TerraScan User Guide

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Navigation: Tool Reference > TerraScan Window > Tools pulldown menu

Compute distance

Compute distance command computes a distance value for points. The distance can be computed based on several references:

•separate reference point cloud to perform detect changes.

•3D vector elements in the CAD file, such as planar shapes, linear elements or surface elements.

•the closest overlapping line in a point cloud.

•the average between overlapping lines.

•ground class(es) or other class(es).

•any surface model loaded in TerraModeler.

•powerline wires that have been produced with the Detect wires tool.

•range which is the distance to the scanner position at the moment of time when a point was captured. This requires trajectory information available in TerraScan.

•built-in vegetation indexes Normalized and Visual band differences. This requires true-color (Visual band difference) and additionally infra-red color information (Normalized difference) stored for the points. The distance value expresses how likely a point belongs to vegetation (larger positive value) or not (larger negative value). Distance values range between -1 and +1. The vegetation index distance values may support the ground classification of photogrammetric point clouds.

•Riegl Extra Byte attributes such as Echo length (= Pulse width) and Deviation (= Pulse shape deviation). The attribute values are translated into a distance value which indicates how likely a point belongs to a hard surface (ground) or not. The distance values range between -1 and +1, values between -1 and 0 are considered to represent ground. The higher the value, the less likely a point represents ground. The distance values may support the ground classification of LiDAR point clouds captured with Riegl scanners.

•curvature of a surface defined by local neighborhood of points. Computes the radius of the surface fit to the points within the neighborhood. If the neighborhood does not represent suitable curved surface, no value is extracted for the point.

•a longitudinal section line in order to detect road bumps and potholes. The trajectories of an MLS survey or any other road center line element may be drawn in the CAD file in order to be used as reference element. The laser data should be classified into hard surface and noise on asphalt up to maximum 1 cm above/below hard surface. The reference element provides the direction of the road. In the computation process, a line equation is fitted to a longitudinal slice of data with a given length and depth. Points above the fitted line get a positive distance value and represent bumps, points below the fitted line get a negative distance value and represent potholes. Use distance coloring with e.g. +-15 cm distance values in order to illustrate road damage.

•surface thickness difference between points inside polygons and neighborhood outside polygon in order to detect paint thickness. Paint thickness could measure thickness of paint layer on road markings. Paint thickness computation requires bounding polygons around objects of interest. Source class points within the defined margin from bounding polygons must be suitable for plane fitting to provide a reliable thickness result.

•object separation for identifying isolated large groups of points. Can be used for filtering noise in the air or below ground.

•to locally fit surface. Distance from the surface defined by local neighbourhood of points can be used for quality assessment. Roughness variations in an point cloud can show system errors, while different results between different sensors can measure system quality.

The distance value can be used for visualization of points and for further processing steps. The following processing commands rely on a distance computed from ground:

•Classify by distance / Classify groups by distance

•Assign groups and corresponding macro actions

•Classify groups by best match

The distance value can be stored in TerraScan FastBinary files. In addition, image files can be created from loaded points or project block files by using the distance attribute of points in order to display the information in other software products.

To compute distance values:

1. If you want to compute distance values for road bumps and potholes, select the vector elements that represent these road damages.

2. Select Compute distance command from the Tools pulldown menu.

This opens the Compute Distance dialog:

compute_distance_elements compute_distance_ground

The Distance setting determines which additional settings are available in the dialog.

2. Define settings and click OK.

This computes the distance values. An information dialog shows the number of effected points when the process is finished.

