These settings are typical, different sonars with different gain characteristics or different transducer beam patterns may require very different settings, you will need to experiment. Other factors are towfish altitude, seabed reflectivity water depth etc.
Before starting with an unknown data set it is a good idea to reset Isis to its default configuration, Click Configure>Reset to Factory config and allow Isis to restart.
After Isis re-starts click the green playback button:
Choose the XTF file from the dialog box and hit OK to playback the file, the main Isis window will display the unprocessed side scan data. Use the Running/Walking buttons to speed up / slow down the display, hit the <spacebar> to pause. The <home> key takes you to the start of the file, the <end> key takes you to the end of the file. The File menu has many other options including Go To Next , Go Back to Start, Go to Time/Ping/Position etc.
IMPORTANT NOTE: In this document we will refer to "conventional" side scan imagery where strong signals (echos) are black and weak signals (shadows) are white. You can reverse this convention if you wish by changing the default palette setting in Isis. Hit the Color>Palette menu and uncheck the box Reverse Palette, you may also choose any of the other color palettes; this document will refer only to the default greyscale palette.
The initial display of the data may look either too dark or (as in this case) too light:
The first basic control is the threshold control. Right click in the main window to open the Waterfall Display dialog:
Use the threshold slider to adjust the darkness of the image, in this case a setting of 3 gave a good result:
Note, as with most sonars the return close to and below the fish (nadir) has a much stronger, and in this display, darker image.
NOTE: Re-tracking the bottom involves writing new data to the XTF file, therefore this will not be possible if the XTF file is write-protected or exists on a CD or DVD.
One of the basic requirements for a mosaic is to correct the imagery for distortion caused by the distance of the sonar above the seabed, known as "watercolumn removal" or "slant range correction" this link has more information on the subject: To make the correction we need to know the towfish altitude above the seabed, in some cases the sonar may measure the altitude directly, send it to Isis during acquisition, and it will be stored in the XTF file, in other cases the bottom may have been tracked in Isis during recording. In the images here neither of those things have happened, as can be seen by the presence of the red bottom track line in the center of the image and from the towfish altitude in the Isis Parameter Display, which shows 0.0m:
NOTE: If the XTF file you have is already correctly bottom tracked you can omit these steps and go to Slant Range Correcting the image.
From the drop down menu in Bottom Track Primary Towfish select Amplitude
And then check Save newly tracked altitude to original file, this will update the XTF file with the new towfish altitude data
There are 3 controls that can be used to control the function the tracking gate:
Channel: - this slider allows you to choose which of the avaialble data channels in the file to use for bottom tracking. Choose the one that has the least amount of noise in the water column and the cleanest first return from the bottom, in the example shown this will be Channel 1 (Port)
Level: - this slider sets the signal level required to trigger the tracking gate expressed as a percentage of the overall signal level; increasing the value causes the tracking gate to look for a higher value (stronger signal) decreasing the value causes the gate to look for a weaker signal. Effectivly this means moving the slider to right will cause the tracked value to be deeper and to the left shallower. At some point the gate should sit right on the actual seabed return (see below)
Holdoff: - this slider sets a delay to prevent the tarcking gate from locking on to noise in the water column or to the sonar transmission pulse, choose a value such that it always less than the smallest altitude value expected for the line or survey.
Check Save newly tracked altitude to original file to track the bottom and write the new value back into the file, then play the file back in Isis.
Remember to turn off saving and tracking (Method to OFF) when done. Isis now displays the new altitude from the XTF file.
In this image the tracking gate is correctly set. And the Altitude is displayed as 22.85m
If you choose the Gradient option then the tracking operation is automatic. The level slider is disabled and the tracking gate searches automatically for the bottom. In this mode the tracking gate is looking for the greatest change in gradient. The hold off value slider works the same as in the Amplitude method. In areas where the bottom is soft or poorly defined the Gradient method can work better than the Amplitude method.
Selecting the Manual option causes the Level slider to change to a simple method of setting a fixed altitude. This can be useful if the water is very shallow and the first return is very close to the fish, setting a fixed value of a few meters can give a good result. When creating detailed images of wrecks or other man-made structures; if the tow fish altitude is changing rapidly due to passing over rocks or wreckage, setting a fixed value for the altitude and ignoring the true altitude can produce a more coherent image.
In this image you can see that there is an area where the bottom tracking has not worked due some noise in the water column:
The P&C (Point and Click) method can be used to correct these errors. Choose P&C from the menu:
Check the Save Newly tracked altitude box and (optionally) the Auto Pause box. The Auto Pause feature is useful if the area to be re-tracked is longer than one screen, it allows you to use the Resume button to scroll one page of data at a time. (you must keep the Alt key depressed)
Hold down the Alt key and click on the waterfall to select a new position for the bottom track.
Each click generates a small "X" and a new red bottom track line joining the "Xs" together.
IMPORTANT: When you are finished, uncheck Save new Altitude... and Auto Pause and also set the Method to OFF
Playback the the file to see the corrected bottom track:
There is always a value set in the XTF file for the altitude. You can set that value to Zero by using the Manual Method, setting the value to Zero and checking the Save newly tracked altitude to original file check box. When the file is played back Isis will write a zero value for the altitude, and the red bottom track line will be in the center of the waterfall.
REMEMBER! You MUST uncheck the Save newly tracked altitude to original file box and also set the Method to OFF in order to read the value of tow fish altitude from the XTF file.
Once the bottom has been tracked you can turn on Slant Range correction, right click again in the waterfall window and check the Slant Correction checkbox:
The waterfall display will change, the water column will disappear and the imagery will properly displayed:
The area directly below the towfish (nadir) will always have poor imagery due to stretching of the data in that area.
Also note that the darker area close to the towfish is now more obvious.
As can be see from the previous image there is a tendency for the signal strength to be greater (darker) close to the towfish and weaker (paler) towards the edges, we need to correct this as far as possible to create a good mosaic.
Click the TVG button:
The TVG and Balance dialog box will open:
There are three options:
Standard - applies a fixed ramp of dB/meter starting at nadir
Custom - which allows some better control of the shape of the TVG curve
Auto - normalizes the entire channel producing a very even, grey record but at the expense of seeing the true contrasts in the seabed type.
The Channel slider allows you to apply different TVG settings to different channels if necessary.
You may use any of these settings to achieve the imagery you need, here is an example of the Custom TVG and its effect on the sample data:
You may either enter the values directly, use the small spin buttons or click/drag the shape of the curve with the mouse:
Here is the result of applying these Custom Settings
Which is very close to the result we are looking for.
An example of the Auto TVG setting:
And the result:
This also looks quite good, but see below:
The Auto TVG setting seems at first sight to be a very "easy answer" and for some types of data it can work quite well. However, notice in the above image how the contrast between the light area near the top of the image has been reduced compared with the image that was obtained with the Custom Setting, the result over a large area is that the mosaiced image is lacking in contrast; the whole survey may look very gray with no highlights or shadows and these are of course the essence of a side scan mosaic. In general the advice is not to use the Auto TVG setting for large areas where the interest is in mapping say, patches of sand, gravel, shell and rocks against a background of low reflectivity fine mud, Using Auto TVG wil cause the whole image to look very "flat". If Auto TVG is turned on when displaying a waterfall without slant range correction the normally "white" water column will turn gray.
However, if the requirement is to pick out small details in the image of a wreck then Auto TVG can be a good option.
The Balance setting has a built in description "What's this?" but is essentially a modification of the Auto TVG setting that attempts to balance one channel against the other, and the Pixel-to-Pixel option is very close to the same effect as Auto TVG.