Taking note of scale

Nautical charts are beautiful examples of the arts of cartography and printing. But a chart’s beauty must never disguise its true purpose it is a navigational aid, designed to facilitate safe travel from one point to another. To use the chart successfully, a mariner should be aware of the chart’s scale and the depth information.

The chart’s scale links the chart to the real world. The scale is the ratio of the distance between two points on the chart and the corresponding distance in the real world. Because the scale is a ratio, both distances must be given in the same units of measurement. The unit is irrelevant; inches, centimeters, boat lengths, anything at hand will do but the same unit must be used in both measurements. For example, assume two points are three inches apart on a chart, and 73,000 inches (one mile) apart in the real world. This chart has a scale of 3/72,000, or 1/24,000. One could measure the same distance on the same chart with the metric side of the ruler, and find a distance of 7.62 cm. Combined with the real-world separation of 182,880 cm, this gives a scale of 7.26 cm /182,880 cm or 1/24,000. The scale isn’t generally written as a fraction, but rather in ratio notation, 1:24,000.

This ratio represents the natural scale of a chart, and is the usual way nautical charts are labeled. This information appears in the title block of a chart, which is generally found near the edge of the chart, superposed on some piece of land. Immediately below the title of the chart, one will find a line of type expressing the scale. As an example, consider Chart 11013, Straits of Florida, which states a “Scale 1:1,200,000.” How does this convert distances to real-world distances? The Miami Beach RW whistle buoy is exactly one inch from the Fowey Rocks “N” can buoy. At a scale of 1:1,200,000, this distance corresponds to 1,200,000 inches or 16.4 nautical miles. This conversion can be found by looking at Table 5 in Bowditch (Table 37 in volume 2 in the previous edition). This is exactly the same distance one would get by using the latitude scale on the margin of the chart: the inch between buoys represents 16.5 miles.

This is the other way of stating the scale of a chart, by giving the distance on a chart and its equivalent distance in the real world, but in different units. Thus the numerical scale of the Straits of Florida chart is one inch equals 16.5 miles. Any number of numerical scales are possible for a given chartin this case the 16.5 miles per inch is the same as 100,000 feet per inch, and so on. It is convenient to translate quickly between the natural and numerical scales of a chart, and of course Bowditch has a table allowing this. Table 5 gives natural and numerical equivalents for a variety of chart scales. Alternatively, one can construct the equivalence oneself, using the two formulas below.

miles per inch = scale/73,000

feet per inch = scale/12

One of the more confusing facets of charts is the concept of large- and small-chart scales. A small scale chart is one that shows a large area of Earth’s surface, and conversely a large-scale chart is one that shows a small area of Earth’s surface. The terminology derives from the natural scale. The natural scale can be written as a fraction (1/1,200,000 for the Straits of Florida, for example) and this number is a rather small one. Thus it is a small-scale chart. Another way of remembering the terminology is to picture the number of charts required, at a given scale, to represent the entire earth at that scale. A small-scale chart would require a small number of charts, while a large-scale chart would require a large number.

Charts produced by NOAA are, for the most part, now standardized into a series of scales. Small-scale, “offshore” charts come in variety of scales of one to a few millions. These charts have numerical scales (one inch equals eight to 40 miles) that preclude their use in near-shore areasthe chart simply lacks the details necessary for precise navigation. They are designed for offshore navigation, tracking offshore progress, and planning landfall. To facilitate this latter purpose, these charts often include the boundaries (shown in purple) of larger-scale charts in the chart area.

These next-larger-scale charts are the one to a few hundred thousands series of charts. These “general” charts typically cover some 200 to 300 miles on a side, cover only coastal areas, and contain sufficient detail for navigation in more than 10 fathoms (60 feet) of water. For navigation in depths of less than 10 fathoms, one should use a “coastal” chart in the 1:80,000 series. These charts typically show areas 40 miles on a side, and show in-shore detail, except within harbors and inland passages. These charts are ideal for the daysailor, as they typically have the navigational detail and geographic range needed for a day-trip. Finally, there are a series of large-scale “harbor” charts available from NOAA that show detailed views or harbors and inland waterways. These charts can be at scales of 1:40,000 to 1:2,500. The later has a numerical scale of one inch equals about 200 feet, detailed enough to represent on the chart a 10-foot-long dock, and water depths of less than one foot.

The descriptions above illustrate the importance of depth measurements reported on charts. Ultimately, the water depth of a point on the sea is the fundamental reason most mariners turn to charts. Water depths on charts can be shown in feet, fathoms, or meters, and in fractions of any of them. The variety of units can be confusing, and potentially dangerous. Chart makers recognize this and most NOAA charts very prominently display the depth unit on opposite corners of the margin of a chart. The Straits of Florida chart, for example, is labeled “SOUNDINGS IN FATHOMS” on the upper left and lower right corners. The same information is given again in the title block, which also gives the datum for the depth measurements. The repetitive announcement of units for depths is not without causeone must carefully determine the depth units of a chart before using it for navigation.

Depths of the sea and heights of land are indicated on charts in two ways. Point measurements are measured depths or heights for that particular point, and are indicated on charts by small, sans-serif type on a horizontal baseline. While a given chart may contain thousands of spot measurements of depths, it would be impossible to indicate the topographic shape of the bottom with spot measurements alone. Instead, chart makers use contour lines to show the shape of the bottom. Contours are lines of equal depth, and from them the practiced mariner can visualize the form of the ocean bottom.

Depth contours are often generalized and usually do not include a label identifying their depth. Instead, NOAA uses a series of different dashed and dotted lines to indicate the depth of a given contourthese symbols are explained in figure 2, on NOAA Chart No. 1. Even on a chart with depths in feet, the depth contours are incremented in fathoms. Shallow depth contours are often given by a dotted line, but contours for four fathoms and deeper have unique patterns, with the number of dots and dashes indicating the water depth. Contours on land are labeled, and are usually given in feet or meters; one must check the title block to find the actual units.

Larry McKenna is a navigation instructor, freelance writer, and sailor who lives in Overland Park, Kansas.

By Ocean Navigator