Data page
The Data page of the Variable Properties dialog box or the Variable Properties - Multiple Variables Selected dialog box is used to enter settings that affect most operations in Echoview including echogram display, integration, schools and fish track detection and data exports. See the Variable Properties flow diagram for details.
The data page for multibeam data (refer to the About data types page) only contains the Minimum threshold and Maximum threshold options.
Applying thresholds affects the data samples in the variables, not just how the data samples display in an echogram.
This page displays settings relevant to the type of variable as follows:
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Sections for all acoustic variables
Sections for Sv or TS variables only
Sections for single target variables only
Sections for H-mode and V-mode variables only
Sections for position variables
Sections for all acoustic variables
Thresholds are a useful data filtering tool, however, consider the impact a threshold may have on the evaluation of virtual variables in your dataflow.
Minimum threshold
The Minimum threshold section of the Data page is used to set the lowest data value for data samples. All samples with a value below the minimum threshold are reassigned a zero value.
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Settings |
Description |
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Apply |
Select this checkbox to apply the entered threshold value to the variable. |
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Lock to color display minimum |
Select this checkbox to lock the minimum threshold to the color display minimum setting on the Echogram Display page. Choose this option to match the echogram display colors to the thresholded data. Once locked, you only need to change the color display settings to change the thresholds. |
Maximum threshold
The Maximum threshold section of the Data page is used to set the highest data value to be displayed on an echogram or included in analyses.
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Settings |
Description |
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Apply |
Select this checkbox to apply the entered value to the variable. |
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Lock to color display maximum |
Select this checkbox to lock the maximum threshold value to the color display maximum value controlled on the Echogram Display page. Choose this option to match the echogram display colors to the thresholded data. Once locked, you only need to change the color display settings to change the thresholds. |
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Set thresholded values to |
Choose a value that will be applied to all samples above the maximum threshold value. Note: This setting will affect the calculation of mean values. If the maximum threshold is set to No data the thresholded values are excluded from mean calculations. If the maximum threshold is set to the Threshold limit, all samples that are above the maximum threshold are converted to the threshold limit value and included in mean calculations. |
Note: Threshold values can only be altered on the Data page if the data threshold is not locked to the color display and the Apply checkbox is checked.
Sections for Sv or TS variables only
Time-varied threshold
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Setting |
Description |
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Apply |
Select this checkbox to apply a time-varied threshold in Sv or TS at 1m (dB). Note: The keyboard shortcuts, Ctrl+F11 and Ctrl+F12, will adjust this value when used in an echogram window. Whilst using the shortcuts the value of TVT is displayed in Panel 1 of the Echoview status bar. |
From Echoview 4.20 onwards, if a TVG range correction is applied on the Calibration 2 page (the Calibration page from Echoview 5.0 onward) of the Variable Properties dialog box, then this correction is also used in the TVT calculations. Previously, Echoview did not account for TVG range correction in such calculations. As a consequence there may be small but significant errors in the TVT data. The errors are of the order of 0.5dB (at 2m) to 0.05 dB (at 17m).
See also
Time-varied threshold
Using a SoundSpeedProfile: Notes
Minimum TS threshold
The Minimum TS threshold section of the Data page is only available for Sv variables.
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Setting |
Description |
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Apply |
Applies a TS threshold to the Sv echogram. |
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TS Threshold (dB) |
Enter a value for the TS threshold (dB). Refer to the Minimum TS threshold algorithm page for more details. |
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Calibration difference offset |
A transducer related value required by the Converted Sv equation and is used when CSv and/or CTS are unavailable. Refer to the Minimum TS threshold algorithm page for more details. |
Sections for single targets
Single target settings
Settings for Single target detection, Single target detections raw, and Single target manipulation variables:
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Settings |
Description |
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Upper detection range |
These settings are available to raw single target variables only. Enter the range in meters. The settings change the (range) display limits of the raw single target variable. |
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Lower detection range |
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Thickness source |
Allows you to select a Thickness source from a list. Together with the Thickness factor, this setting controls the display thickness of single target detections on an echogram.
