to true before laying it off on the chart.
Each position line (LOP) should be identified by one arrow at the top of the road faraway from the thing observed. When position lines intercept at (or nearly at) some extent , that time should be encircled and therefore the time of the fix noted alongside.
When position lines fail to intercept at some extent and a “cocked hat” results, it's going to flow from to any of the following:
a.Too long a delay between taking bearings
b.Wrong identification of an object
c.Error in plotting
d.Compass error wrongly applied
e.Unknown compass error
f.Poor survey of the area
Ranges of Two or More Points
This is the well-liked method when fixing the position by radar observations. Ranges taken from the radar are generally more accurate than radar bearings. Avoiding the steps necessary to convert relative or compass bearings to true also reduces the prospect of error.
Ranges must be began the adjacent latitude scale and therefore the relevant arc plotted on the chart using compasses. Both ends of the arcs should be marked with one arrow, the purpose of intersection circled, and therefore the time of the fix written alongside.
Selection of objects for ranges is as important because it is with bearings, and any hat should be treated in much an equivalent way.
Combined Range and Bearing
When just one suitable object is out there the position could also be fixed with one bearing of that object combined with its range. Although the range would usually be measured by radar it's still preferable to get the bearing visually.
The bearing can usually be taken more accurately by visual means.
Bearing and Sounding
This method may be used providing :
Allowance is formed to scale back the sounding to chart datum.
The depth contours are well defined.
The contour in question only crosses the position line in one possible place.
The depth contour crosses the position line at a good angle.
Running Fix
Under some circumstances, like low visibility, just one line of position are often obtained at a time. In this event, a line of position obtained at an earlier time could also be advanced to the time of the later LOP. These two LOPs shouldn't be parallel to every other; remember that the optimal angular spread is 90°. The position obtained is termed a running fix because the ship has “run” a particular distance during the interval between the 2 LOPs.
It is more commonly used when just one object is out there for bearings and there's no means of measuring the range. In this case there's a planned delay between bearings in order that the change in bearing will provide a suitable angle of cut.
Doubling the Angle on tcompas
This is a refinement of the running fix which takes advantage of the properties of isosceles triangles.
As we all know that the angle on the bow when the primary bearing is taken is 35°. The time of this bearing is noted and therefore the bearing then carefully watched until the angle on the bow doubles to 70°. The triangle formed by the 2 position lines and therefore the course line is isosceles, therefore the range at the time of the second bearing is adequate to the space run between bearings.
The Four-Point Bearing
This is an extra refinement of the running fix during which the primary bearing is taken when the thing is at four points (45°) on the bow. When the thing is on the beam the range are going to be an equivalent because the distance run since the primary bearing was taken. The disadvantage of the four point bearing is that the range of the only object isn't known until it's abeam. This is of little help in passing at a safe distance.
The Transit Bearing
When two charted objects inherit line they're said to be in transit. It has already been shown how a transit are often wont to check the compass error. A transit also can be wont to obtain a fix in conjunction with another position line like a variety (or be wont to obtain a fix in conjunction with another position line like a variety (or even a sounding) without use of the compass.
Danger or Clearing Bearings
Many ports have provided leading lights or shapes to guide mariners safely into harbour, avoiding shoals and other dangers.
In places where such aids aren't provided, the navigator should be ready to select a number one line provided by the transit of natural features. Thus approaching an anchorage with a coastal hill in transit with a more distant peak may make sure that the vessel clears dangerous rocks.
The advantage of a transit is that the mariner is assured of a secure approach no matter any compass error
Vertical Sextant Angle
The distance off a light-weight are often found by taking the angle the sunshine subtends at the vessel above water level .
Six common methods are used :
01.Latitude by Polaris (Pole Star)
02.Latitude by meridian altitude
03.Latitude by ex-meridian
04.Longitude by meridian passage of the sun
05.Longitude by chronometer
06.Intercept
Latitude by Polaris (Pole Star)
As the Pole Star is usually round the North Pole at radius of 1°, so it's always on or near the meridian passage. The latitude of the observer can be determined. The position line runs in an east-west, or nearly east-west, direction.
Latitude by Meridian Altitude
This method is employed to get the position line by taking the altitude of the heavenly body when it's instantly on an equivalent meridian because the observer’s. In this case, the position line runs in an east-west direction (90°-270°), and coincides with a parallel of latitude.
Latitude by Ex- Meridian Altitude
It is sometimes impossible to get the altitude of the heavenly body when it's on same observer’s meridian thanks to cloud, environmental factors, etc. If the altitude of the heavenly body are often obtained a couple of minutes before or after meridian passage, the Ex-Meridian method are often wont to reduce the observed altitude to meridian altitude. The latitude of the observer can be determined. The position line runs nearly in an east-west direction.
Longitude by Meridian Passage of the Sun
By knowing that the sun orbits with one completed circle in 24 hours, or 15° for each hour, the observer can determine position at noon by using the chronometer. The advantage of this method is that the DR position isn't required.
Longitude by Chronometer
This method is additionally wont to determine the longitude of the observer. The position line runs through the position at DR latitude and observed longitude during a direction perpendicular to the azimuth of the heavenly body from the observer.
Intercept
Since it's impractical to draw the massive circle of an edge circle on the chart, only the a part of it within the vicinity of the ship that's perpendicular to the bearing of the body from the ship is drawn. When observing a heavenly body , we will obtain its azimuth and altitude. The azimuth is that the bearing of the body and therefore the altitude of the body, giving us the zenith distance. As long because the altitude is corrected, the observed zenith distance is that the true zenith distance, which is named Observed Zenith Distance or True Zenith Distance. With the D.R. position of the observer at the time of observing, the altitude are often calculated to get the zenith distance, which is named Calculated Zenith Distance. The difference between the observer or true zenith distance and therefore the calculated zenith distance is that the intercept.
If truth zenith distance is smaller than the calculated zenith distance, then the observer is nearer toward the geographical position of the heavenly body compared with the DR position, and the intercept is called TOWARD.
If truth zenith distance is bigger than calculated zenith distance, then the observer is further faraway from the geographical position of the heavenly body compared with the DR position, and the intercept is called AWAY.
