Page 735 



THE SMOOTH SHEET 



7822 



7822. General Definitions 



Three types of rocks may be encountered in a hydrographic survey, characterized 

 as bare, awash, and sunken. By convention and for general purposes, they are defined 

 in relation to the tidal datums of the locality, as follows: 



(a) Bare rocks are those extending above the plane of mean high water. 



(6) Rocks awash are those exposed at any stage of the tide between mean high water and the 

 sounding datum, or that are exactly awash at these planes. 



(c) Sunken rocks are those covered at the sounding datum, that are potentially dangerous to 

 navigation. 



For cartographic purposes, however, the general definitions have been modified 

 in order that the charted symbols may 



Atlantic Coast Pacific Coast 



BARE ROCKS 



y7'/T/~^ '' 



reflect the most probable condition of 

 the rock as seen by the mariner. On 

 smooth sheets, therefore, the rules in 

 7823 shall be followed in the delineation 

 of the data relative to rocks, reefs, and 

 ledges. (See also fig. 168). The rules 

 are, of course, not inflexible, and in 

 their application consideration should 

 be given to the character of the area, 

 whether exposed or protected; the 

 proximity to shore; the range of the 

 tide ; and the extent of the minus tides ; 

 the controlling factors in all cases being 

 the emphasis on dangers and the 

 probable visibility of the rock at some 

 stage of the tide. (See 9334a..) 



Regardless of the sounding datum, 

 references to Atlantic Coast include 

 all areas in the Atlantic Ocean and 

 the Gulf of Mexico, and references to 

 Pacific Coast include all areas in the Pacific Ocean and in Alaska 



Pacific entrance 

 Panama Canal 



U N K E N 



ROCKS 



Figure 168.— Relationship of rocks to tidal datum planes. 



1823. Rules of Application 



a. Bare rocks.- — Rocks with elevations of 1 foot or more above mean high water on the Atlantic 

 Coast, or 2 feet or more on the Pacific Coast, shall be shown as bare rocks. The actual shape of 

 the rock should be shown if it is more than one-half millimeter in diameter on the survey sheet, other- 

 wise it should be indicated by a dot. (See fig. 169.) 



b. Rocks awash. — Rocks shall be represented by the awash symbol where their summits are in the 

 zone between 1 foot above mean high water and 1 foot below the sounding datum on the Atlantic 

 Coast. On the Pacific Coast the limits are 2 feet. The same applies to reduced soundings in rocky 

 areas, that are surrounded bj- depths considerably greater than the soundings. These limits are for 

 reduced depths in the Sounding Record. 



The symbol for a rock awash is three crossed lines, one of which is parallel to the lines of latitude. 



c. Sunken rocks. — Rocks potentially dangerous to navigation, whose summits are below the 

 lower limit of the zone for rocks awash, are classed as sunken rocks, and are represented on the smooth 

 sheet, either by symbol (a simple cross), or by a sounding accompanied by the legend "Rk" depending 

 on the nature of the available information. 



(1) The symbol should be used where the depth over the rock has not been definitely deter- 

 mined, as in case of transfer from the topographic survey, or where breakers have been cut in but 

 no soundings taken, or where the depth obtained does not represent the least depth over the rock. 



