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Introduction

This command will generate classificated points for the selected chart(s).
Input to the classification algorithms are the grids called Seafloor that are located under each "chart name" folder.reduce the amount of points from one or more input data sets. The selection method can for instance be priority of the Minimum or Maximum depth values. The selection area can be by looking at points inside a Rectangle or Circle. The selected points are always with the original position and not snapped into any grid structure. 

The output will be a new point data set which in Geocap will have a schema of type Pointcloud.

The command is a folder command and is implemented for folders with schema types Charts, Multibeam, Soundings and Generic. 

         
The Statistical Points Reduction dialogs

The content of the menu

The classification options are: Shoals, Deeps, Saddles, Ridges and Valleys

The points ar defined as:

• Shoal point: The shallowest point (node or sounding) in an area. All surrounding nodes are deeper.
• Deep point: The deepest point (node or sounding) in an area. All surrounding nodes are shallower.
• Saddle point: The shallowest point (node) in a fairway. The surrounding nodes are deeper on two sides and shallower on the two perpendicular sides.
• Ridge point: A break point (node) on a ridge. The surrounding nodes are deeper on three sides and shallower on the fourth side.
• Valley point: A break point (node) in a valley. The surrounding nodes are shallower on three sides and deeper on the fourth side.

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Classification points

The result is a set of points that tells where the selected classification points are located. The classification points are saved under the <chart name> folder and a classification folder with the name of the option and the parameters selected.

The parameters are set in the four tabs: Shoals/Deeps, Saddles, Ridges and Valleys

When the correct parameters for the selected options  are set, click Execute to perform the points classification.

The Shoals/Deeps tab

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Specify settings for shoal objects

Specify how to find those grids nodes that are the shallowest within the Minimum distance. The parameters are:

• Mask width (grid cells): Is a mask moving over the grid and checking the centre node with the nodes around. Default value is 2.
• Minimum distance (m): A shoal is detected when the centre node is shallower than the rest of the nodes in the mask. After the detection the points are sorted on increasing depth values, and by starting with the first (and shallowest) point, all points within a distance smaller than the Minimum distance will be removed. Then the same test is performed for the next valid point, etc. Default value is 25.

The shoals are saved as <chart name> / classification / shoals_<mask width>_<distance>.

Specify settings for deep objects

Specify how to find those grids nodes that are the deepest within the Minimum distance. The parameters are:

• Mask width (grid cells): Is a mask moving over the grid and checking the centre node with the nodes around. Default value is 2.
• Minimum distance (m): A deep is detected when the centre node is deeper than the rest of the nodes in the mask. After the detection the points are sorted on decreasing depth values, and by starting with the first (and deepest) point, all points within a distance smaller than the Minimum distance will be removed. Then the same test is performed for the next valid point, etc. Default value is 25.

The deeps are saved as <chart name> / classification / deeps_<mask width>_<distance>.

Find corresponding Shoals/Deeps in Soundings

This option enables a search in the Soundings dataset for the nearest point that was found in the Seafloor grid. The corresponding points are saved as <chart name> / classification / <type>_in_soundings_<mask width>_<distance>.

The Saddles tab

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Spscify how to find those grids nodes that have two low and two high neighbour points. The parameters are:

• Mask width (grid cells): A filter moving over the grid and checking the centre node with the nodes around. Default value is 2. 
• Minimum depth difference (m): The minimum depth difference between the actual node and the neighboring topography. Default value is 0.1.

The saddle points are found by searching the grid nodes along the x and y axis, and then the diagonals within the mask. The criteria for a saddle point is that along one of the directions, the neighboring terrain should be shallower in both directions, while in the perpendicular axis the terrain should be deeper in both directions.

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The search directions for saddles, ridges and valleys for mask width 2 (5x5).

After the detection the points are sorted on increasing depth values, and by starting with the first (and shallowest) point, all points within a distance smaller than the Mask width will be removed. All the remaining points ar checked the same way.

The result dataset is saved as <chart name> / classification / saddles_<mask width>.

The Ridges tab

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Will find those grids nodes that define a ridge. The parameters are:

• Mask width (grid cells): A filter moving over the grid and checking the centre node with the nodes around. Default value is 2.
• Slope angle towards shallower (deg): The vertical angle towards shallower terrain. This angle should not be too steep for a ridge point. Default value is 2.
• Slope angle towards deeper (deg): The vertical angle towards shallower terrain. This angle should be steeper than the upward angle. Default value is 5.
• Maximum contour angle (deg): The angle of a contour line passing through the selected point. A smaller angle gives a sharper feature. Default value is 120.

The ridge points are found by searching the grid nodes along the x and y axis, and then the diagonals within the mask. The criteria for a ridge point is that along one of the directions the neighboring terrain should be shallower in one direction and deeper in the other, while in the perpendicular axis the terrain should be deeper in both directions.

