The depth sounder measures the distance from the transducer to the seafloor. What is indicated on the instrument depends on choices we make.
If no offset is introduced the instrument will indicate the distance between the sensor and the seafloor.
There are two schools of thought, one is to introduce an offset so the depth sounder indicates water below the keel, the other is to offset so that the indication is from the water line.
An indication of water below the keel is very useful in shallow waters with the associated risk of running aground. Quite often an additional offset is introduced (for example by the owner) to give an extra margin (Indicating 0.0 could mean 20 cm margin below the keel).
An indication of depth reduced to the water line is handy when using the depth as an additional navigation tool. This indication can then be compared with a chart (chart depth + rise of tide) as an aid in navigation. Following the 2m contour with a rise of tide of 1.5m means that your looking at 3.5m on the indicator.
Both settings have their advantages, as a skipper you need to know what setting is used on the yacht you sail on. Using an improvised lead line, you can easily check the offset on a particular yacht.
(Article written by Albert de Nijs for Dutch Offshore Sailing Academy / De Zeezeilers.nl).