TerraScan User Guide

Extract Echoes

Not Lite, Not UAV

Extract Echoes tool extracts additional points from a return signal.

When scanner system software is generating laser points, it follows a certain logic. It may generate a point from the strongest, first, or last return but usually, it extracts one point from a multiple-return signal. In general, the extraction method of system software works well for laser point clouds.

However, in some places, the generated points might not be optimal. Examples are missing returns from wires or from ground below dense vegetation. In both cases, the system software might extract a point from the return signal, but possibly not the point of biggest interest for certain applications. The Extract Echoes tool can be used at such places in order to extract additional points from return signals.

There are several extraction methods available:

•First possible - looks only at rising start of the return signal.

•Last possible - looks only at trailing end of the return signal. This should be used, for example, to extract additional ground points.

•All distinct - constant fraction discriminator, more reliable result than All possible method.

•All possible - Gaussian decomposition, can generate multiple points from overlapping signals. This should be used, for example, to extract additional points on wires.

The extraction of additional points should only be performed in limited areas where the extraction method of the system software did not provide optimal results. The following methods can be used to limit the processing area for point extraction:

•Place a fence in a section view in order to specify a 3D slice of space where to generate new points. The section depth defines the XY area and the fence the elevation range for point extraction.

•Draw a fence or select polygon(s) to specify a 2D area where to generate points. In this case, the new points can be located at any elevation, only the XY area is defined.

The process creates new points only if it finds returns in the waveform that match the settings for the extraction. For example, if there is no part of the laser beam that penetrated to the ground because of dense vegetation, the Last possible method will probably not generate a point on the ground level.

The extraction of additional points requires that trajectories are active and laser points are loaded in TerraScan. The points must be linked to the trajectories and the trajectories must reference the waveform files and the scanner systems. The scanner waveform profile must be available and linked to the scanner system. See Waveform processing principles and Workflow summary for more information.

To extract additional points:

1. Draw a fence or polygon(s) around the area(s) for which you want to extract points as described above. Select the polygon(s).

2. Select Extract Echoes tool.

The Extract Echoes dialog opens:

3. Define settings and click OK.

This generates new points if the software finds return signals in the waveforms that match the settings. The points are added to the laser points in TerraScan memory.

4. Use Save points as commands in order to save the laser points into a file.

The new points are created as inactive points. You must save the points with setting Points set to All points in the Save points dialog. Otherwise, the additional points extracted by Extract Echoes tool are not stored.