Bottom Composition

The bottom composition module provides a way to visualise changes in the nature of the sea, lake or river bed. The relative hardness and roughness of the bottom can be determined from sonar log files in Humminbird or Lowrance formats, and processed to produce overlays on map projects. Bottom composition overlays can then be exported in a range of formats suitable for viewing on GPS devices or use in other GIS applications.

The Bottom Composition module is an additional module to the ReefMaster PRO edition. See ReefMaster license activation for information on how to upgrade to, and install, the bottom composition module.

3D map image with an overlay showing harder sea bed areas

Overview

Bottom composition is determined by analysing the data within sonar log files and looking at signal returns at various positions within the returned "ping". By calculating and averaging a range of values, ReefMaster is able to determine to relative hardness and roughness of areas within the map project. It is important to note that the values that ReefMaster calculates for bottom hardness are unit-less and provide only an indication of relative changes in bottom type across the mapped area.

Hardness Layers

Information about the bottom type is extracted from several places in each sonar return, each of which provides a different value and each of which can be shown separately as different layers within the bottom composition module.

The image above shows a typical sonar return. Information that is useful for determining the relative bottom type is extracted from three distinct areas, labelled in the image above as PeakSV, E1 (first return) and E2 (second return). Each layer can tell us something slightly different about the nature of the bottom.

Peak SV

Peak SV simply measures the strength of the sonar return as it is reflected off the bottom, and is highly correlated to the hardness of the bottom. For various reasons, simply measuring the peak signal return at the bottom is not always as reliable as using the second echo return (E2, see below) but in many cases the results can be very useful. Peak SV is included as an layer in ReefMaster because many sonar logs are not recorded with a sufficient depth-range to include a usable E2 layer.

The E1 layer is derived from the sonar returns that immediately follow the peak return of the first echo return. This value is commonly referred to as roughness or rugosity and is a measure of the roughness of the bottom. Although this value is not a direct measurement of hardness, the two are often closely correlated.

The E2 layer is derived from the full second echo return of the bottom and is commonly referred to as hardness. The second echo return is generated when the sonar echos once again off the bottom, after having returned once to the surface and bounced off the underside of the boat. This second echo return is particularly useful for determining the relative hardness of the bottom. For example, in the image above, the second return shows a hard area in the middle much more clearly than the first.

Collecting data for use by the bottom composition module

·The process of calculating bottom composition values can be very sensitive to noise. Run slowly over areas of interest to minimise the amount of noise in the sonar recording.

"hardness" values extracted from the PeakSV and E2 layers

·The E2 layer is the most reliable source of relative hardness information. The image above shows track-points coloured according to hardness values extracted from the PeakSV (1) and E2 (2) layers (darker colours indicate a higher return). Notice how much more clearly the harder area in the centre of the image is defined by the E2 layer. Where possible, always collect data with the sonar depth-range configured so that it is large enough range to include the full E2 return. In practice, this should be a good 10m more than double the current depth. Auto-range on many devices often truncates the E2 layer, or even omits it entirely, which means that a large amount of usable information is lost.

Importing bottom composition data

Sonar log files from Lowrance and Humminbird may be used to provide data for the bottom composition module. Any sonar file which is able to be displayed in the sonar viewer is capable of providing the required information. If the file cannot be seen in the sonar viewer (this applies to some early Lowrance slg files), then hardness data cannot be extracted.

When the bottom composition module is installed, hardness data will automatically be calculated whenever a compatible sonar log file is imported. Bottom composition values are assigned to each track-point within the imported track, for each of the three layers.

Note that sonar log files must be imported again after activation of the bottom composition module.

Importing Humminbird sonar log files

It is very important to set the water type and temperature parameters in the active GPS Equipment Profile (1). Temperature should be accurate to within 5 degrees C.

Scale and Offset

Scale and offset are provided as import parameters, and can also be edited in the individual track properties. Scale and offset simply scale and shift the calculated hardness values by the specified amounts; scale is applied first, followed by the offset. Raw values for hardness are in the range 0 - 255; values that become less than zero after scale and offset are applied are ignored in all bottom composition calculations.

The scale and offset parameters are provided to assist in calibrating files recorded using different units or transducers and should be left to their default values unless combining data from different sources.

Viewing Bottom Composition in Tracks

The show hardness values button (1) is enabled on tracks that have associated bottom composition values. Click this button to change the colouring of the track-points to reflect their relative bottom composition values, instead of depth.

Track-points overlaid in the sonar viewer are also shown with their hardness values when the show hardness values button is selected.

The bottom classification edit section (2) in the track properties panel will also be enabled when bottom composition values are present.

(1) Layer Selector

Choose the active layer using the drop-down layer selector. The track-point colours displayed in the graphical edit area will update to reflect the new layer values, along with the rest of the values in the bottom classification edit area.

(2) Palette Selector

The palette used for the track-point bottom composition colours can be selected using the drop-down palette selector.

(3) Signal Histograms

Two charts are shown, displaying information about the signal returns in the selected layer. The top chart displays the average return across the depth range of the track. The second histogram shows the distribution of signal returns across the range of return strengths. This chart in particular can be useful when adjusting range, scale and offset parameters.

(4) Range

Use a defined range to truncate values to specified lower and/or upper values. When a range is applied, any values falling above or below the maximum or minimum values specified are set to be equal to the upper or lower value; the range values do not act as a filter.

To apply a range, select the Range button and adjust the upper and lower range values using the sliders. Click Apply to apply the chosen values. Setting a range is not a destructive operation; the range can be set back to the full limits of the track, or the Range button can be de-selected so that the range is no longer applied.

(5) Scale and Offset

Scale and offset parameters, as described above. These should be left to their default values unless required to calibrate tracks from different devices.

Bottom Composition in Map Projects

Bottom composition is shown as a vector layer on top of the depth map, and is available in both the 2D and 3D views.

If bottom composition information is present in at least one of the map project's component tracks then the Show Bottom Type button (1) is enabled. Clicking this button will cause the bottom composition vector overlay to be generated, using the currently selected layer. Once the bottom composition layer has been generated, this button can be used to toggle the visibility of the layer without having to regenerate it.

The Bottom Classification section (2) of the project properties window contains further options for generating and displaying the bottom composition layers.

(1) Layer and palette selectors

Select the active layer using the layer selector.

Projects have an additional layer, Composite, which is an average of the normalised values of all of the layers present.

(2) Smoothing

The smoothing parameter relates to the amount of smoothing carried on the grid of calculated bottom composition values, before the contouring process is carried out. Higher values give smoother looking contours, at the loss of some detail.

(3) Number of contours

The number of contour levels to generate. Higher values equate to a smaller contour interval and will give a larger number of hardness areas in the finished map.

(4) Hardness area display range

Once the hardness areas have been generated, the displayed area ranges can be adjusted using the Display lower and Display upper sliders.

Higher value hardness areas shown on left, with lower values on the right

Adjust the sliders so that just the required areas are shown; this could be higher areas, as shown on the left in the image above, or lower areas, or any contiguous subset of areas within the range. Showing just the areas on interest is very useful when exporting maps; for example, it may be that only harder or rougher areas are of interest, so the resulting map will be much less cluttered if only those areas are exported.

(5) Opacity

Adjust the opacity slider to vary the transparency of the bottom composition layer.

(6) Range

Range can be adjusted and applied using the Range controls, as described above. Adjusting range in map projects can be very useful to increase the level of discrimination within a smaller range of bottom composition values.

(7) Regenerate

The bottom composition must be regenerated when parameters are changed. The Regenerate button will become enabled when changes have been made to parameters which affect the finished layer.