Exporting Sidescan Mosaic Images

Sidescan mosaics can be exported from either Sidescan Track Review or a Sidescan Mosaic, in a range of image formats. Exported images can then be used within ReefMaster or a wide range of other applications.

Exporting Images from the Sidescan Track Review

To export an image from the Sidescan Track Review window, an image region must first be selected:

To select a region for export, put the mouse into region select mode (1) and draw a rectangle that defines the area to be exported by holding the left mouse button down and moving the mouse. One the region has been defined, release the left mouse button. The export mosaic image button (2) will be shown. Clicking the export mosaic image button will show the Export Mosaic Image options window (see below).

Exporting Images from a Sidescan Mosaic

Images can be exported from a sidescan mosaic by using the region select method described above.

To export the entire mosaic without first selecting a region, click the Export Mosaic Image button in the graphical toolbar (circled):

The Export Mosaic Image Window

The Image Export Window is where the image file format and a number of image export options can be specified:

An export format can be chosen by clicking a button in the export format list (1) (export formats are described in detail, below). Further options for an export format may be presented in the main panel (2), which updates to reflect the currently selected format. When the required options have been set, click the Save button (3). A file (or folder) chooser window will be shown, and the export started.

Exporting large mosaic images can take a long time, and is done in the "background" of ReefMaster. This means that it is still possible to use ReefMaster whilst the export is in progress, although it is not possible to start another image export for the same mosaic during this time. The progress of the export is updated in the Application Status Bar:

Fade Edges

Fade Edges is an export option common to all image export formats, that fades the extreme port and starboard edges of the sonar swath from fully opaque to fully transparent over a distance of 4 metres (about 12 feet). This option is useful when exporting mosaic images with overlapping swathes, and can be very effective at hiding the "join" where one swath meets another:

Detail of mosaic export image without the Fade Edges option checked

The same area as above, exported with using the Fade Edges option

Note that when exporting mosaics using the fade edges option, swaths should have an overlap of at least 4 metres.

Image Export Formats

ReefMaster supports a range of different image export formats, each of which has benefits for certain applications.

The problem with large images

Image size is a major consideration when considering the export of sidescan mosaics. Mosaics within ReefMaster can be very large - perhaps a mile or more across - and the resolution of sidescan data is very high, in the order of 15 - 20 pixels per metre. Simply saving an image of a large mosaic at a high resolution would result in a huge single image file; if we consider the example of a one mile square mosaic exported with a 20 pixel per metre resolution, the resulting image file would be approximately 32000 pixels across and take up 4GB of memory (uncompressed, at 4 bytes per pixel). Images of this size are not practical, and very few applications are able to display or process them.

Tiled image formats

ReefMaster tackles the problem of exporting large mosaic images by supporting the MBTiles and KML Super-overlay tiled image formats. Tiling an image involves splitting the image into a large number of smaller image files, that combine together to form the complete image. The advantage of this technique is that an application only needs to load visible portions of the image for display. In addition to splitting the source image, the mosaic image is also saved at a number of lower resolutions (which can themselves be tiled), which allows large areas of the mosaic image to be viewed with minimal memory and performance overhead.

MBTiles

MBTiles is a tiled image database format used in ReefMaster and an increasing number of other GIS applications.

Background images within ReefMaster are stored in this format, and this format should be selected when creating a mosaic image for use within ReefMaster.

In addition to the Fade swath edges option, described above, MBTiles has one further options: Add exported image to workspace (1). This option simply adds the exported image file back into the current ReefMaster as a Background Image, allowing the image to be used as a Image Overlay or for tracing in a User Map.

Tracing sidescan images within ReefMaster

Although it is possible to export complete sidescan mosaic images for use in chartplotters, it is often more useful to trace just the areas of interest and create a map using these features. This provides a much less cluttered map that is easier to read on a typical chartplotter screen.

To create custom path and/or polygon features for use in a map, a User Map is used. An sidescan mosaic image that has been exported in the MBTiles format and added to the current ReefMaster workspace can be viewed within a user map edit window, and features traced as either custom paths or custom polygons. In the example above, a submerged road and railway have been traced using a yellow-filled polygon. Various creek beds have been traced using red paths. Once features have been created, they can be exported in a range of map and data formats; see Exporting Data.

PNG Image

The Portable Network Graphics (PNG) format is a lossless, compressed image format supported by a very wide range of applications.

Mosaics can be exported as a single PNG image, or as a set of PNG image tiles. A Google Earth calibration (.KML) file is created for each PNG image (or tile), which can be used to view the exported in Google Earth and other GIS applications.

Exporting a mosaic as single image files is not recommended when the exported image dimensions are very large, although ReefMaster does support the writing of such large files if required -the MBTiles or KML Super-overlay formats are suggested for large image exports.

(1) Pixels per metre

Pixels per metre can be selected between the range 0.5 - 20 and determines the number of pixels per metre in the exported image. For example, if the image is 1000m across and 10m per pixel is selected, the width of the exported image will be 10,000 pixels.

The size of the image (in pixels) is shown, and updates as the pixels per metre value is changed. A warning background is shown when the potential image size becomes very large (2).

(3) Show grid lines

Latitude and longitude lines can be exported by selecting this option.

(4) Split image into tiles

Splits the image into tiles of 1024 x 1024 pixels. Resulting tiles are given names of the form <filename>_x.png, where x is the tile number. A Google Earth calibration file is created for each tile.

KML Super-overlay

KML super-overlays are a tiled image format suitable for use in Google Earth.

A KML super-overlay export consists of a single, top-level.kml file and an associated _files folder that contains a large number of image tiles in various resolutions, with further associated .kml files:

Note that the top-level KML file is itself very small in size; this file simply contains a link to a top-level image file within the _file folder.

Hosting KML super-overlays online

KML super-overlays can be hosted online very easily, and provide a very simple and effective way to share large sidescan mosaic images. Image data is only downloaded for for the required region and resolution, which means that performance is usually very good when viewing online images.

To host a super-overlay, the _files folder and contents should be copied to an appropriate online location. The top-level .kml file must be edited so that it points to the top-level image file in the _files folder:

·Modify the <href> field so that it points to the URL of the top-level kml file within the _files directory. For example:

·Save the KML file. This (small) file can then be distributed very easily, and will link to the online image source when opened in Google Earth.

AT5 Raster Overlay

The AT5 format is a Navico™ map format compatible with Lowrance™ and Simard™ chartplotters. An AT5 raster layer is an image background that can be shown on a charplotter and combined with vector components such as contour lines. To use a sidescan mosaic as a background image, choose the AT5 Raster Overlay option and generate a raster layer. The generated files are contained in a directory called ShadedRelief, which should be copied to the root directory of the memory card to be used in the chartplotter. Vector components, such as isobaths and contours, should be exported separately from either a Map Project or User Map, and the resulting Large.at5 and at5.xml files should also be copied to the root directory of the memory card.

ReefMaster includes a default map style that produces semi-transparent vector depth areas specifically designed for use over raster backgrounds. See Exporting Preset Map Styles.