The assumption is that the better the correlation between the size of the echo, the size of the shadow and knowing the expected echo/shadow distance the more likely the object is to be of interest. For the module to work efficiently the side scan data must be of high quality with good navigation. Testing has shown that the module works best with Side Scan data acquired from an AUV where the platform dynamics (altitude, heading and stability) are good. The sample file in the following guide came from a Klein 3500 sonar mounted on an AUV. You can download this file from the Triton FTP site by following this link. The zip file includes both the XTF file and the IDX file with processed navigation.
The guide makes reference to a mosaic of the line being processed. It is not strictly necessary to make a mosaic, the TargetOne CAD module can be run directly on an XTF file (or a number of XTF files) but it can be helpful to understand the process and to see where the detected Targets are located in the mosaic. Also detected Targets can be imported immediately into a Perspective project that has a mosaic of the XTF file(s) being used.
Here is a mosaic of the XTF file being processed:
There are a number of well-defined targets, also note that the line runs from the lower right to the upper left (approx 315degs). The CAD process breaks the line into tiles starting with the Port Channel. The numbered boxes show the order of tiles being analyzed.
Even if a mosaic is not being used it is important to set up the Contacts tab in Perspective, check the Targeting Paths, Contact Usage, Contact Number and Threshold settings:
Launch TargetOne by clicking the Icon in the Perspective toolbar:
Launch the CAD module by clicking Tools>CAD in the TargetOne menu:
Choose the XTF file(s) that you want:
Once an XTF file has been chosen the following dialog will open:
Some modern sonars have a very large number of samples per channel, automatic analysis of this data can be very time consuming, you have the option to downsample the data to something more realistic, the suggested resolution is 0.1m. The original number of samples per channel is shown, also the sonar range and the new number of samples/channel.
There are 3 options for the downsampling method, Maximum, Minimum or the Average. It has been found that using Minimum gives good results (favoring the detection of shadows).
Click OK - the downsampling process will run for the whole of the first XTF file, when complete the "Object Detect" dialog will open showing the first sample Port 1 Tile 1: Pings 1 to (nnn)
Initially the image may look very poor with no or a poor display of the Histogram. The object detection method requires a good grayscale distribution Histogram..
In the Object Detect window, click the CAD Settings Tab:
Adjust the Threshold slider to get the best image possible, (there may be areas where the image is brighter or darker).
Check the box Auto TVG:
Accept the default settings by clicking Apply and OK (but see below)
You should see a definite improvement in the image, go back to the Settings tab:
Note that the Histogram graph of grayscales in the image has is close to a normal distribution.
Click the Detect button to see which objects can be detected:
Echoes (areas of strong returns) are delineated by a Green outline, Shadows are delineated by a Cyan outline. Using Shadow and Echo thresholds of 15% the default Max number of objects have been detected, including one obvious one (3), however the large object on the left did not make it into the "Top 3", the Echo/Shadow Distance is set to 5m, this is an unrealistic value for the detection of smaller (MLOs), change the Max Echo/Shadow distance to 1m and click the Detect button again.
Notice that the two previous targets 1 and 2 no longer meet the requirements, they have been dropped and now the targets with the closer echo shadow distances are promoted.
To save all these settings as the future default settings click Set Defaults
Gives some more insight into the detection results:
Controls how Targets are Saved:
In early versions only TVG dB/m was available, the user needed to sample several different areas within the data set to determine a setting that would work for the whole data set. In many cases this was unsatisfactory due to large changes of gain due to change in seabed type Auto TVG in general works much better.
The default settings have been shown to work in most cases; the Contrast setting is available only when Mean or RMS is chosen.
A smaller Window size allows the adjustment to be made over a smaller number of samples (= shorter sonar range) If the image has large changes in gain level over shorter sonar range a smaller window size may be better, however decreasing the Window size too much may result in banding artifacts.
When the setup has been completed and the settings are correct be sure to click Set Defaults, if the program has been closed click the Use Defaults button to reload the settings.
It is not necessary to have a mosaic of the XTF files to be analyzed displayed in Perspective, but it makes it easier to see which contacts have been identified and their position.
Check AUTO and click START
The program will step through the XTF file(s) detecting objects, at the start of each new file the downsampling settings will be used automatically. The following dialog will be displayed showing progress and the number of detections:
Click Exit Auto CAD Mode to quit immediately, or check the box Stop at End of File to complete the current file before stopping.
When the last XTF file has been processed the following will be the display in TargetOne:
Use the Next-> and <-Prev buttons in the Page File dialog to page through the detected objects. Highlighting a file will display the image at the top left. At this time the contact files exist only in the Temporary contact folder selected earlier and each file is prepended with the ~ character. Once a contact is accepted by saving (Save Selected or Save All) the contact is moved to the Saved Contacts folder and the ~ character is removed. The default locations for these folders is:
Temporary Contacts : C:\Users\<username>\AppData\Local\Temp\TritonTempContacts
Saved Contacts: C:\Users\<username>\My Documents\Triton Saved Contacts
If Perspective is open and displaying the relevant Mosaic at the time the contacts are saved they will be immediately imported into Perspective, added to the list of contacts, and their positions identified in the Map Window:
NOTE for ECA / MWD implementation:
The above events (importing to Perspective) will not occur if the ECA / MWD addons are in use. In that case the .conimg check box should be checked and the contacts will only appear in the MWD module.