#Gnomonic projection.# In this projection the eye is assumed to be at the center of the earth and the features are projected upon a plane tangent to some point on the earth's surface. It is practicable to use this projection for oceanic areas, and it has the very important quality that every straight line on it represents a great circle of the earth. To obtain the great circle or shortest course between two points it is therefore only necessary to draw a straight line between the points on a gnomonic chart. Because of the great distortion near the edges this projection is not otherwise adapted to navigational use, and it is employed only to mark out the general course, and sufficient points are then transferred to a mercator chart. The gnomonic chart is therefore useful in supplementing the mercator chart, supplying its deficiencies as to convenience in marking out great circle courses. The great circle course can be derived not only more easily and quickly from the gnomonic chart than by computation, but the chart is also to be preferred because the course marked out on it will show at once if any obstruction, as an island or danger, is met or too high a latitude is reached. A modified or composite course can readily be laid out on a gnomonic chart. [Illustration: FIG. 25. NORTH POLAR CHART ON ARBITRARY PROJECTION.] #Arbitrary projection.# The few charts published of the polar regions are sometimes on an arbitrary projection, in which the meridians are straight lines radiating from the pole and the parallels are equidistant circles with the pole as center. The latitude scale is uniform. At some distance from the pole the longitude scale becomes very much distorted, but the projection is a practicable and convenient one for the immediate polar regions. Gnomonic and conical projections are also used for the polar charts, differing little from the foregoing for moderate areas. #Scales.# Charts are published on a variety of scales to suit different needs of navigation, and the usual classification depends on scale. In addition to the ocean charts covering a single ocean in either one or several sheets and intended for navigation on the high seas, there are for our Atlantic coast the following series: Sailing charts, scale about 1/1200000, for general coastwise navigation. General coast charts, scale 1/400000, for local coastwise navigation. Coast charts, scale 1/80000, for approaching the coast at any point and for inside passages. Harbor and channel charts, of various large scales from 1/5000 to 1/60000, for entering harbors and rivers and passing through channels.