Notebook

ALS HD demo scene¶

Notebook creator: Sina Zumstein & Hannah Weiser, 2022

This demo scene uses a digital terrain model (DTM) of Heidelberg, Germany, which will be scanned by airborne laser scanning (ALS). We will use the command-line access of HELIOS++ to run the simulation, and use Python just for displaying the input XMLs and the resulting point clouds.

In [1]:

from pathlib import Path
from IPython.display import Code
from pyhelios.util.xmldisplayer import display_xml, find_playback_dir

In [2]:

import os
os.chdir("..")

Survey¶

Let us look at the XML files in the simulation. First, we investigate the survey XML file, als_hd_demo_tiff.xml:

In [3]:

Code(display_xml('data/surveys/demo/als_hd_demo_tiff.xml'), language='XML')

Out[3]:

<document>
	<!-- default scanner settings -->
	<scannerSettings id="set" active="true" pulseFreq_hz="70000" scanAngle_deg="60" scanFreq_hz="50" />
    <survey name="als_hd_demo" scene="data/scenes/demo/hd_demo.xml#hd_demo" platform="data/platforms.xml#sr22" scanner="data/scanners_als.xml#leica_als50-ii">
	<!-- platform: Plane, deflector: oscillating -->
        <FWFSettings beamSampleQuality="3" binSize_ns="0.25" winSize_ns="1" />
        <detectorSettings rangeMax_m="1700" />

		<leg>
			<platformSettings x="474500" y="5474500" z="1500.000" onGround="false" movePerSec_m="150" />
			<scannerSettings template="set" trajectoryTimeInterval_s="0.05" />
		</leg>
		
		<leg>
			<platformSettings x="490000" y="5474500" z="1500.000" onGround="false" movePerSec_m="150" />
			<scannerSettings template="set" trajectoryTimeInterval_s="0.05" />
		</leg>

		<leg>
			<platformSettings x="474500" y="5473500" z="1500.000" onGround="false" movePerSec_m="150" />
			<scannerSettings template="set" trajectoryTimeInterval_s="0.05" />
		</leg>
		
		<leg>
			<platformSettings x="490000" y="5473500" z="1500.000" onGround="false" movePerSec_m="150" />
			<scannerSettings active="false" />
		</leg>

    </survey>
</document>

We can see that there are four leg elements which define the waypoints of the airplane trajectory with x, y, z coordinates and the speed between these waypoints (movePerSec_m). This results in three flight lines. Furthermore, we see that the sr22 platform in data/platforms.xml is referenced, so let's have a look at that next:

Platform¶

In [4]:

Code(display_xml('data/platforms.xml', 'sr22'))

Out[4]:

<platform id="sr22" name="Cirrus SR-22" type="linearpath">
		<scannerMount z="0.7">
			<rot axis="x" angle_deg="-90" />
			<rot axis="z" angle_deg="90" />
		</scannerMount>
		<!--<positionXNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.02"/>
		<positionYNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.02"/>
		<positionZNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.02"/>
		<attitudeXNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.001"/>
		<attitudeYNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.001"/>
		<attitudeZNoise
			clipMin="0.0" clipMax="0.0" clipEnabled="false" fixedLifespan="1"
			type="NORMAL" mean="0.0" stdev="0.001"/>-->
	</platform>

This is a linearpath type platform, a mobile platform which moves in a straight line between consecutive legs with a constant speed provided by the user.

Without any rotations applied to the scannerMount in the platform XML or the beamOrigin in the scanner XML, scanners in HELIOS would face towards to Y-axis (forward) and scan in vertical scan lines (in the plane created by the Y- and Z-axis).

For the sr22, the scannerMount is configured so that the scanner is rotated -90° around the X-axis (facing downwards) and 90° around the Z-axis (scanning from left to right, i.e., in the plane created by the X- and Z-axes).

For more information and examples, check out the rotations section of the Wiki page on platforms.

Sanner¶

Next we will have a look at the scanner that is placed on the platform. Here it is the leica_als50-ii defined in data/scanners_als.xml as shown in the survey XML.

In [5]:

Code(display_xml('data/scanners_als.xml', 'leica_als50-ii'), language='XML')

Out[5]:

<scanner id="leica_als50-ii" accuracy_m="0.05" beamDivergence_rad="0.00022" name="Leica ALS50-II" optics="oscillating" pulseFreqs_Hz="20000,60000,150000" pulseLength_ns="10" rangeMin_m="200" scanAngleMax_deg="37.5" scanFreqMin_Hz="0" scanFreqMax_Hz="90" maxNOR="4" />

Here we can see the scanner-specific settings, for example the beamDivergence_rad, the accuracy or the possible pulse frequencies (pulseFreq_Hz). The Leica ALS50-II has an oscillating beam deflector (optics) and is capable of recording up to 4 returns per emitted pulse (maxNOR).

Scene¶

Now we will have a look at the scene, hd_demo.xmlin data/scenes/demo/hd_demo.xml:

In [6]:

Code(display_xml('data/scenes/demo/hd_demo.xml', 'hd_demo'))

Out[6]:

<scene id="hd_demo" name="Heidelberg">
        <part>
            <filter type="geotiffloader">
                <param type="string" key="filepath" value="data/sceneparts/tiff/dem_hd.tif" />
            </filter>
        </part>
    </scene>

Here we see one object, which is a digital elevation model (DEM) of Heidelberg (Germany) in GeoTIFF file format. To load it, we use the geotiffloader filter and give the relative path to the file in the filepath parameter.

Executing the Simulation¶

Next, we will run the simulation. In Jupyter Notebooks, we can run external commands with the !command syntax, but you can also just run it from the command line.

In [8]:

!helios data/surveys/demo/als_hd_demo_tiff.xml -q

The results¶

Now we can display a couple of 2D plots of the simulated point cloud.

