Data manipulation operators

Settings

Description

Horizontal extent (pings)

Specifies the horizontal size of the averaging cell (number of pings). The cell width defines an averaging ping interval that examines samples for the noise estimate for the middle ping of the interval.

The default value is 20 pings. The value is cited in the Sensitivity to grid size discussion in De Robertis and Higginbottom (2007).

Vertical units

Specifies the units for the vertical axis of the averaging cell. Available units are Samples or Meters.

Notes:

• The number of samples within the Vertical extent of the averaging cell can affect the statistics of the noise estimate. Too few samples may result in a poor noise estimate.
• Under Meters, a value for samples per meter is evaluated once (only), for the first group of calibrated pings and is applied to all pings in the echogram. In the case where the ping calibration changes within the data, the evaluated value may be inaccurate. Also be aware that too few samples per meter may result in a poor noise estimate.
• Meters are used in De Robertis and Higginbottom (2007).

Vertical extent (samples)

Vertical extent (meters)

Specifies the vertical size of the averaging cell. The averaging ping interval averages its samples in vertical averaging intervals.

Note: De Robertis and Higginbottom (2007) use a vertical cell size of 10 m for their data.

Vertical overlap (%)

Specifies the percentage vertical overlap of the averaging cell down the samples of the averaging ping interval.

• The default overlap is zero. De Robertis and Higginbottom (2007) use non-overlapping averaging cells.
• Calculation time increases with an increase in Vertical overlap.
• Specify an overlap, when the calculated noise estimate is too high and the wanted signal in the echogram is undesirably affected by the operator.

Settings

Description

Maximum noise (dB)

Specifies the maximum acceptable noise (dB) for the operand data. The algorithm uses this value when the calculated noise is greater than the Maximum Noise.

The default value is -125 dB. De Robertis and Higginbottom (2007) use this value for their data.

Maximum Noise is equivalent to Noisemax used by De Robertis and Higginbottom (2007).

"...Noisemax represents an upper limit for background noise expected under the operating conditions. Noisemax must be determined empirically and will depend on the echosounder, its installation, and the radiated noise of the vessel."

Minimum SNR

Specifies the acceptable signal to noise ratio (dB) for samples in the operand data. The algorithm evaluates the noise-corrected-signal to calculated-noise ratio and tests to see it satisfies a minimum desired SNR. If the ratio is less than Minimum SNR then the signal is deemed to be unacceptably distinguishable from the noise. In this case, the corrected Sv is set to -999.

The default value is 10 dB.

Minimum SNR is equivalent to thresholdSNR used by De Robertis and Higginbottom (2007).

"...SNR is a measure of the relative contribution of signal and noise and can be used objectively to identify data that contain sufficient signal to warrant further analysis, such as echo integration or multifrequency comparisons...The minimum thresholdSNR required to suppress the contribution from these pixels can be estimated by examining distributions of Powercal from areas dominated by noise and selecting a threshold above the mean value that will exclude contributions from it. This value will depend on the equipment used, the operational settings, and the extent to which data are averaged..."

Settings

Description

Algorithm

Available Cell statistic algorithms are selected from the Algorithm list.

Fill empty cells with the mean value of surrounding cells

Select this setting to fill empty cells with the mean value of surrounding cells.

Clear this setting to leave empty cells empty.

Mean calculation

The mean is calculated by considering the original cell values of the nearest-neighbor cells (up to 8 cells). Cells that have a no-data value do not contribute to the mean calculation (in the numerator or divisor).

Settings

Description

Acoustic bottom

Select the line that denotes the start of the bottom echo trace.

True bottom

Select the line that denotes the end of the bottom echo trace.

Averaging layer thickness (m)

Enter the thickness of the layer (on the transducer side of the acoustic bottom line) for which the average volume backscatter should be calculated. The calculated value will be used to fill the layer between the acoustic bottom and true bottom.

Setting

Description

Algorithm

Select one of the following algorithms for the Data Generator operator:

• Constant

• TVG Curve

• Constant + TVG Curve

When selecting the TVG curve or Constant + TVG Curve algorithms check to see if the Logged draft warning applies to the input operand.

Sv at 1m (dB)

Specifies Sv (dB) at 1 meter.

Sv (dB)

Specifies Sv (dB).

Apply TVG override

When checked, specifies the value used to override the TVG spread for the algorithm.

Setting

Description

Algorithm

Select one of the following algorithms for the Data Generator operator:

• Constant

• TVG Curve

• Constant + TVG Curve

When selecting the TVG curve or Constant + TVG Curve algorithms check to see if the Logged draft warning applies to the input operand.

TS at 1m (dB)

Specifies TS (dB) at 1 meter.

TS (dB)

Specifies TS (dB).

Apply TVG override

When checked, specifies the value used to override the TVG spread for the algorithm.

Setting

Description

Algorithm

The Constant algorithm applies a value specified by Constant (dB), to all data samples in the virtual variable.

Constant (dB)

Specifies a value (dB). 

Setting

Description

Algorithm

The Constant algorithm applies a value specified by Constant (linear), to all data samples in the virtual variable.

Constant (linear)

Specifies a value (linear).

Setting

Description

Allowed slop

The maximum allowable difference in ping times between the two operands for a successful match. If no successful match is found, then that ping from the second operand does not appear in the resulting echogram.

