#Aids to navigation.# Each series of charts has a definite system of representing the aids to navigation; these are similar in principle but differ as to detail. The characteristics of the lights, light-vessels, buoys, and beacons are usually explained by abbreviations placed by the side of each, and the entire system of representation is given on the explanatory sheet for the charts. Various methods of coloring lights and sectors and buoys are in use on different charts. It is evidently of importance that the user of the chart should readily understand the significance of the navigational aids as shown. For details regarding lights it is of course desirable to refer to the light lists; for the coasts of the United States detailed buoy lists are also published. Range and channel lines when shown are represented by distinctive symbols with bearings indicated. Danger ranges for the avoidance of shoals are sometimes shown. On the British charts bearings as stated on range and channel lines are magnetic; the custom varies on other charts and must be carefully noted in each case. #Plane of reference.# The soundings given on the chart express the depth of water when the tide is at the height adopted for the plane of reference; this same plane is used in the tide tables, which thus will indicate the amount to be added to the soundings when the tide is above the plane, or to be subtracted when it is below. In order to be on the safe side the plane of reference adopted is always some low stage of the tide, so that there is usually more water than shown on the chart. On the British and German charts the soundings are reduced to the mean low water of ordinary spring tides, unless otherwise stated. On the charts of the Coast and Geodetic Survey the following are the planes of reference: for the Atlantic and Gulf coasts, the mean of the low waters; for the Pacific coast, Alaska, and the Philippines, the mean of the lower low waters, except for Puget Sound and Wrangell Narrows, where planes two and three feet lower respectively have been adopted. According to the Tide Tables for 1908, at New York (Sandy Hook) the tide will fall below the plane of reference on 135 days during the year, but the extreme low tide will be only one foot below the plane. At Portland, Maine, in 1908, the extreme low water is 2.1 feet below the plane, and at San Francisco 1.5 feet. Of course when the tide is below the plane of reference the amount must be subtracted from the depths shown on the chart. Strong winds and unusual barometric pressure may have a marked effect on the height of tide, so that it may differ appreciably from the predicted height, which is of course based on normal conditions. At Baltimore and at Willets Point observation shows that a heavy wind may reduce the tide four feet below the predicted heights.