The depths given on a Nautical Chart (whether in meters, foot or fathoms etc) are measured from the Chart datum. Common chart datums are the Lowest Astronomical Tide or Mean Low Low water tide. For Non-tidal areas Mean Sea level (MSL) values are used.

The echo sounder gives the depth BELOW KEEL. Let us assume that Echo Sounder is giving you a fairly accurate reading.

In order to compare both (Echo sounder and Chart readings) following shall be used -

Depth on Chart = Depth obtained from Echo Sounder + draft of the vessel - Value of Tide at the given place

Above is just an assumption and the values could be affected by the rolling/pitching, sea conditions and other meteorological phenomenon such as High-low pressure systems.

The depths given on a Nautical Chart (whether in meters, foot or fathoms etc) are measured from the Chart datum. Common chart datums are the Lowest Astronomical Tide or Mean Low Low water tide. For Non-tidal areas Mean Sea level (MSL) values are used.

The echo sounder gives the depth BELOW KEEL. Let us assume that Echo Sounder is giving you a fairly accurate reading.

In order to compare both (Echo sounder and Chart readings) following shall be used -

Depth on Chart = Depth obtained from Echo Sounder + draft of the vessel - Value of Tide at the given place

Above is just an assumption and the values could be affected by the rolling/pitching, sea conditions and other meteorological phenomenon such as High-low pressure systems.