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 called static or fixed draft. Dynamic draft (depending 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 which is above the bottom of the transducer has to be added to the echo sounder output, and can be entered here.
Most echo sounders have the option to enter the distance between the water surface and the transducer, so in this case you do have to disable this option (otherwise it is applied twice!), or you can use it for other corrections.
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 should 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 the minus sign "-". To disable the correction, just remove the tick from the "Apply transducer offset" checkbox.
Please make sure transducer offset is 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.
When you are measuring the elevation of the river bed 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).
When your echo sounder transducer is mounted directly below the GNSS antenna, for instance when both the antenna and transducer are mounted to the same pole, there is no need to set a horizontal offset. When they aren't, you should also enter an offset in easting and northing direction 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".
When using a NMEA0183 compatible echo sounder, whether to enter 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.
| Sentence | Depth Value | Correction |
|---|---|---|
| $SDDBK | Depth below keel | Enter the distance between the keel of the ship and the water surface. |
| $SDDBS | Depth below surface | No correction is needed, as the depth is measured from the water surface. |
| $SDDBT | Depth below transducer | Enter the distance between the transducer and the water surface. |
| $SDDPT | Depth | The offset configured in the echo sounder is used. |
