ASA 105: Skill 15, Charting
15.
Plot a chart position from terrestrial objects using:
- Two or more bearings on different objects taken at one time.
- Bearings at different times (i.e. a running fix).
- One bearing and transit range.
- One distance (i.e. a sounding or dipping a light) and one bearing.
Returning to the Menemsha to Brenton Reef trip, the following will discuss plotting a position on a chart using a hand bearing compass and a depth meter.
The depth measurements will be in feet and correspond to the charted depths. This is done for simplicity. Cruising in regions of significant tidal range tide tables and the Rule of Twelfths would be applied. Refer to Eldridge 2022 for an explanation of the Rule. This link is helpful: eOceanic
Hand Bearing Compass measurements are given in "degrees Magnetic." That is a common practice. A general assumption is that a hand bearing compass may be used in places aboard a yacht that are free of magnetic influences. It is also assumed that one would not use a hand bearing compass in the presence of magnetic influences. The most common influence is from steel rimmed eyewear. Be aware.
A. Two or more bearings on different objects taken at one time:
The dark blue yarn represents the Rhumb Line from Menemsha to Brenton Point buoy, Red "2" Fl R 4 sec Whistle, just inshore of the Brenton Reef Tower. It is not a bearing used for this fix.
The dark brown yarn line represents a bearing taken off the tower on Cutty Hunk Island, 002 deg M.
The light brown yarn line represents a bearing taken off Gay Head Light, 130 deg M.
The fix shows a track of 282 deg T or 297 deg M and a leeway of 0.3 nm due to current, steering, and/or sideslipping. Note the depth and compare to the charted depths of 100+ feet. If the two Lines of Position and the vertical depth measurement match the fix has a high degree of confidence.
B. Bearings at different times, Running Fix:
The Running Fix is used extensively in offshore celestial navigation. It is less common in coastal navigation since there are often multiple landmarks available for two and three bearing fixes. It is useful when sailing along a long straight coastline such as the coast of New Jersy or outside of Cape Cod. Off both coasts only one lighthouse may be visible at the time.
Working with the example we have before us on chart 1210 Tr as we sail towards Newport, RI.
Rhumb Line and boat's heading is blue yarn, 280 deg T.
Hand bearing compass sight at time T=0 is dark brown yarn.
Hand bearing compass sight at time T=30 min is light brown yarn, 020 deg M.
Advanced LOP from T=0 is grey yarn parallel to dark brown yarn.
Dividers show the estimated direction and distance travelled in 30 min, 2.5 nm at 5 knots.
The process is to follow these steps:
First take a bearing on an object that will be visible at the time you wish to take a second bearing. Note the time of the first bearing, the bearing, your heading and your speed.
The second step is to take a second bearing at a convenient time, 30 minutes later in this case. Record the bearing and time, 020 deg M at T=30 min.
The third step is to plot the two LOPs, T=0 and T=30min.
The fourth step is to advance the T=0 LOP 2.5 nm in the direction of the course steered, 280 deg T. Mark that LOP as "T=0 adv."
The final step is to note the intersection of T=0 adv with T=30 min and that is a running fix. Errors with running fixes occur if the speed and/or direction are incorrect. In this case we can check the depth measurement which my be helpful if the bottom is contoured.
Tips for advancing an LOP:
To advance an LOP:
1. Draw a line from the point of reference (blue arrow tab) in the direction of the vessel's heading (course plotter)
2. Mark off the distance travelled along that line (see dividers in prior image)
3. Draw a line parallel to the T=0 LOP through the advanced point (green arrow tab) which is "T=0 adv." .
4. The intersection of LOP "T=0 adv" and LOP "T=30 min" is your position as determined by a running fix.
There are many opportunities in coastal navigation to use impromptu ranges. In section 10 we used a charted range in the Cleveland Ledge Channel to check ship's compass deviation. Ranges may be formed much less formally by lining up a pair of buoys or other charted landmarks. Generally, ranges from buoys are considered less reliable than charted landmarks. Sometimes that is the best available.
The dark blue yarn is the Rhumb Line between Menemsha and Brenton Point Red Whistle.
The dark brown yarn line is a hand bearing compass reading off Brenton Reef Tower.
The light brown yarn line is along a range formed by the Abandoned Light House on Sakonnet Point Reef and a Spire in Little Compton.
To establish a fix cross the hand bearing compass bearing with the line formed by the range.
D. One distance/depth and one bearing:
There are opportunities to fix one's position with only one bearing plus a reliable distance or depth measurement.
Distance and bearing:
Looking at chart 1210 Tr we find five major light houses. Each has a nominal range (in clear weather) of visibility. Point Judith Light has a height of 84 ft and a range of 16 nm to the horizon. Assume your height of eye from your vessel is 8 ft. Referring to Eldridge 2022 page 221 we find a table of distances and heights of eye. Always add your height of eye range to the height of the object being viewed's range.
At 8 ft, the horizon is 3.2 nm away. When we are 16 nm plus 3.2 nm away from Point Judith Light, or 19.2 nm, the top of the Light will be just visible. This gives us the range from the Light. Use a hand bearing compass to get a direction and we can plot our position.
Point Judith Light is marked with a green arrow. The blue arrow marks a distance of 19.2 nm from the Light on a bearing of 277 deg T.
This method is particularly useful at night when the light just becomes visible over the horizon.
Bearing a depth:
There are conditions when the bottom contours are distinct enough that they can be an aid to fixing one's position.
A bearing off Naushon Island's Tarpaulin Cove Light is taken and the depth meter shows 15 ft of water. Along the LOP from the Light there is only one place that shows 15 feet of water depth. That's where you are.
A significant limitation to this method is the range of the tide. Depths are considered accurate at Mean Low Water. Tidal Information for the area is show at the top of the chart. Vineyard Sound has a tidal range of 1 1/2 feet so the charted depths are very close to what your depth meter reads. At some point the tidal range requires the navigator to apply the rule of twelfths to correct the charted depth for time of day. See Eldridge 2022 page 204 for Rule of 12ths information.
E. Danger Bearings
A danger bearing represents a line of position you do not wish to cross. The two images below show two different danger bearings.
Danger Bearing on Gay Head Light to avoid a shallow area on Lucas Shoal.
The Line of Position from the Light through the 12 ft shallow area is 219 deg True. If the bearing on the light from your vessel is less than 219 deg True you are in safe water.
Danger bearing on Buzzards Bay Light Tower to avoid Sow and Pigs Reef.
The Line of Position from the Tower along the edge of Sow and Pigs Reef is 272 deg True. If the bearing on the Tower from your vessel is greater than 272 deg True you are in safe water.
Danger bearings may be formed by a bearing ahead or astern. For example, sailing away from the Tower keep the back bearing at 092 deg True or greater to keep the Reef clear on your port hand. Similarly, sailing away from Gay Head Light keep the back bearing less than 039 deg True to keep Lucas Shoal clear on your starboard hand.
Comments
Post a Comment