Part 4. Gnomonic Chart Problems
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Transcript of Part 4. Gnomonic Chart Problems
The Cutterman’s Guide to Navigation Problems
Part Four: Gnomonic Chart Problems Gnomonic charts are used for ocean voyage planning and are oblique representations of the Earth’s surface, with only one point of tangency. Typically, waypoints are transferred from the gnomonic chart to the Mercator chart before a voyage. However, gnomonic charts are also useful for general decision making prior to a voyage. The provide a quick reference guide to trans-‐oceanic voyages.
Chart Examples
WOCZX-‐5274 via Google
WOCZX-‐5270 via Google.
Gnomic Chart Problems Problem 4-‐1 (CG-‐664). The following question is taken directly from the USCG test bank and illustrates how to solve qualitative gnomonic chart problems. You are planning a voyage by great circle from latitude 38° 00’ N, longitude 73° 00’ W to latitude 49° 00’ N, longitude 06° 00’ W. Which of the following statements are true? Use Gnomonic tracking chart WOXZC 5274). Answer: The shoals with a 25-‐mile radius around Sable Island will be a hazard.
Step 1: Plot the origin and arrival positions on the tracking chart and draw the trackline.
Step 2: Read each answer option carefully to eliminate all but the correct
answer.
a. You will pass to the south of icebergs reported extending to 100 miles south of Cape Race, Newfoundland. (Incorrect – You will pass through the iceberg area)
b. The shoals with a 25 mile radius around Sable Island will be a hazard. (Correct – The track passes within the shoal area)
c. You will reach the maximum northerly latitude at longitude 29° 45’ W.
(Incorrect – The maximum northerly latitude will be significantly further north than the stated position.)
d. The distance is measured in 60-‐mile segments using the length of a
degree of latitude at the vertex. (Incorrect – Gnomonic charts are not used in this way.)
Problem 4-‐2 (CG-‐666). The following question is taken directly from the USCG test bank and illustrates how to solve qualitative gnomonic chart problems. You are planning a voyage by great circle from Reykjavik (latitude 63° 30’ N, longitude 24° 00’ W) to the Azores (latitude 39° 30’ N, longitude 29° 00’ W). Which of the following statements are true? Use Gnomonic tracking chart WOXZC 5274). Answer: The great circle track is not appreciably shorter than a rhumb line track.
Step 1: Plot the origin and arrival positions on the tracking chart and draw the trackline.
Step 2: Read each answer option carefully to eliminate all but the correct
answer.
a. The distance is measured in sixty-‐mile segments based on the length
of a degree of latitude at the mid-‐latitude and mid-‐longitude positions. (Incorrect – Gnomonic charts are not used in this way.)
b. The northern hemisphere vertex lies south of Reykjavik. (Incorrect – The course is virtually North/South, so the vertexes would be near the poles.)
c. The great circle track is not appreciably shorter than a rhumb
line track. (Correct – The course is virtually North/South, yielding no significant savings by great circle.)
d. When plotted on a Mercator chart, the great circle track will be convex
to the British Isles. (Incorrect – The course would be concave to the British Isles.)
Additional Problems and Answers All of the following questions labeled “CG” were taken directly from the 2013 USCG test bank and illustrate the concepts in this Part. Note – not all problems have been worked and are subject to occasional errors in the database. For more problems and answers, see the USCG database of questions (database information located in the preface). Problem CG-‐1. A great circle track would be most advantageous when compared to the rhumb line track on which route? (Use gnomonic tracking chart WOXZC 5274).
a) Cayenne (4° 40’ N, 52° 15’ W) to Sao Tome (0° 00’, 6° 45’ E) b) Palm Beach, FL to the English Channel-‐ correct c) Natal, Brazil, to Reykjavik, Iceland d) Recife, Brazil, to Monrovia
Problem CG-‐545. On which voyage would a great circle track be significantly shorter than a rhumb line track? (Use gnomonic tracking chart WOXZC 5274).