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Thickness factor |
Controls the display thickness of single target detections on an echogram. Use in conjunction with the Thickness source. See Configuring single target display thickness for more information. |
See also:
Sections for H-mode and V-mode variables only
Target-locking
A variable can be made target-locked by specifying its partner under Target-locked variable. If this is an H-mode variable, a list of V-mode variables will be available for selection. If this is a V-mode variable, a list of H-mode variables will be available for selection.
You must select a Target-locked variable here if you intend to detect schools on this variable using the target-locked scanning algorithm or use it as an Analysis variable when integrating from a scene using the Target-locked scanning algorithm (see the Analysis page of the Scene properties dialog box)
Sections for position variables only
GPS Processing
The following settings are only available for position variables and provide basic filtering (quality control) for GPS fixes. Echoview uses the settings to determine whether the status of GPS fixes is good or bad (see Processing GPS data for more information).
Hint: You may have to experiment to find optimal settings. To view the results achieved using different settings: ensure Show good fixes and Show bad fixes are selected on the Cruisetrack Display page of the Variable Properties dialog box; arrange the cruise track window and Variable Properties dialog box so both windows are visible; enter settings; then click Apply and observe the status assigned to the GPS fixes and the resulting cruise track. If you have optimized the settings but are not satisfied that the cruise track accurately represents the path followed by the logging vessel, you can create a smoothed cruise track using the Kalman GPS smoothing filter
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Settings |
Description |
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Hemisphere |
Used to force the logged GPS latitudes to be treated as northern or southern hemisphere if required. The default setting, "As recorded", is normally used unless there is a problem with the GPS data. |
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Maximum apparent speed |
Enter a speed, in knots, of the logging vessel's maximum speed through the water. A GPS fix will be marked bad if the calculated speed traveled between the previous good fix and the current fix (calculated from the difference in position and time) is faster than the maximum speed entered. Such fixes are discarded as errors on the basis that the vessel could not have moved this quickly. |
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Maximum acceptable gap |
Enter a value, in seconds, for the maximum time interval allowed between two good GPS fixes, for the positions between the fixes to be given a good status. If the gap between two fixes exceeds this time, the ping positions between these two fixes will be given a status of uncertain. This setting is included to flag occasions when there are gaps in the stream of GPS fixes. Echoview assumes the vessel traveled in a straight line between two fixes. If those fixes are too far apart, this assumption becomes unreliable. The uncertain position status does not affect calculations made with the ping positions, but is displayed on the echogram and exported bathymetric data can be filtered on the basis of this position status. |
| Minimum acceptable gap |
Enter a value, in seconds, for the minimum time interval between good GPS fixes. If the gap between two fixes is less than this time, the second GPS fix will be given a bad status. This setting is used to remove random variation in position from GPS fixes where it is large compared to the actual change in the ship's position between fixes. Hint: You may wish to choose an initial value based on the known error radius of the GPS and the known speed of the vessel. Example: Assume the error radius of a given GPS is 50 m (your system may have a higher or lower error radius) and the ship is moving in straight line at a two meters per second. Based on this data, it is possible that fixes received within 25 seconds of the previous good fix could fall within the error radius of the previous fix. This can result in a cruise track with frequent changes in direction (it may even loop back on itself) and in which the along track distance is longer than the actual distance traveled by the vessel. In this example a Minimum acceptable gap of 25 seconds would remove all fixes that could fall within the error radius of the previous good fix. |
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Processing method |
Select which method Echoview will use to determine the start of your cruise track. Method 1 uses the first good GPS fix as the start of the cruise track. This method can result in the cruise track not being displayed if the first GPS fix in your data set is a bad fix. Method 2 groups the good GPS fixes and displays a cruise track starting with the first good GPS fix. Use this method when your first GPS fix is bad. |