Point Group Tracing and Squaring Point Cloud Task – LP360

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

LP360 includes a Point Cloud Task (PCT) for vectorizing clusters of points of a particular class. A squaring option is included for constraining the vectors to straight lines with 90° vertex angles (for example, when tracing an object with known straight edges and 90° such as a building roof). The link below provides a detailed overview of using this tool. LP360 Tracing and Squaring User Guide.

Introduction

This PCT is used to vectorize clusters of classified points. A squaring option is included for constraining the vectors to straight lines with 90° vertex angles (for example, when tracing an object with known straight edges and 90° such as a building roof). An example of tracing High Vegetation is depicted in Figure 1.

Figure 1: Traced High Vegetation

For this vegetation example, the following settings were used:

Grow Window: 30
Trace Window: 60
Minimum Area: 300
Max Grow Area: 5000

Preparation

Before executing this task, the points to be vectorize must be classified into a distinct class. Note: multiple classes can be used for input.

Example: Buildings (class 6) ; Medium Vegetation (class 4) and High Vegetation (class 5); etc.

It is also recommended to have your ground classification prior to execution.

Options Setting Summary

For a general workflow for best performance using this tool, refer to the Building Filter and Extraction for dense datasets page.

A Default Building Extractor PCT is available for use in LP360 in the PCT Manager. Settings can be adjusted as needed (Perform Squaring enabled, etc.).

Figure 2: The Point Group Tracing and Squaring settings dialog

A quick summary of all settings for the Point Group Tracing and Squaring PCT is provided in Table 1. For additional details, review the Point Group Tracing and Squaring Property page.

Table 1: Quick Setting Guide

Option	Setting	Notes
Boundary Points	Class of points you wish to trace (e.g. High Vegetation)	Points must be classified prior to execution. Multiple classes can be selected for input.
Units	Should be the same as Map Units	May be adjusted for advanced users
Grow Window	Use the "eye dropper" tool to digitize a representative polygon over a data region in the map view.	This controls clustering. Larger values make courser clusters. If set to less than about twice the point spacing, no clusters will be created. If set to more than about 4X the nominal point spacing, you will note a significant decrease in performance.
Trace Window	Start with 2X the Grow Window size (automatically set when you use the dropper tool).	Controls the "jaggedness" of the polygon outline. The smaller the value, the more likely to cause clusters to not be fully enclosed.
Minimum Area	Set this to the minimum area that you want traced.	Filters out small regions from being polygonised.
Reclassify Min Points	Reclassifies any group of three or less boundary points to another class	Optional. This can be useful if the boundary class is noisy or sparse and you need to remove isolated groups. Only use for homogeneous structures (e.g. building roofs)
Should Undo Minimum Area	Reclassified points inside any polygon (or group) smaller than the Minimum Area Threshold	Optional. This can be useful to prevent small polygons from being created and clean-up the point cloud in certain workflows.
Min Class	Destination class for reclassified points for the Reclassify Min Points option.	Points in clusters smaller than the Minimum Area will be reclassified to this class if the Reclassify Min Points is enabled.
Classify Interior Points	Check if you want all points within a trace polygon to be reclassified to the Boundary Class.	Optional. You must set LAS files to Read/Write if this option is used. Since it classifies all points, it is only used with homogeneous structures (roofs), not with canopy or natural features.
Perform Squaring	Select this option if you want all polygon corners to be 90° (as in buildings)	Optional. This option is used to force orthogonal corners to the polygon for buildings or similar features. For natural features such as vegetation, this option should be disabled.
Squaring Angle (Deg)	Adjusts the detail of a squared outline. The smaller the value, the more detailed the outline.	45° is a good starting value
Dissolve Overlapping Polygons	Select this option if you want non-square outlines	This option does two things - creates detailed outlines and ensures that these detailed outlines do not overlap.
Maximum Grow Window Area	Maximum size of any single polygon. In general, this parameter should be kept large (5,000 units) unless there are issues with the results.	This will produce overlapping polygons if the boundary class region is larger than the maximum area, so these are dissolved into a single polygon as a final step in the tracing.

Troubleshooting

Nothing is traced

Grow Window is too small (use the eye-dropper tool to get a good initial approximation).
There are no points of the type selected in the Boundary Trace Class (test this by setting a display filter to the same parameters and observe the Map View).
The Trace Window is less than 2X the Grow Window (always start your parameter adjustment process with the Trace Window at least 2X the Grow Window. If you use the eye dropper tool, this parameter will be automatically set for you.
The Minimum Area parameter is set larger than the largest cluster in your data set.

Process runs extremely slowly

Grow Window is too large. As soon as the Grow Window exceeds about 4X the nominal point spacing of the Boundary Trace Class, you will see a significant decrease in performance without much change in clustering. Keep this parameter in the range of 2 to 4 times the nominal point spacing.
Reset Maximum Grow Window Area to 5,000 and do not adjust.
Dataset is very dense. Review the Building Filter and Extraction for dense datasets page for steps to address this.

Buildings have too many facets

Increase the Squaring Angle parameter (45° is a good starting point)

Building footprints are rotated

Decrease the Squaring Angle parameter
We have a known defect with properly orienting simple (e.g. 4 sided) outlines to the true orientation.

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