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Scales are expressed as ratios and differ from map to map. The scale, printed in the upper right corner of the map, will look something like 1:100,000. A ratio of 1:10,000 indicates that every 1in (2.5cm) the map equals 10,000in (25,000cm) in the real world. Charts with smaller scales provide an up-close view, depicting more area details and markers. If you're familiar with land maps, the scale works the same way there. A harbor chart is an example of a small scale chart. Navigators need a close view of the harbor with all the obstructions and other details. Larger charts show land and waterways but few additional details. Look in the upper right corner of the map near the scale. You will see the unit of measurement printed in big letters. You need to know this because charts use all sorts of different measurements. Your chart could include feet, fathoms, or meters. In the U.S., charts generally use feet or fathoms. However, the National Oceanic and Atmospheric Administration (NOAA) has begun converting its charts to meters for the sake of international consistency. The black numbers printed on the chart represent water depth. Each number indicates the "mean lower low water" (MLLW) in an area. This is the average water depth at low tide, so most of the time the water is deeper than what you see on a chart. If you see a negative number, that means the water depth in the area is typically less than what is listed on the chart. Numbers written in fathoms have a unique look to them, so they are easy to spot. A fathom listing often consists of a pair of numbers. The first number indicates the number of fathoms. The second number, which will be a small subscript, is the number of additional feet. For example, a listing of 0 and 3 means a depth of 0 fathoms, 3 feet. A listing of 3 and 2 means 3 fathoms, 2 feet, or 20 feet total. For uniformity and simplicity, many modern charts use meters. This can be tough to understand if you're used to measurements in feet or fathoms. Multiplying by 3 is a quick way to get a relatively accurate estimate of the water's depth. For example, multiply a listing of 6 meters by 3 to estimate it as being at least 18 feet deep. The actual depth is 6 metres (20ft), so it isn't completely accurate, but it is helpful for avoiding shallow areas. If the depths are listed in feet, you can also divide them by 3 to quickly convert them to meters.
Use the scale indicator to track distance on a chart. Find the unit of measurement used to indicate water depth. Read the chart numbers to figure out the water's lowest depth. Multiply fathoms by 6 to measure depths by feet. Multiply meter measurements by 3 to convert them to feet.