Setting

Description

 3dB beam angle factor

A multiplying factor for the 3dB beam angle used to threshold the motion correction output. This setting enables you to specify the degree to which the Dunford algorithm is applied to your data.

Values in the Information section of the Motion correction page show the impact of the threshold on your data.

Refer to the Motion correction (Dunford) algorithm for more information.

Settings

Description

Ping range(s)

Select whether the ranges will be specified by ping number, time or distance in meters or nautical miles. Specify the pings in the operand to be used in the virtual variable. Ranges are separated by commas. The actual range of the operand and an example specification are shown.

Number

Specify pings by their numbers, in order. Single numbers or ranges may be used. An unterminated range spans to the last ping.

e.g.: 5,7, 9-20, 25-

Time

Specify pings by one or more times ranges in abbreviated ISO 8601 format.

e.g.: 2012-6-28 12:30 = 13:45, 16:00 =

You can write times in full or you use these abbreviation rules:

Distance (m)

Distance (m) specification is entered as a start distance (m), a dash and a stop distance (m).

e.g. 0-9.5

Distance (nmi)

Distance (nmi) specification is entered as start distance (nmi), a dash and a stop distance (nmi).

e.g. 2.58-4.00

Ping Inclusion

Select whether the specified ranges are to be included or excluded.

Pings

The Pings section lists the actual pings included or excluded, next to the corresponding ranges.

Setting

Description

Assign ping times from the relative ping index

Choose to shift the ping times relative to the ping index entered. A positive value shifts pings forward in time, a negative value shifts pings back in time.

Settings

Description

Region class

Choose one or all region classes to apply the statistic.

Statistic to calculate

Select the statistic to be calculated for the selected region class. You can choose from one of:

• Minimum
• Mean
• Maximum
• Standard Deviation

All will produce a virtual variable of the same data type as the operand. The choice of standard deviation will result in a linear variable which with all samples outside of regions receiving the no data value.

Settings

Description

Ping selection

Choose to apply statistical functions to all pings in interval, or to the middle ping in the sample interval.

Note: In the Select middle ping mode, if there is an original ping in the time interval corresponding to the resampled ping, the closest one to the ping center is selected. If not, no ping is selected and a no data ping is output on the echogram.

Distance mode

For resampling by distance there is a choice of resampling using GPS or Vessel logs (if available) in units of either nautical miles or meters.

Distance interval

Number of pings in interval

Time interval

Specify the distance (nautical miles) / number of pings / time (seconds) sample interval for resampling.

Settings

Description

Resampling operation

Choose the statistical operation to use in resampling your data.

See Resample operator algorithms for more information.

Percentile

For resampling using the percentile operation, enter the required percentile. For example a percentile of 90 will return that value below which 90% of the sampled datapoints lie.

Mean algorithm in range dimension

This setting is only accessible when resampling using the Mean resampling operation. Four Mean resampling algorithms are available:

• Weighted mean (recommended)
• Unweighted mean
• Linear interpolation
• Linear interpolation | Weighted mean

Weighted mean (recommended) is selected by default (in new EV files). When you change the data range (from the original range), is is advised to use this setting.

If the sample thickness is increased, the resampled value of samples is calculated using a weighted mean.

If the sample thickness is decreased, the resampled value of samples is calculated using interpolation.

Weighting along-track

This setting is only accessible when resampling using the Mean resampling operation on the Resample by time interval and Resample by distance interval operators.

If selected, the resampled value is calculated using a weighted mean, otherwise an ordinary mean (unweighted) is used.

Settings

Description

Use ranges from operand 1

Click Use ranges from operand 1 to use the ranges specified by operand 1.

Note: This setting is unsuitable to use where Operand 1 contains pings with no samples in them. In this case, Echoview encounters problems with the Start range and Stop range of such empty pings.

Custom ranges

Click Custom ranges to enter and use your own ranges.

Custom ranges together with Start range and Stop range define the Start sample range and End sample range boundaries of the output ping. Number of data points defines the resolution of the samples within those bounds. In the range dimension the resampling can sometimes look like a zoom.

	Resample variable Not resampled in the along-track direction. Start range = 50 m Stop range = 100 m Data points = 20
	Input variable Start range = 0.096 m Stop range = 249.9 m Data points = 1301

Start range

Enter the start range in meters, i.e. the range of the first sample in the output echogram.

Stop range

Enter the stop range in meters, i.e. the range of the last sample in the output echogram.

Number of datapoints

Specify the desired data resolution to output for the depth range specified.

Setting

Description

Use Operand 2 as the Maximum threshold

When this setting is selected, this operator behaves as a Maximum threshold operator.

The value of a datapoint in Operand 1 is compared to the value of the corresponding datapoint in Operand 2.

When the value of Operand 1 is greater than that of Operand 2, the output datapoint is set to the value specified for Set thresholded values to, otherwise it is set the the value of the datapoint from Operand 1.

Use Operand 2 as the Minimum threshold

When this setting is selected, this operator behaves as a Minimum threshold operator.

The value of a datapoint in Operand 1 is compared to the value of the corresponding datapoint in Operand 2.

When the value of Operand 1 is less than that of Operand 2, the output datapoint is set to the value specified for Set thresholded values to, otherwise it is set the the value of the datapoint from Operand 1.

Set thresholded values to

Select required output for Operand 1 thresholded values.

Setting

Description

Using line

Select the line that represents the path of the towed body. The depth of this line at each ping will be added to the depth of all samples in that ping to create the virtual echogram.