The slope angles towards shallower and deeper terrain are the test criteria for defining the the limit for the slope.

When a candidate is detected, the next check is the contour angle between the grid node and two interpolated points with the same depth value. If the sharpness of the angle is smaller than the value in the parameter, the node is accepted as a ridge point. 

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The contour angle for ridge and valley points.

After the detection the points are sorted on increasing depth values, and by starting with the first (and shallowest) point, all points within a distance smaller than the Mask width will be removed. All the remaining points ar checked the same way.

The result is saved as <chart name> / classification / ridges_'mask width'_'shallower angle'_'deeper angle'_'contour angle'. 

To see all ridge points set Slope angle towards shallower = 1, Slope angle towards deeper = 1 and Maximum contour angle = 180. 

Test and tune the parameters to get the proper ridge points.

The Valleys tab

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Will find those grids nodes that define a valley (which is the opposite of a ridge). The parameters are: 

• Mask width (grid cells): A filter moving over the grid and checking the centre node with the nodes around. Default value is 2.
• Slope angle towards shallower (deg): The vertical angle towards shallower terrain. This angle should be steeper than the downward angle for a valley point. Default value is 5.
• Slope angle towards deeper (deg): The vertical angle towards shallower terrain. This angle should be not be too steep for a valley point. Default value is 2.
• Maximum contour angle (deg): The angle of a contour line passing through the selected point. A smaller angle gives a sharper feature. Default value is 120.

The valley points are found by searching the grid nodes along the x and y axis, and then the diagonals within the mask. The criteria for a valley point is that along one of the directions the neighboring terrain should be deeper in one direction and shallower in the other, while in the perpendicular axis the terrain should be shallower in both directions.

The slope angles towards shallower and deeper terrain are the test criteria for defining the the limit for the slope.

When a candidate is detected, the next check is the contour angle between the grid node and two interpolated points with the same depth value. If the sharpness of the angle is smaller than the value in the parameter, the node is accepted as a valley point. 

After the detection the points are sorted on increasing depth values, and by starting with the first (and shallowest) point, all points within a distance smaller than the Mask width will be removed. All the remaining points ar checked the same way.

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Test and tune the parameters to get the proper valley points.

The Results

After executing the point classification upper part of the menu contains the project name and the name of the active folder.

The Selection Area part

Set parameters for the geometry of the area to select points within.

• Rectangle: Divide the area into cells and analyse the points which ara inside the cell. Normally the cell size will be a square.

• Circle: First collect all points into cells with half the size as the circle radius. Then sort on increasing or decreasing values and then start removing points within the given radius. 

The Rectangle definition part

Set parameters for the rectangle to select points within.

The Width (X size) (m) and the Height (Y size) (m) are the definitions of the selection cell size.
Use Height=Width means that the value of the Height should be the same as the Width.

The Circle definition part

Set parameters for the circle to select points within.

The Radius (m) is the definition of the selection circle size.

The Point selection method part:

Set parameters for the method for selecting the points. Several options can be selected for each run, but be aware of possible memory capacity problems.

• Minimum means the shallowest (smallest) value inside the cell or circle.
• Maximum means the deepest (largest) value inside the cell or circle.

The next options are implemented for the Rectangle method only:

• Shallower than Mean means the first point shallower (smaller) than the mean value in the cell.
• Deeper than Mean means select the first point deeper (larger) than the mean value in the cell.
• Closest to Mean means the point with depth value closest to the mean values of the points in the cell.

• Minimum number of points in cell: If the cell has less points than the minimum number, the cell will not contribute with a point to the result data set.

The Dataset part or Survey Line part

Select in the list which datasets to do the point reduction from. The area for the selection is taken from the extension of the input files. The corner points are justified to fit the selected increments for the rectangles.

The Chart Name part

Select in the list for which charts to do the point reduction from. The area for the section is taken from the extension of the input file, and the input data is the Soundings data set. The corner points are justified to fit the selected increments for the rectangles.

The Results

After executing the statistical point reduction a dialog will pop up showing the result from the classificationsselection. The report itself is saved in the project structure in the folder named named Reports  located located in the same folder as the terrain surfacecommand. The report itself is itself is named as YYYYMMDD Points Classification as YYYYMMDD Statistical Point Reduction (the name of the executed command).
All reports from all runs will be saved in the Reports folder.the Reports folder.

The resulting points file will be a combination of the inut file name, the selection area and selection parameters.

For the Rectangle method the file name will be: 'filename'_'selectionType'_'width'_'height'_'minimumPoints'.
For the Circle method the file name will be: 'filename'_'selectionType'_Radius_'radius'.