Session II Nautical Charts

Objectives F To become familiar with: – data found on Nautical Chart and how it relates to GPS and the AtoN Program. –how depths are depicted on a chart. –the horizontal and vertical datum on chart

Expectation You as members of Auxiliary are familiar with or will become familiar with the information on a chart.

4 Horizontal Datum Refer to the General Information Block on your Nautical Chart for the horizontal datum. Update your GPS with the HORIZONTAL DATUM referenced on the nautical chart that you are using. WGS84

Depth Soundings Mean Lower Low Water F MLLW - Mean Lower Low Water is the mean or average of the lower tide projections for the area. –The lowest of the lows per day. –All US charts are being changed to this datum. F Always calibrate (verify) the accuracy of your depth sounder.

Heights of Objects Heights of Objects are measured in vertical datum above Mean High Water (MHW). –Clearance under bridges are measured up from the MHW projection. –Heights of lights are measured up from the MHW projection to the focal plane of the light. Special Abbreviations and Notes Special Abbreviations and Notes are shown that are peculiar to the area.

Electronic Equipment Notes F Set your Depth Sounder to the vertical datum from your chart—feet, meters, or fathoms. F Set your GPS to the horizontal datum from your chart– usually WGS 84 in the US. F If your GPS does not have an automatic variation adjustment, set it for the Variation Error found in the nearest Compass Rose.

End of Session II