Session IISession IINautical ChartsNautical Charts

ObjectivesObjectives 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

ExpectationYou as members of

Auxiliary are familiar with or will become familiar

with the information on a chart.

4

Horizontal DatumHorizontal 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 SoundingsMLLW - Mean Lower Low WaterMean 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.

Always calibrate (verify) the accuracy of your depth sounder.

Heights of ObjectsHeights 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 NotesSpecial Abbreviations and Notes are shown that are peculiar to the area.

Electronic Equipment Notes

Set your Depth Sounder to the vertical datum from your chart—feet, meters, or fathoms.

Set your GPS to the horizontal datum from your chart– usually WGS 84 in the US.

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