Depth curves are shown on charts in order to bring clearly to the eye the different depth areas and the limits for navigation of vessels of various drafts. The shoaler areas are usually indicated by sanding the outer limit or the entire area within the depth curve. For the curves of greater depths various standard symbols are used which vary slightly in the different series but which may readily be recognized by the soundings on either side of them. On the British charts the 1 and 3 fathom curves are usually indicated by sanding the outer edge of the areas of these depths respectively; beyond these the standard curves shown on these charts are the 5, 10, 20, and 100 fathom curves. Similar curves are used on the United States charts. The German charts show the 2, 4, 6, 10, and 20 meter and various deeper curves, and the French the 2, 5, 10, and 20 meter and deeper curves. On the United States Lake Survey charts the areas included within the 6, 12, and 18 foot curves are shaded with a blue tint, heavy along the outer edge, which brings out strongly the shoal areas. Depth curves if clearly shown are a great aid in interpreting the hydrography and making plain the shoals and passages. The system of curves should always be understood when using a chart, and it may sometimes aid the navigator to trace out with a pencil an additional curve, if needed, beyond the draft of his vessel. The abbreviations used for the bottom characteristics are explained either on the chart or on the sheet of chart symbols, and give information which is useful in anchoring, and may be helpful in identifying a position by soundings. When a sounding is made without the lead reaching bottom, the depth obtained is sometimes shown on the chart by a short line and zero above the figure, indicating that at the depth stated, bottom was not obtained (no bottom). There are a few important symbols shown in the water area of charts. The sunken rock symbol indicates a dangerous area, or a danger having a moderate depth of water over it, or a rock the least water over which is not known; ordinarily on the United States charts the least depth will be stated when known, and the symbol omitted. The rock awash symbol indicates a rock awash at some stage of the tide, unless more definitely stated. The position of a wreck is indicated by a special symbol. P. D. (position doubtful) and E. D. (existence doubtful) are placed after soundings or rocks or other features which depend on some doubtful report not yet verified. The following are the relations between depth units found on various charts: 1 meter = 3.281 English feet = 0.547 English fathoms. 1 sajene (Russian) = 7 English feet = 1.167 English fathoms. 1 braza (old Spanish) = 5.484 English feet = 0.914 English fathom. 1.829 meters = 6 English feet = 1.000 English fathom.