Monday, February 9, 2009

Using Gnomonic Charts for Voyage Planning

There are three types of gnomonic charts depending on the location of the point of tangency. In equatorial gnomomic charts, the point where the plane is tangent is located at some point on the equator. A polar gnomonic chart is tangent at one of the poles. Oblique gnomonic charts have their point of tangency at some latitude and longitude between the equator and the poles.
The land features of gnomonic charts become more distorted as the distance from the point of tangency increases. The most often used gnomonic charts are oblique charts with a point of tangency located in the center of the ocean basin. Gnomonic tracking charts WOXZC 5270 and WOXZC 5274, have their points of tangency in the North Pacific and North Atlantic Ocean basins.

The advantage of gnomonic charts is that a straight line between two points represents the great circle between the points. If it is desired to proceed by great circle, all you need do is connect the points of departure and arrival with a straight line. The latitudes and longitudes of points at intervals along this line can be measured on the gnomonic chart and then transferred to a Mercator chart. When these points are connected with straight lines (rhumb lines), the navigator has the legs of the great circle track. Once transferred to the Mercator chart, the course on each leg may be determined, and the sum of the distances along each leg gives the total great circle distance. The main disadvantages of the gnomomic projection are that courses, direction and distance can not be readily measured. For these reasons, gnomonic projections are not used for day to day navigation and plotting, and should be regarded as an aid in voyage planning.

Using Gnomonic Charts For Track Planning
The use of gnomonic charts for voyage planning provides for quick checking of planned great circle tracks for intervening land, iceberg hazards, and other factors. For example, if you wanted to determine the advisability of proceeding by great circle from Cape Flattery, at the entrance to Puget Sound, to the entrance to Tokyo Bay, Japan, you would draw a straight line between the two places on chart WOXZC 5270. The chart would show that such a track would require your vessel to pass through the Aleutian Chain twice, and part of the voyage would be in the Bering Sea. Knowledge of the average poor weather, violent storms, uncharted rocks, and fog in this far northern waters makes the saving in distance of a great circle over a rhumb line less attractive. The great circle chart can be a big help to plan composite sailing tracks. A composite sailing consists of proceeding from the point of departure until the great circle reaches a limiting latitude. You then proceed by rhumb line (parallel sailing) along that latitude to the point where a great circle from the point of arrival is tangent to the limiting latitude line.

Great Circle Tracking Chart Questions On The License Exam
The Coast Guard uses questions which refer to great circle tracking charts. An applicant testing for Mates, or Master unlimited could easily have one of these questions on the exam. I have six example problems in another blog dated on 7 February 2009, titled "Great Circle Tracking Charts" with a full explanation. These examples should give a idea of how to reach the answers to problems of this type. If you can get copies of charts WOXZC 5270 and WOXZC 5274 so that you can follow the explanations with each example problem.