We first load the output files to numpy arrays.

In [9]:

import numpy as np
import matplotlib.pyplot as plt

output_path = find_playback_dir('data/surveys/demo/als_hd_demo_tiff.xml')

print('Loading points from', Path(output_path))

strip_1 = np.loadtxt(Path(output_path) / 'leg000_points.xyz')
strip_2 = np.loadtxt(Path(output_path) / 'leg001_points.xyz')
strip_3 = np.loadtxt(Path(output_path) / 'leg002_points.xyz')
traj_1 = np.loadtxt(Path(output_path) / 'leg000_trajectory.txt')
traj_2 = np.loadtxt(Path(output_path) / 'leg001_trajectory.txt')
traj_3 = np.loadtxt(Path(output_path) / 'leg002_trajectory.txt')
traj = np.vstack((traj_1[:, :3], traj_2[:, :3], traj_3[:, :3]))

Loading points from /home/dkempf/helios/output/als_hd_demo/2024-02-13_10-34-20

Now we create two plots - one from above and one from the side - showing the point cloud colored by flight strip and the trajectory.

In [10]:

# two subplots
fig, (ax1, ax2) = plt.subplots(2, figsize=(15, 12))

# view from above, colored by strip, including trajectory - for faster display, show only every 25th measurement
ax1.scatter(strip_1[::25, 0], strip_1[::25, 1], s=0.1, alpha=0.4, label="Points from strip 1") # select X and Y coordinates
ax1.scatter(strip_2[::25, 0], strip_2[::25, 1], s=0.1, alpha=0.4, label="Points from strip 2")
ax1.scatter(strip_3[::25, 0], strip_3[::25, 1], s=0.1, alpha=0.4, label="Points from strip 3")
ax1.scatter(traj[:, 0], traj[:, 1], s=0.1, label="Trajectory", color="black")
ax1.tick_params(labelsize=16)
ax1.set_xlabel('X', fontsize=18)
ax1.set_ylabel('Y', fontsize=18, rotation=0)
ax1.legend(fontsize=18, markerscale=20, loc="lower right", bbox_to_anchor=(1.01, 1))

# use only every 50th measurement for better display
ax2.scatter(strip_1[::50, 0], strip_1[::50, 2], alpha=0.05, s=0.1, label="Points from strip 1") # select X and Z coordinates
ax2.scatter(strip_2[::50, 0], strip_2[::50, 2], alpha=0.05, s=0.1, label="Points from strip 2")
ax2.scatter(strip_3[::50, 0], strip_3[::50, 2], alpha=0.05, s=0.1, label="Points from strip 3")
ax2.scatter(traj[:, 0], traj[:, 2], s=0.05, label="Trajectory", color="black")
ax2.tick_params(labelsize=16)
ax2.set_xlabel('X', fontsize=18)
ax2.set_ylabel('Z', fontsize=18, rotation=0)

plt.axis('equal')
#plt.legend(fontsize=18, markerscale=20)
plt.show()

Let's create another plot from above which is colored by altitude.

In [11]:

#view from above - colored by altitude

fig, ax = plt.subplots(figsize=(15, 5))

# select X and y coordinates
plot1 = ax.scatter(strip_1[::20, 0], strip_1[::20, 1], s=0.1, c=strip_1[::20,2], cmap= 'terrain', vmin=50, vmax=500)
plot2 = ax.scatter(strip_2[::20, 0], strip_2[::20, 1], s=0.1, c=strip_2[::20,2], cmap= 'terrain', vmin=50, vmax=500)
plot3 = ax.scatter(strip_3[::20, 0], strip_3[::20, 1], s=0.1, c=strip_3[::20,2], cmap= 'terrain', vmin=50, vmax=500)
plt.axis('equal')
ax.tick_params(labelsize=16)
plt.xlabel('X', fontsize=18)
plt.ylabel('Y', fontsize=18, rotation=0)

cbar = plt.colorbar(plot1)
cbar.ax.tick_params(labelsize=16)
cbar.set_label('Altitude [m]', fontsize=18)
plt.show()

Finally, plot only a section of the point cloud to get a profile of the Neckar valley in Heidelberg.

In [12]:

# Section in direction of Y 

xmin, ymin, xmax, ymax = [480000, 5472000, 480100, 5476000]

section_1 = strip_1[(strip_1[:, 0] > xmin) & 
                    (strip_1[:, 0] < xmax) & 
                    (strip_1[:, 1] > ymin) & 
                    (strip_1[:, 1] < ymax)]
section_2 = strip_2[(strip_2[:, 0] > xmin) & 
                    (strip_2[:, 0] < xmax) & 
                    (strip_2[:, 1] > ymin) & 
                    (strip_2[:, 1] < ymax)]
section_3 = strip_3[(strip_3[:, 0] > xmin) & 
                    (strip_3[:, 0] < xmax) & 
                    (strip_3[:, 1] > ymin) & 
                    (strip_3[:, 1] < ymax)]

In [11]:

fig = plt.figure(figsize=(15,5))
ax = fig.add_subplot()

ax.scatter(section_1[:, 1], section_1[:, 2], c=section_1[:, 2], cmap='terrain', s=0.01, vmin=50, vmax=500)
ax.scatter(section_2[:, 1], section_2[:, 2], c=section_2[:, 2], cmap='terrain', s=0.01, vmin=50, vmax=500)
ax.scatter(section_3[:, 1], section_3[:, 2], c=section_3[:, 2], cmap='terrain', s=0.01, vmin=50, vmax=500)
ax.tick_params(labelsize=16)

plt.xlabel('X', fontsize=18)
plt.ylabel('Height (Z)', fontsize=18, rotation=90)
plt.axis('equal')
plt.show()