This guide can be read from the start following each step or jump to a heading using these links:
The only file format currently supported for mosaicing is .XTF files containing sidescan sonar data. Existing mosaic files with the .tmap_moz file extension can imported.
Start Perspective and click on File>Import>Raw Data File...
The Open Survey File dialog opens:
You can used the normal Windows Shift+Click to select multiple files, hit Open.
The following dialog will appear:
Indicating that Perspective has detected that these files contain sidescan data, and giving you the opportunity to create a mosaic or simply import the navigation lines.
If you respond No - then only the navigation lines will be imported and plotted.
Use Wizard lets you choose how the mosaic will be imported and displayed.
Use Defaults will use the previously saved parameters to automatically create and display the mosaic.
The Use Wizard option will first plot all the navigation lines and then open the Choose/Create Mosaic Layer Dialog:
Click Create and enter a name for the new mosaic file:
Hit Save.
The Choose Mosaic Settings dialog opens:
Resolution (m/pixel)- choose the resolution of the mosaic (the size, in meters of each pixel in the image). Use any of the predefined values or enter a custom value.
Combine Using: Maximum - will cause stronger returns to "shine-through" an overlapping line. (default)
Minimum - will cause weaker returns to be preserved when lines overlap.
Over - Lines will be displayed based only on the order in which they were mosaiced.
No Step - No merging will take place, existing imagery will be preserved, new lines will not overwrite.
Constrain to map view - only generate the mosaic in the visible map view window.
Antenna to Sidescan transducer Offsets (hull mounted)
If the side scan transducers are mounted directly on the vessels hull then you should enter the X and Y offsets here.
On - enables the feature.
Use geometry from XTF header - if supported the X and Y sidescan transducer offsets can be read directly from the the XTF file (not Isis)
Sidescan offsets are the same as Bathy - sonars that have both Muti-Beam and Sidescan (SwathBathymetry) capability will usually have the same offsets for both data types. Both data types can be processed by Perspective / Bathy One/ Mosaic One. Checking this box causes the Bathy offsets to be read from the XTF file. Note that only the Antenna to Transducer offsets are used and are computed from the Transducer > Reference Point and Antenna > Reference Point values. For example the, offsets shown above would results for this offset diagram:
Examples of using the Antenna to Transducer Offsets feature when creating a Mosaic.
Single Channel Auto Flip - where a mosaic has sufficient raw data overlap it may be possible to mosaic only one channel on lines running in opposite directions. Perspective detects the line direction and selects the correct channel, - the resulting mosaic will have shadows only on one side of all objects, which can give superior results by removing the "double shadow" problem. The user can select the starting line and also the starting channel.
Click Next to proceed.
The Choose Line Settings Dialog opens:
Downsampling and Combine Overlapping - both options refer to the method by which overlapping data from within one line is handled. Downsampling refers to the across track element (sample to sample within a ping) and Combine Overlapping refers to the merging of samples when a line overlaps itself. Each has the following options:
Average - the average pixel value is calculated and used
Maximum - the pixel with the highest value (maximum) is used
Minimum - the pixel with the lowest value (minimum) is used
RMS - Root Mean Square is a statistical measure of the magnitude of pixel values. It is the square root of the mean of the squares of the pixel values.
Skip -
Navigation > Settings
Allows the user to modify the type of smoothing that will be applied to the navigation.
For more information on Navigation Processing settings follow this link.
Fill Between Pings
None - do not fill
Lerp - calculate a smooth transition from the pixels in one swath to the next. (default)
Source of Heading
This will tell Perspective what to use as the source for the heading when creating the mosaic. Sensor - meaning a heading sensor that is integrated into the towfish, Ship - meaning use the heading that is recorded for the ship and Line CMG - the mean line heading, useful when the survey lines are very straight. CMG will be available in later version and will compute the heading from the vessel track.
