Scale, Types of charts and some Terminology
A chart with a scale (marked on the chart) of 1:500000 implies that every unit of measurement on the chart is equivalent to 500000 units of measurement on the earth’s surface. (eg 1 cm on this chart will be 500000 cm on the earth’s surface). Charts are made with all kinds of scale, depending on how much detail is required for navigational safety.
Sailing charts are open sea charts. Land may be depicted as a rough contour and these charts are used far from land in ocean regions.
General charts cover coastal areas outside the immediate areas of reefs or shoals (where the water depth is very low and navigation more dangerous)
Coastal charts cover areas closer to the coast and give greater detail of outlying dangers.
Harbour charts are intended for use in approaching harbours and in mooring ships to berths, usually under pilotage. They have the greatest detail.
Soundings refer to the depth of the water at a location and is marked on the chart after hydrographic surveys.
A compass rose is a easy means- for a navigator- of finding direction on the chart and also plotting a ship’s course, besides taking bearings (angles) of objects with a compass. Called because it looks like a rose petal opened up, it is graded from 0 degrees to 360 degrees, with 0 being North, 90 being E, 180 being S and 270 being W.
Routeing and Passage Planning charts are small scale (cover large areas) and give information on winds, currents, icebergs and general information which assists us in planning a voyage at sea. These charts are not directly used for navigation.
There are some other kinds of charts, but the ones above are the main types.