SETTING	EFFECT
Class	Point class(es) for which dimensions and normal vectors are computed.
	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 Class field.
Distance	Method for computing the distance: •Active vs reference - distance between a point and the closest point in reference points or Project information / Reference project, and vice versa. •To elements - distance between a point and vector elements drawn in the CAD file. •To closest line 3D - 3D distance between a point and its closest neighbour point of another line. The computation takes the normal vector direction of points into account. •To closest line dz - vertical distance between a point and its closest neighbour point in XY of another line. •To line average Z - vertical distance from a point to the average elevation between overlapping lines. The computation is based on TINs created from points of one line. •To ground class - vertical distance from a point to the ground class(es). A temporary TIN is created from points in the ground class(es) in order to compute the distance. •To ground and roof classes - vertical distance from a point to the ground and roof classes. A temporary TIN is created from points in the classes in order to compute the distance. In overlapping parts of the ground and roof classes, the TIN prioritizes the roof class. •Neighbour point count - number of points in the selected class(es) within the search radius. •To another class - 3D distance from a point to points in the given Other class. •To another line - 3D distance between a point and its closest neighbour point of another line. •To wires - distance from a point to the closest powerline wire. Line strings for wires must be produced with the Detect wires tool. •Range 3D - 3D distance from the scanner position to a point. •Range xy - horizontal distance from the scanner position to a point. •Range offset - left/right distance from the scanner position to a point. •Range forward - forward/backward distance from the scanner position to a point. •Range dz - vertical distance from the scanner position to a point. •Normalized difference - built-in vegetation index. •Visible band difference - built-in vegetation index. •Echo Length - Riegl Extra Byte attribute Pulse width. •Deviation - Riegl Extra Byte attribute Pulse shape deviation. •Curvature - curvature radius of the points inside local neighborhood. •Paint thickness - distance to local plane fit to neighbor points outside polygon margin. •Road bumps & potholes - distance to longitudinal section line. •Separation - jump required to form a point group of defined size. •Roughness - distance from locally fitted plane. •<TIN> - vertical distance from a point to a surface model loaded in TerraModeler.
Store into	The attribute field where to store the result: •Distance - store the distance to the TerraScan known distance attribute field. Designed for storing the distance from ground. •<Extra attribute> - store the distance to a defined Extra attribute.
Levels	Number(s) of CAD file level(s) on which the elements are drawn. Level ranges and several levels can be define with minus and comma, for example: 1-5,10,22. This is active only if Distance is set to Elements.
Max distance	Maximum distance of points from elements for which a distance value is computed. This is active only if Distance is set to Elements.
Planar shapes	If on, planar shapes are considered as reference elements. This is active only if Distance is set to Elements.
Tolerance	Defines a stroking tolerance for curved element types. This determines what shapes are considered as planar. This is active only if Distance is set to Elements and Planar shapes is switched on.
Above is	Defines which direction is considered as positive distance from planar shapes: •Upwards - points above a shape. •Clockwise side - points in clockwise direction of a shape. This is active only if Distance is set to Elements and Planar shapes is switched on.
Linear elements	If on, linear elements are used for computing a distance. This is active only if Distance is set to Elements.
Surface elements	If on, surface elements are used for computing a distance. This is active only if Distance is set to Elements.
Max distance	Maximum distance between overlapping lines for which a distance value is computed. This is active only if Distance is set to Closest line 3D, Closest line dz, or Another line.
Ground	Point class(es) used as reference ground level. This is active only if Distance is set to Ground class or Ground and roof classes.
Roof	Point class(es) used as reference roof level. This is active only if Distance is set to Ground and roof classes.
Search type	Whether to consider 2D or 3D radius around a point to define neighbour points. This is active only if Distance is set to Neighbour point count.
Search radius	Search radius around a point for counting neighbour points. This is active only if Distance is set to Neighbour point count, or Another line. Points outside search radius are remain without distance value when computing distance To another line.
Other class	Point class(es) used as reference class(es). This is active only if Distance is set to Another class.
Max distance	Maximum distance between a point and reference points in another class. The distance value is not stored for a point if it is larger than the given maximum value. This is active only if Distance is set to Another class or Another line.
Skip single line xy locations	If on, distance to the other line is calculated only if several points on the other line appear within the Search radius. Helps to reduce the effect of noise points. This is active only if Distance is set to Another line.
Wire levels	Number(s) of CAD file level(s) on which powerline wires are drawn. Several level number can be separated by comma, a range of level numbers can be defined by a minus sign, for example 10,14,22 or 3-5. This is active only if Distance is set to Wires.
Find using	Distance definition: •Vertical distance to wire - vertical distance from a wire. •3D distance to wire - 3D distance from a wire. •Falling tree logic - each point is considered as the tip of a tree with its trunk at the xy location of the point and the elevation of the base point on the ground. The shortest distance of the point “falling like a tree” to a wire is computed. This is active only if Distance is set to Wires.
Within offset	Maximum horizontal distance from a wire within which a distance value is computed. Basically, this value defines the corridor left and right of the wires for computing distance values. This is active only if Distance is set to Wires.
Ground	Point class(es) used as ground level. This is active only if Distance is set to Wires and Find using to Fallen tree logic.
Limit	Threshold value that separates vegetation from non-vegetation by using a vegetation index. This is active only if Distance is set to Normalized difference or Visual band difference.
Neutral length	Echo length/Deviation value that is translated into a distance value of 0. This is active only if Distance is set to Echo length or Deviation.
Spread	Echo length/Deviation value that is translated into the minimum and maximum distance value of ±1. This is active only if Distance is set to Echo length or Deviation.
Max offset	Maximum horizontal distance of a point from the longitudinal section line. Only point within the given offset are considered in the distance computation. This is active only if Distance is set to Road bumps & potholes.
Fit length	Length of a longitudinal slice of data used for fitting a line equation for each point. Together with the Fit depth value it determines, how big damages can be detected. This should be bigger than the largest bump/pothole to find. If the value is too small, bigger damages may be missed. If the value is too big, smaller details of damages may be missed. This is active only if Distance is set to Road bumps & potholes.
Fit depth	Depth of a longitudinal slice of data used for fitting a line equation for each point. Together with the Fit length value it determines, how big damages can be detected. This is active only if Distance is set to Road bumps & potholes.
Polygon levels	Number(s) of CAD file level(s) on which paint marking polygons are drawn. Several level number can be separated by comma, a range of level numbers can be defined by a minus sign, for example 10,14,22 or 3-5. This is active only if Distance is set to Paint thickness.
Surface	Type of surface equation to fit to points outside Polygon margin of paint marking polygon and within Max distance radius from the point. This is active only if Distance is set to Paint thickness.
Polygon margin	Additional margin outside polygon to filter paint points from plane fitting in case marking polygon is slightly misplaced. This is active only if Distance is set to Paint thickness.
Max distance	Maximum radius around point for selecting points used to extract the local plane. This is active only if Distance is set to Paint thickness.
Valid count	Required point group size. This is active only if Distance is set to Separation.
Min separation	Minimum separation value to store. Groups requiring smaller value are skipped. This is active only if Distance is set to Separation.
Max separation	Maximum separation value to store. Groups requiring bigger value are assigned this maximum value. This is active only if Distance is set to Separation.
Set sign by relative elevation	If on, point groups below ground are assigned negative distance values, and groups above ground positive values. The ground extraction is done in simple and robust manner, and included in the distance computation.
Search radius	Radius for selecting points for local surface extraction. This is active only if Distance is set to Roughness.
Time tolerance	Time interval for selecting points for surface extraction. Can be applied to avoid fitting surface to points from different passes during data collection. This is active only if Distance is set to Roughness.

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TerraScan User Guide