Port Channel / Starboard Channel
Select which channels of data to use in the mosaic. Typically in a dual frequency system Ch 3 and Ch 4 will be a higher frequency with better resolution but shorter range than Ch 1 and Ch 2
Boundary Clipping
Allows parts of the swath to be blanked out, for example the outer ends of a sidescan swath may be noisy and contain no data, likewise the area directly beneath the towfish is often not used.
Apply To
The parameters set in Choose Mosaic Settings and Choose Line Settings will be applied to either All lines or only to a user specified line.
Click Finish to generate the mosaic:
Several stages will occur Preparation, Threshold Balancing, Geo-Referencing and Merging.
The mosaic will be displayed:
Note that we have vectors (green lines) showing the navigation track, these can be turned off under the Navigation Node, the mosaic is under the Imagery>Sidescan>"k5000_new" node. The lines included in the mosaic are displayed under the mosaic node.
The method used to map the grey scale values in the mosaic is dB based. The application views the overall greyscale values for the whole image and chooses a suitable 0dB level. All other greyscale levels are mapped in dBs relative to that level. This means that there should never be areas that are completely black (0,0,0) or completely white (255,255,255). It also means that the user does not need to choose a the best level using a threshold or "gain" control, the best levels are automatically chosen.
However the initial appearance of the mosaic can be rather "pale" or "washed-out" compared with an intensity- based mosaic; also there may be some bias in the original computation of the 0dB level. The user has the opportunity to modify the automatic mapping using the Histogram tool.:
Right - Click on the side scan image layer and choose Histogram from the drop down menu:
The Histogram window opens:
Use the handles to drag the Histogram limits and observer the effect on the mosaic. Moving the left handle to the right will cause the mosaic image to become darker or have more contrast however you may find that some detail at the very low (black) end of the signal spectrum is lost. The other handle has the opposite effect and you may find that some information in the lighter areas is lost.
The graphs represent the original distribution of the greyscale values (grey) and the modified distribution (green)
The most common correction is to require a darker image with more contrast. When the OK button is clicked the Histogram dialog closes and the changes to the mosaic will be saved, the changes will also be preserved in any GeoTIFFs that are generated.
There may also be a requirement to spread the available greyscale values over a smaller zoomed area of interest, for this there is also a "Spot Histogram" tool:
The "Spot Histogram" changes are only temporary and will disappear when the Spot Histogram dialog is closed.
Click on File>Import>Mosaic File...
The Open GRL Files dialog opens:
Select mosaic file, .tmap_moz, then hit Open.
A mosaic can be exported to GeoTiff or KML.
GeoTIFF
GeoTiff - there are two ways to export to GeoTiff; File>Export>Composite GeoTiff or by Right-Clicking on the mosaic name in the tree view.
File>Export>Composite GeoTiff.
This method exports everything in the current map view window (grid lines, tracklines, bookmarks, annotation, bathymetry, targets, etc).
Use the slider to "chop" the image if needed into 1, 4, 8, 16, 32 etc smaller TIFs (each has a .TFW) this can help with handling large high resolution images, you can also downsample the image using the Resolution pixels/meter setting.
This shows that the slider has been set to 4. This indicates that the composite exported TIF will be "chopped" into 4 TIF files, each 717 x 648 pixels, rather than the default of 1 TIF with a size of 1434 x 1296. Perspective automatically numbers the 4 exported TIFs.
The other method of exporting to a GeoTiff is to Right-Click on the mosaic name in the tree view.
Exporting a Geotiff this way exports only the items under the imagery node (sidescan mosaics and/or bathymetry) it does not include grid lines or any other nodes. The user can select which imagery node item to be displayed by simply turning off the undesired bathymetry file in tree view or an undesired mosaic file within the Geo TIFF Exporter.
KML
KML files are used by Google™ Earth, if Google Earth is installed on your system double-click the .KML file to launch Google Earth and see the imagery.
To export a mosaic to a KML file Right-Click on an existing mosaic name in the tree view, then select Export KML.
The KML Exporter is opened:
The KML Exporter works just like the Right-Click Geo TIFF Exporter.