The Sonic Layer Depth (SLD) analysis chart as shown in figure 4-2-14 on a Mercator map projection uses shading instead of contours to indicate the depth of the maximum speed of sound near the surface. The different types of shading indicate ranges of the SLD in feet: no horizontal shading lines (clear) indicate an SLD between the surface and 50 feet; widely spaced horizontal lines (light) indicate SLD depths between 50 and 100 feet; closely spaced lines (medium) indicate depths from 100 to 350 feet; and very closely spaced lines, forming the darkest (heavy) shading, indicate depths deeper than 350 feet. Since the SLD may coincide with the MLD as deep as 1,500 feet, the MLD analysis, which contours down to 280 meters (about 900 feet), may be more useful for many applications. Reanalysis of this chart by drawing in the 50-, 100-, and 350-foot contours makes it a better briefing tool and equates roughly to the 15-, 30-, and 100-meter contours on the MLD analysis, FNOC also produces this chart with the contours in feet vice meters.  

You have seen a few of the charts that are available for oceanographic applications on many different map projections and scales. The basic presentation of features on these charts are all very similar, regardless of the resolution used. Just about every data field that is presented in chart form is also available as a message of gridded point values, which may be transmitted over various communications circuits to suit your requirements.

Learning Objective: Identify the sources to obtain information on the request data fields.

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