Transducer offset (draft)

The transducer offset can be used to correct the difference between the depth outputted by the echo sounder, and the real depth. This offset is also known as static or fixed draft. Dynamic draft, which depends on the speed of the vessel, can be corrected when processing your raw data files.

The transducer of the echo sounder measures the water level from the bottom of the transducer to the bottom of the river. The height of the water above the transducer must be added to the echo sounder output and can be entered here.

Most echo sounders provide the option to enter the distance between the water surface and the transducer. In this case, you must disable this option (otherwise, it will be applied twice), or you can use it for other corrections.

Entering the transducer offset

Before starting your hydrographic survey, you have to enter an offset for the echo sounder. To do so, select "Preferences..." from the "Option" menu. In the dialog that appears, select the "Calibration" tab. You will see the following dialog:

The static draft or transducer offset can be entered in the "Calibration" tab.

The value entered is added to the depth (both high and low frequency depths) reported by the echo sounder. To subtract from the sounders value (in case of depth below keel), just prefix the value with a minus sign "-". To disable the correction, just remove the tick from the "Apply transducer offset" checkbox.

Raw data files

Ensure the transducer offset has been measured and entered into the software before recording any data. The depth written in the raw data files is the depth corrected with the transducer offset !

The depth data received is corrected with the static draft before saving.

Using RTK

When you are measuring the elevation of the riverbed using a RTK receiver, and the offset between the transducer and GNSS antenna is fixed, you can just enter this offset in the RTK settings tab. However, we recommend to fill out both the antenna and transducer offset separately in case you ever need the absolute water depths (and the tide readings).

When using RTK, set the GNSS antenna height as well (in the RTK tab).

Horizontal offset

If your echo sounder transducer is mounted directly below the GNSS antenna (e.g., when both are mounted on the same pole), there is no need to set a horizontal offset. If they are not, you should enter an offset in the Easting and Northing directions so the depth measurement is recorded at the correct GNSS position This offset can be set in the "Vessel Designer" utility.

The position relative to the GNSS antenna can be set in the "Vessel Designer".

NMEA0183 compatible echo sounders

When using a NMEA0183 compatible echo sounder, Entering a transducer offset depends on the NMEA0183 sentence(s) used to retrieve the depth data, as well as the settings entered into the echo sounder