Logged draft warning

Some Simrad echosounders support the definition of a transducer depth, or draft, which is then embedded in Q and E telegrams by adding below-threshold samples to the beginning of the data. In Echoview, this results in a band of below-threshold samples at the top of the echogram, representing the blind zone between the face of the transducer and the water surface which has not been sampled.

In Echoview, this embedded draft is referred to as 'logged draft'. Simrad Q and E telegrams are the only sources of data that may include a logged draft that we are aware of.

For Simrad data that does not include a logged draft, draft is dealt with in a straightforward manner by setting up transducer geometry.

For data that does include a logged draft, care is required. It is necessary to apply a calibration correction in Echoview called logged draft which will remove the below-threshold samples representing the embedded draft from the echogram. The draft should then be applied through the transducer geometry settings.

The SimradDraftLogging calibration setting is displayed on the Calibration page of the Variable Properties dialog box for all variables based on Q and E telegrams. This includes Sv and TS variables from Q telegrams and single target variables based on E telegrams.

Determining the correct value for SimradDraftLogging is not always straightforward. See How to determine Logged draft.

The SimradDraftLogging value must be accurate for any range-related calculations and for integration analyses of the whole water column to be correct. Further explanations are included below.

Integration of data with logged drafts

Simrad data that includes a logged draft appears on an echogram with a band of below-threshold samples at the beginning of each ping, representing the transducer depth. In Echoview, below-threshold samples on Sv echograms have a zero value of -999.0 dB which, when integrated, contribute to calculations of mean Sv and other analysis variables. As the transducer depth, or draft, represents a blind zone that has not been sampled, integrations or analyses that include these samples are not truly valid.

Therefore, we recommend removing the logged draft below-threshold samples with the SimradDraftLogging calibration setting. You can then re-apply the draft through Echoview's transducer geometry. See How to determine Logged draft.

Range values should not include draft

Echograms in Echoview can be displayed and analyzed in either depth or range mode.

For data that includes a logged draft, Echoview cannot differentiate between the logged draft and other below-threshold samples, so ranges will be incorrectly reported with the logged draft samples included. For range values and range-related variables to be reported correctly, it is necessary to remove the logged draft samples from the echograms and model the draft through Echoview's transducer geometry.

In addition, versions of Echoview prior to Echoview 3 supported only depth mode, and the introduction of range mode in version 3.00 onwards requires that transducer depth, or draft,  be accounted for if range dependent variables are to return the same results as in previous versions of Echoview. See Opening Version 2 EV files in Echoview 3 and higher for more details.

Although only one transducer depth setting is entered into the Simrad echosounder, you may need to use different logged draft values with different Echoview variables because the data resolution is different with different variables. See How to determine Logged draft for further details.

Logged draft tip

Echoview Software recommends logging data with transducer depth set to 0 and using Echoview's transducer geometry to model the draft when live-viewing and/or for data analysis. This will result in:

• result in fewer data points per ping for Q telegram data because samples with depths above the transducer face are set to the below threshold value.

• require a calibration correction in Echoview if range values and integrations are to be calculated correctly.

Contact Echoview support for further information.

See also

About draft
How to determine Logged draft
About depth, range and altitude
About transducer geometry