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han16nah edited this page Jun 4, 2021 · 3 revisions

Platform definition

HELIOS++ supports static and dynamic as well as terrestrial and airborne platforms. All platforms are defined in data/platforms.xml. For an overview of all possible tags and parameters in this XML, go to XML Tag and Parameter Summary at the bottom of the page.

The platform is assigned in the <survey> tag of the survey.xml together with the survey name, the scene, the scanner and optionally a seed. Referencing the different components is done by specifying the absolute or relative path of the XML file, followed by a hash (#) and the ID of the entry.

<survey name="toyblocks_als" 
        platform="data/platforms.xml#sr22"
        scanner="data/scanners_als.xml#riegl_vq-880g"
        scene="data/scenes/toyblocks/toyblocks_scene.xml#toyblocks_scene">

For each scan position or way point, a <leg> is defined in the survey.xml. Within this tag, platform position is provided using the tag <platformSettings>.

Static platforms

Terrestrial laser scanning (TLS) simulations are conducted from the static platform tripod. Per default, it is mounted at a height of 1.5 m. Each leg is considered a single scan position. Defining the horizontal and vertical field of view is explained at Scanner Settings.

Dynamic platforms

There are three types of platforms available.

  • linearpath: This platform type simply moves on a straight line from one waypoint (leg) to the next with constant speed (no speed-ups or slow-downs).
  • groundvehicle: This platform type performs different turns at the waypoints depending on the curve angles. For sharp turns with turn angles larger than 36°, the vehicle performs a two-point turn (1). For wide angle-turns, it performs a curved turn (2). groundvehicle_trajectory Figure: Trajectory output for a groundvehicle survey, coloured by GpsTime.

IMPORTANT NOTE: There are several bugs with the multicopter platform, like legs not ending when using slowdownEnabled (#22, #37) or the user-defined speed (movePerSec_m) not being used. We advise to use the linearpath platform instead. In the platforms.xml, this could look like that:

	<platform id="copter_linearpath" name="Quadrocopter UAV" type="linearpath">
		<scannerMount x="0" y="0" z="0.2">
			<rot axis="x" angle_deg="180" />
			<rot axis="z" angle_deg="175" />
		</scannerMount>
	</platform>

This platform would be compatible with e.g., the riegl_vux-1uav. Using the rotations in the scannerMount, the copter pitch during forward movement is simulated.

  • multicopter: This platform type simulates the behaviour of a multicopter by tilting into the movement direction and tilting backwards for slow-down. By default, the copter approaches the waypoint in a straight line, stops both movement and scanning, turns towards the next waypoint and then accelerates and starts scanning again (stopAndTurn). If the parameter smoothTurn is set to "true" instead, the copter turns smoothly at the waypoints while moving and scanning (so-called bank turns). Optionally, yawAtDeparture can be set explicitly if the copter is supposed to face in another direction than the next waypoint. stopAndTurn_c
    Figure: Trajectory output for a multicopter survey in stopAndTurn mode, colored by the aircraft principal axis. The flight starts on the bottom right and ends at the top left.
    smoothTurn_c
    Figure: Trajectory output for a multicopter survey in smoothTurn mode, colored by the aircraft principal axis. The flight starts on the bottom right and ends at the top left.

The multicopter has multiple parameters to control slowdown and speedup as well as roll and pitch angles. For an overview, go to XML Tag and Parameter Summary.

Mobile laser scanning (MLS) is simulated using a dynamic terrestrial platform, such as a car or tractor. Two tractor and two car platforms are currently implemented with different scanner mounts using either the linearpath or the groundvehicle platform type.

Note that for terrestrial (static or dynamic) surveys, the onGround parameter comes in handy which is set in the <platformSettings> tag in the survey.xml. It forces the platform to be placed at the lowest scene point (i.e. the ground) and any specified z-value will be ignored.

For dynamic simulations, at least two legs have to be defined, corresponding to the start and stop way point, and the moveperSec_m parameter controls the speed of the platform moving between the waypoints.

Airborne laser scanning (ALS) simulations are performed from an airplane (with platform type linearpath) or multicopter. Currently, one aircraft for conventional ALS surveys and a quadcopter for UAV-borne surveys are implemented.

Custom platforms

Users can extend the platform.xml with custom platforms. For this, the id, name, and type have to be specified. Optionally, a value for the drag may be given.

