Requires an LP360 Geospatial Advanced, or a BIT add-on with LP360 Drone license.

About Point Group Tracing and Squaring

The Point Group Tracing and Squaring point cloud task is used to extract traced outlines around points of a particular classification value (e.g., buildings classification), and optionally the traced outlines can be squared producing an approximation of objects such as the roof outlines of buildings.

Extracted outlines are saved to shapefiles with attributes that describe the traced objects. Attributes captured for each object include Area, and if squaring the polygons fit metrics (i.e., RmsError, MaxErr, ForceFit). If squaring is performed, two shapefiles will be generated; one for the traced outlines, and other containing a "_sqr" suffix representing the squared outlines.

The Point Group Tracing and Squaring point cloud task is both an extractor and a filter. The filtering capabilities are limited to clean-up duties as described by the various reclassification options on the property page. When using any of the reclassifying options the point cloud data must be opened for read/write when using the tools on the LP360 Point Cloud Tasks Tab.

The Point Group Tracing and Squaring task assumes that points exist in the point cloud data on the selected boundary trace class, such as tracing outlines for buildings requires points classified on the building classification. If you do not already have points on your boundary trace class such as for the buildings example, use the Planar Point Filter to classify the points before executing the Point Group Tracing and Squaring extractor. These two point cloud tasks (i.e., Planar Point Filter, and Point Group Tracing and Squaring) can be combined into a one-step process by creating a macro and including a Planar Point Filter as the first task and then the Point Group Tracing and Squaring as the second task.

How to use the Point Group Tracing and Squaring Point Cloud Task

1. Use the Add Files tool to open the LAS files that contain the data you want to use.  
2. Use the Add button in the Point Cloud Task Manager to create a new point cloud task.
   • Task Type: Point Group Tracing and Squaring. 
3. Specify the settings for the Point Group Tracing and Squaring operation in the Properties page that displays.

Setting Options

Boundary Trace Class

This is the class of points that will be traced by the function.  Currently you can trace only a single class (we intend to make this a multi-select option some time in 2013.  It requires not only a dialog change but quite a bit of changes to the underlying algorithms).

Note that the class that you intend to trace must have "clustering" properties.  For example, attempting to trace the Ground class might simply yield a polygon around the entire data set.

Units

This should normally be set to the same units as the data in the map view.  As with all PCT's, this parameter is present to allow you to create unit independent parameters.  For example, if you carefully craft parameters in a metric map (units of meters), then setting this value to "meters" will cause the parameters to be automatically translated to the proper values if you apply the filter to a data set with units of feet.

Grow Window

The parameter controls point clustering.  A point (in all discussions we assume that when we say a point we mean a point that is of the Boundary Trace Class) is selected.  If at least one other point is located within a window, centered on this point and of size 2*(Grow Window) by 2*(Grow Window), the window is re-centered on the new point(s) and the process is repeated.  This algorithm continues until either no new point is located, or the Maximum Grow Area parameter is exceeded. 

It should be obvious that if the Grow Window parameter is less than half the average distance between points in the Boundary Trace Class, no points will be collected into a cluster.  On the other hand, if the Grow Window parameter is a few times the point spacing in the Boundary Trace Class or larger, clusters will begin to be grouped together.  Thus set this parameter to a minimum of about the point spacing of the Boundary Trace Class.   

If you need a starting point, use the dropper tool (located beside the Grow Window input box) to trace a polygon over the Map View in a region containing points of the Boundary Trace Class.  This will provide initial values for both the Grow Window and Trace Window parameters.

Note that using a value for the Grow Window that is significantly larger than the Boundary Trace Class point spacing will result in very slow running of the algorithm.  

Trace Window

This parameter sets the minimum distance between vertices of the polygon created around the clusters (recall that clusters are determined by the Grow Window parameter).  Thus it is a smoothness parameter for the traced outlines.  Note that if it is made smaller than the distance between points, no outlines will be drawn.  A good starting value is twice the Grow Window size (this will be the default value if you use the eye dropper tool).  If you set this parameter to less than twice the Grow Window size, you may notice that clusters become truncated.

Minimum Area

This parameter determines the minimum size of cluster that will be traced, in square units of the Units setting (e.g. if you set this parameter to 50 and the Units are set to feet, clusters smaller than 50² will not be traced).

Reclassify Min Points, Should Undo Minimum Area, Min Class

This, in theory, is supposed to classify points that do not meet a cluster criterion into a user selectable class.

Classify Interior Points

Setting this option will cause all points within a traced boundary to be reclassified to the Boundary Trace Class.  This can be useful when performing an operation such as tracing building roofs.  Note, however, that it currently does not allow a vertical distance parameter and hence will classify points that are vertically 'far' from the Boundary Trace Class.  An example of this problem is vegetation overhanging a set of roof (building) points.  Similarly, classifying the interior of vegetation polygons will cause all ground points within the polygons to be reclassified to the Boundary Trace Class so be sure of the residual effects of this option.  I recommend that you always use a copy of your LAS data if you are going to enable this option.

Perform Squaring

This option will cause the PCT to generate a second set of polygons that represent a squaring of the trace polygons.  The Squaring Angle (Deg) setting determines the detail of these squared polygons.  The algorithm functions by fitting lines with corners restricted to 90° to the trace polygon.  If, at a particular point on the original trace polygon, the angle between the polygon and the squared polygon is more than

Squaring Angle, a new corner is created in the squared polygon.  Thus the larger the Squaring Angle, the fewer corners and less detail in the fitted, squared polygons.  NOTE:  There is currently a defect in our algorithm for rotating the squared polygon relative to the detailed polygon when the Squaring Angle is large (say greater that 50°.  This can result in the squared footprints being skewed relative to the detail model.  This defect is on our list for correction.  

Dissolve Overlapping Polygons

This is effectively the detailed (as opposed to Squared) polygon option since it is a radio button control.  It generates detailed polygons.  

Maximum Grow Window Area

This parameter is in affect only when creating detailed polygons (by the Dissolve Overlapping Polygons option).  It limits the maximum cluster size when grouping points prior to creating the polygons.  In practice, it is only useful in very specialized circumstances.  Making this value small can cause excessive run times with no noticeable change in the output.  We intend, in a future release, to remove this parameter.  Always set this parameter to 5,000.

Setting the output file

When you run this PCT, you will be prompted for an output shape file.  This file will contain the vector results of the operation.  If you choose Perform Squaring, two vectors files will be output.  The detailed outline will be contained in the file whose name you specify.  The squared outlines will be contained in a file with the same base name but post-fixed with a "_sqr" in the name.  Note that if you run the command multiple times, you can choose the same file.  You will be asked if you wish to append to the existing file.  If you choose Yes, the results will be added to the current results.  If you choose No, the file will be rewritten and contain only the latest results.  If you have already inserted the file into your Table of Contents, the display will be automatically updated. 

Learn how to select a boundary trace class

Learn how to modify the units

Learn how to modify the grow window

Learn how to modify the trace window

Learn how to modify the minimum area

Learn how to classify point groups that are too small

Learn how classify points within small areas

Learn how select a classification for minimum area points

Learn how to classify points within the interior of outlines

Learn how to square the outlines

Learn how to modify the squaring angle

Learn how to dissolve overlapping polygons (e.g. vegetation outlines)

Learn how to modify the maximum patch area