a) Savannah, GA to Lisbon, Portugal-‐ correct b) Dublin, Ireland (Irish Sea) to La Coruna, Spain (latitude 43° 22’ N, longitude 8°
24’ W) c) Reykjavik, Iceland to Lisbon, Portugal d) Boston to Sable Island
Problem CG-‐546. On which voyage would a great circle track provide a significant savings in distance to steam, when compared to a rhumb line track? (Use gnomonic tracking chart WOXZC 5270).
a) Valdez, AK to the Marquesas Islands (latitude 8° S, longitude 140° W) b) San Francisco to Kodiak, AK c) Christmas Island (latitude 2° N, longitude 157° 30’ W) to Singapore via latitude
3° N, longitude 126° E d) Guam (latitude 14° N, longitude 145° E) to Seattle via latitude 47° 30’ N,
longitude 125° 20’ W-‐ correct
Problem CG-‐541. On a voyage via the southern tip of Nova Scotia (latitude 43° 20’ N, longitude 65° 35’ W), you wish to sail the shortest route to La Coruna, Spain (latitude 43° 20’ N, longitude 8° 24’ W). Which of the following will require you to plot a composite sailing? (Use gnomonic tracking chart WOXZC 5274).
a) Shoals extending 15 miles from Sable Island b) Sea ice reported 68 miles ESE of St. John’s, Newfoundland c) Icebergs reported extending west to west-‐northwest from latitude 47° N,
longitude 35° W-‐ correct d) Naval exercises using live ammunition being conducted with a 150 mile radiues
of latitude 49° N, longitude 20° W Problem CG-‐593. Using gnomonic tracking chart WOXZC 5270, determine which of the following statements about a voyage from San Francisco to San Bernardino Strait (latitude 13° N, longitude 125° 30’ E) is true.
a) A composite sailing should be used to avoid the Bonin Islands. b) Distance is measured using the length of a degree of longitude at the point of
tangency. c) You will cross the Northern Hemisphere vertex at the approximate longitude of
159° W-‐ correct d) The entire track line is west of the Northern Hemisphere vertex.
Problem CG-‐594. Using gnomonic tracking chart WOXZC 5270, determine which of the following statements about a voyage from Valdez, AK to Hilo, HI is true.
a) A great circle track is not significantly shorter than a rhumb line track-‐ correct b) You will cross the Northern Hemisphere vertex where the track line crosses
latitude 45° N. c) Distance is measured using the length of a degree of longitude at the mid-‐
latitude line. d) When plotted on a Mercator chart, the track line will be convex to San
Francisco. Problem CG-‐656. You are on a voyage via position latitude 44° N, longitude 150° E to latitude 46° 15’ N, longitude 124° W. Using gnomonic tracking chart WOXZC 5270, determine which statement is true.
a) A composite sailing with a limiting latitude of 51° N will clear the Aleutian Islands-‐ correct
b) The northern hemisphere vertex is east of the arrival position. c) The Aleutian Islands are not a navigational hazard on the direct great circle
track. d) The final course angle lies in the northeast quadrant.
Problem CG-‐667. You are planning a voyage by great circle from the mouth of the Delaware River (latitude 38° 40’ N, longitude 75° W to Lisbon, Portugal. Which statement is true? (Use gnomonic tracking chart WOXZC 5274).
a) You will reach the northernmost latitude of the voyage in the vicinity of longitude 42° 30’ W-‐ correct
b) The Northern Hemisphere vertex lies to the east of Lisbon. c) You must plot a composite sailing to remain south of icebergs reported north of
44° N. d) The distance is measured in 60-‐mile segments using the length of the degree of
latitude crossed by the track line. Problem CG-‐672. You are planning a voyage from Jacksonville, FL to the Strait of Gibraltar. Using chart WOXZC 5274, determine which statement is true.
a) All of the courses lie in the northeast quadrant of the compass. b) You will be east of the Northern Hemisphere vertex during the entire voyage. c) The great circle track approximates a rhumb line track because there is little
difference in the latitudes. d) None of the above are true-‐ correct.