<platform id="quadcopter" name="Quadrocopter UAV" type="multicopter" drag="0.01">

How the scanner is mounted relative to the platform is defined in the scannerMount-tag. For example, the following XML snippet creates a tripod platform with the scanner mounted at 2 m (the height of the default tripod is 1.5 m):

<platform id="tripod_2m" name="TLS Tripod (2 m)" type="static" drag="0">
	<scannerMount x="0" y="0" z="2.0">
	</scannerMount>
</platform>

Rotations

Additionally, rotations can be defined, for instance to specify that the scanner is facing down and scanning orthogonally to the platform orientation (as is the case for ALS):

<platform id="sr22" name="Cirrus SR-22" type="linearpath">
    <scannerMount z="0.7" rotation="global">
        <rot axis="Z" angle_deg="-90" /> <!-- scan plane: left-to-right -->
        <rot axis="Y" angle_deg="90" /> <!-- main direction: downwards -->
    </scannerMount>
</platform>

The scanner's coordinate system is defined as R-F-U, i.e. the first axis (X) is pointing to the right wingtip, the second axis (Y) is pointing towards the front of the platform and the third axis (Z) is pointing upwards, i.e. to zenith. For rotating mirror, fibre array and oscillating scanners, the scan plane is created by the second and the third axis, and the main scan direction is the Y-Axis (forward). Without rotation, this results in a scan pattern normal to the horizon: image

Note that there are two modes of rotation, as defined in the scannerMount-Tag: rotations="global", where the axes of rotation are kept fixed in space, and rotations="local", where the axes are rotated with the scanner. The difference is prominent when completing turns about multiple axes, and is shown in the following example. If no mode is provided, global mode assumed.

The task here is to create a scanner that looks backwards, 20° off-nadir.

Rotations Global mode Local mode
None image image
Y-axis: 90° image image
Y-axis: 90°, X-axis: -110° image image
Y-axis: 90°, Z-axis: -110° image image

The orange triangle shows the scan plane. Note, how the axes in the local mode change their direction with respect to the platform after every rotation. For the global mode, the following definition gives the desired result:

<scannerMount x="0.0" y="0.0" z="0.0" rotations="global">		
    <rot axis="y" angle_deg="90" />	
    <rot axis="x" angle_deg="-110" />	
</scannerMount>

and for the local mode, this definition is correct:

<scannerMount x="0.0" y="0.0" z="0.0" rotations="local">		
    <rot axis="y" angle_deg="90" />	
    <rot axis="z" angle_deg="-110" />	
</scannerMount>

Note that an additional rotation is applied in the scanner's beamOrigin tag, which uses the same syntax. We suggest to use the beamOrigin rotation to point the scanner downwards and scan left-to-right (as this is the most common usecase) and the scannerMount (which is applied thereafter) for specialized mounts such as 23° off-nadir for airborne bathymetry (although this is usually solved by using a conical scan pattern with an opening angle of 46°).

XML Tag and Parameter Summary

<platform [...]> tag

Attribute Default value Comment
id REQUIRED
name
type "static" possible values: "multicopter", "linearpath", "groundvehicle";
any other entries will use the default static platform
drag
engine_max_force 0.1 specific to multicopter
speedup_magnitude 2.0 specific to multicopter
slowdown_magnitude 2.0 specific to multicopter
slowdown_distance 5.0 specific to multicopter
base_pitch_deg -5.0 specific to multicopter
roll_speed_deg 28.65 specific to multicopter
pitch_speed_deg 85.94 specific to multicopter
yaw_speed_deg 85.94 specific to multicopter
roll_offset_deg 25.0 specific to multicopter
pitch_offset_deg 35.0 specific to multicopter

<scannerMount [...]> tag within <platform> tag

Attribute Default value Comment
x 0
y 0
z 0
rotations "global" possible values: "global", "local"

<rot [...]> tag within <scannerMount> tag

Attribute Default value Comment
angle_deg
axis possible values: "x", "y", "z" (upper or lower case)

Platform noise tags within the <platform> tag

  • <positionXNoise [...]>
  • <positionYNoise [...]>
  • <positionZNoise [...]>
  • <attitudeXNoise [...]>
  • <attitudeYNoise [...]>
  • <attitudeZNoise [...]>

Parameters within the noise tags

Attribute Default value Comment
type "NORMAL" possible values: "NORMAL", "UNIFORM"
mean 0.0 specific to NORMAL distribution
stdev 1.0 specific to NORMAL distribution
min 0.0 specific to UNIFORM distribution
max 1.0 specific to UNIFORM distribution
clipEnabled "false"
clipMin 0.0
clipMax 1.0
fixedLifespan 1 see Error sources and randomness control

References

Airplane model CC-BY Emmanuel Baranger

Overview

Main page

First steps and examples

Survey

Scene

Platforms

Scanners

Fullwave

Output

Error sources and randomness control

Python bindings 🐍 Getting started

Python bindings 🐍 Details

pyhelios 🐍 The util subpackage

Dealing with large scenes

Visualising the simulation

Tutorial videos

References

Research using HELIOS or HELIOS++

Change log

Supplementary tools for scene generation

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