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TerraScan User Guide

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Cut overlap

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Cut overlap

Not Lite

Cut overlap command removes laser points from locations where laser data from multiple lines overlap. The points of overlapping areas can be classified to a specific class or classes, or they can be deleted from the data set. In LAS 1.4 files, the overlap bit can be set instead of classifying or deleting points.

There are several methods for cutting off overlap. If you want to apply several methods to a data set, run the one method after another in separate processing steps in order to avoid incorrect results.

General procedure to cut overlap:

1. Select Cut overlap command from the Line pulldown menu.

This opens the Cut Overlap Points dialog:

cut_overlap

2. Define settings and click OK.

This classifies/removes points from overlapping areas.

SETTING

EFFECT

Coverage classes

List of point classes to consider when determining if a line covers an area. Use 0-255 to cover all classes.

Select

Opens the Select classes dialog which contains the list of active classes in TerraScan. You can select multiple source classes from the list that are then used in the Coverage classes field.

Action

Action to perform on points in overlapping areas:

Add constant to class - add a given value to the class number.

Classify to single class - classify points into one target class.

Delete - remove points from the data set.

Set overlap bit - sets the overlap bit for points in LAS 1.4 files.

Add

Value to add to class numbers if Action is set to Add constant to class.

To class

Target class if Action is set to Classify to single class.

Cut by quality

Cut by quality method removes lower quality laser points from locations where there is laser data from a higher quality line. The method is suited if:

there is laser data from two different flying altitudes. Data of a lower flight line is normally more accurate than data of a higher line.

the hardware was not working at the best level for some lines.

the GPS trajectory is weak for some lines.

there is laser data from single crossing lines while the majority of lines follows another direction.

You can define the quality of lines as an attribute of imported trajectories. See Manage Trajectories tool and Edit information command from the Trajectory window for more information. If you do not have imported trajectories, you can assign quality values to different line number sequences in the Default flightline qualities category of TerraScan Settings.

cut_by_quality_before

Before cut

cut_by_quality_after

After cut by quality

Point color: red points are from a lower quality line, blue points from a higher quality line

SETTING

EFFECT

Cut by quality

If on, cut by quality method is applied.

Hole size

Approximate maximum diameter of hole. If a larger area is not covered by points of a higher quality line, points from the lower quality line are kept.

Cut single scanner edges

Cut single scanner edges method is designed for removing overlap in data sets collect by airborne dual-scanner systems. It removes edges of lines that are captured by one scanner only.

SETTING

EFFECT

Cut single scanner edges

If on, cut single scanner edges method is applied.

Keep minimum

Minimum central part of a line and scanner to keep. A value of 20 would keep a +10.. -10 degree corridor of each line and scanner.

Cut by offset

Cut by offset method removes laser points from the edges of lines if the same location is covered more vertically from another line. This method serves two purposes:

Removing edges of lines produces a more uniform point density and point pattern.

The magnitude of error sources grows with increasing scan angle. Removing edges of lines removes less accurate points and keeps the more accurate central part of a line.

The method is suited for cutting off overlap between parallel lines in an airborne data set. It requires trajectory information and matching line numbers of points and trajectories. See Deduce using time command for more information about matching line numbers of points and trajectories easily. If time stamps are not available for laser points, the application uses a perpendicular projection from a laser point to the trajectory.

cut_by_offset_before

Before cut

cut_by_offset_after

After cut by offset

Point color: different colors represent points of different lines

SETTING

EFFECT

Cut by offset

If on, cut by offset method is applied.

Scan lines

Scan line pattern of the data set:

Perpendicular to flight - parallel or zigzag scan lines.

Elliptical - elliptical scan lines.

Keep minimum

Minimum central part of a line to keep. A value of 20 would keep a +10.. -10 degree corridor of each line.

Cut by scan angle

Cut by scan angle method removes points with a scan angle that is larger than a given value. It removes the edges of lines similar to the Cut by offset method but does not rely on trajectory information.

SETTING

EFFECT

Cut by scan angle

If on, cut by scan angle is applied.

Keep angles <=

Points with scan anlges smaller or equal to the given value are kept.

Cut by range

Cut by range method removes laser points from the edges of a line if the same location is covered by points from a shorter measurement distance. This method is designed for laser data from mobile ground-based systems.

The software searches for points inside a sphere and cuts off points resulting from long measurements if points from a shorter range within the search radius are present. The range can be defined as 3D range or offset range.

The method requires trajectory information and matching line numbers of points and trajectories. See Deduce using time command for more information about matching line numbers of points and trajectories easily.

cut_by_range_3d_before

Before cut

cut_by_range_3d_after

After cut by 3D range

cut_by_range_offset_after

After cut by offset range

Point color: different colors represent points of different lines

SETTING

EFFECT

Cut by range

If on, cut by range method is applied.

Use

Method to use for cut by range: 3D range or Offset range.

Search radius

Radius of a sphere within which the software searches for closer range points from another line. The radius can be automatically increased as a factor of the range. By increasing the radius at longer ranges, the method is more eager to keep points from more distant lines.

Keep range

Range from scanner within which all points are kept.

Cut by density

Cut by density method is developed for removing overlap in merged point clouds from different sensors. The merged point cloud is a result of several point clouds with significantly different point densities. Examples are point clouds from mobile and airborne laser scanners, or photogrammetric airborne and terrestrial laser point clouds. Some areas may be covered by both sensors but other areas just by one sensor. In overlap areas, the method keeps the data from the sensor with the higher point density and removes more sparse data from the other sensor.

The cut overlap method requires scanner numbers assigned to the points. The data from different sensors is identified by a unique scanner number.

SETTING

EFFECT

Cut by density

If on, cut by density method is applied.

Search radius

Determines the area in which the local density is computed.

Keep classes

Point class(es) from which all points are kept even if the density is lower than in data from another sensor. Examples: roofs from airborne laser data, hard surface areas from mobile laser data.

Scanner groups

If a range of scanner numbers is given, the corresponding sensors are treated as a group. For example, if there is data from two mobile laser scanners with numbers 1 and 2, and the scanners are calibrated, use 1-2 in order to define a group for these 2 scanners. Separate different groups by semicolon, for example 1-2;3-4.

If the field is empty, each sensor is treated individually.

TerraScan User Guide   26.07.2021   © 2021 Terrasolid Ltd