Mapping with laser scanning

GIS-E1020 From measurements to maps Lecture 7 Mapping with laser scanning Petri Rönnholm Aalto University 1 Learning objectives Basics of airborne laser scanning Intensity and its calibration Applications of airborne laser scanning Airborne laser scanning process in practice 2 1

Laser scanning alternatives for mapping Airborne laser scanning (in this course we focus on this) Airplane Helicopter Unmanned aerial vehicle (UAV) Terrestrial laser scanning (more in the course GIS- E1040 Photogrammetry, Laser Scanning and Remote Sensing) Mobile laser scanning (terrestrial perspective, more in the course GIS-E3030 Advanced laser scanning) Vehicle-based laser scanning Backpack laser scanning 3 Select the best alternative for your mapping case Airborne laser scanning Mapping of large areas Creation of digital elevation models (DEM) Buildings, structures, vegetation, cars etc. Terrestrial laser scanning Mapping of relatively small areas (relatively slow to measure) Indoor mapping Mobile laser scanning Mapping of large areas (both outdoors and indoors) Vehicle mobile mapping is limited with accessibility (you cannot measure areas in which you cannot drive) Backpack laser scanning allows practically access to all areas but is relatively slow (walking speed) 4 2

Properties of laser Laser light is well structured while, for example, plumb lights produce light waves randomly Laser light is coherent, which means light waves are in the same phase compared to each other In addition, laser light is highly directional (with a narrow beam width) 5 Properties of laser Unlike normal light, laser light contains only one wavelength of the light spectrum (monochromatic) white laser / supercontinuum laser is an exception and it can include several areas of spectrum, but has all other properties of laser 6 3

Laser scanning measurements (airborne laser scanning) An active method An equipment sends a signal (pulse of light) to the known direction Returning signal (echo) is recorded Traveling time of the signal is measured Distance to the target is calculated using the traveling time of the signal 7 Almost all airborne laser scanners use pulsed laser rays A short laser ray pulse is sent to the target and the time and strength of returning echo are recorded Distance/range (R ) from the scanner to the target is measured using time t and the speed of light c t R = 2 n= a correction factor of the speed of light (depends on air temperature, air pressure and humidity) c n 8 4

The speed of light The first approximate value was discovered in 1676, Olaus Roemer using Jupiter s moons eclipses In vacuum: 299 792 458 m/s Infrared laser light in normal atmosphere, in 12 Celsius, in 100 mbar air pressure and in 60 % air humidity travels only at the speed of 299 710 484 m/s. Actually, it is possible to stop light using a proper medium substance and low temperatures http://www.jupiterscientific.org/sciinfo/stoplight.html 9 Scanner 3D point from a laser scanner Scanner coordinate system Location and attitude of scanner need to be known from each time when a distant measurement occurs Light ray The angle of the outcoming light ray need to be known Traveling time of light ray to the target and back is measured Ground coordinate system X, Y, Z The location of 3D point is calculated according to observations 10 5

Principle of airborne laser scanner Laser Scanner DGPS Laser Range Finder (LRF) Camera Controlling, monitoring and recording unit IMU Differential GPS (DGPS) gives location. Inertial Measurement Unit (IMU) gives location and attitude. (X,Y,Z), Center of the footprint of the laser beam 11 Airborne laser scanning Image: Petri Rönnholm 12 6

Laser ray A footprint of a laser ray is not an infinitely small dot at the target, but laser illuminates an area. (diameter of 0.1-3.8 m in airborne systems) 13 Laser ray A beam divergence of a laser ray ( β ), together with range (R) defines how large a laser footprint on a target surface is Area of footprint 2 π ( Rβ ) A laser 4 Typical beam divergence for aerial laser scanners is 0.3 2 mrad In some systems, you can change the beam divergence 14 7

Laser ray Aperture (D) Lens β / 2 = θ Aperture (D) D footprint R β = D + 2R tan 2 β 2R tan 2 Rβ For long distances we can round it: 15 Typically laser scanners use TEM 00 laser beam TEM 00 is the lowest order transverse mode Gaussian intensity profile Mathematically the strength of intensity follows a Gaussian function 2 P r I( r, z) = exp 2 2 2 πw( z) / 2 w( z) where the beam radius w(z) is the distance from the middle axis of the beam to the distance where the intensity drops to 1/e 2 ( 13.5%) of the maximum power P (r = distance in the transverse plane). 16 8

Laser ray Laser beam divergence can be calculated using the wavelength of light λ and the diameter of an aperture D from which laser comes out. Beam divergence angle β is measured using the distance where the power of the main beam is half of the maximum λ β = D When light goes through a round hole (aperture) some diffraction happens. The diffraction causes the light beam to diverge. The minimum beam divergence is λ β 2. 44 D 17 Laser ray A laser scanner is constantly changing the outcoming angle of laser light If a laser beam goes directly towards the ground the footprint of the laser is smaller than if the laser ray has been tilted Previously, it was shown that Dfootprint Rβ If the angle of an outgoing light ray θ is known, it is possible to estimate the footprint size of a light ray using the flying height (H ) (accurate only if the terrain is flat) H D footprint Rβ = β cosθ 18 9

An example H D footprint Rβ = β cosθ The flying height is 1000 m and the beam divergence is 1 mrad Directly under the aircraft θ = 0, and therefore the diameter of the footprint of the light ray is 1 m At the side of the laser scanning swath, angle is θ = 20, and the diameter of the footprint is 1000 m 0.001 rad 1.06 m cos 20 19 Footprint of the laser ray The footprint is a circle only if the ray hits perpendicularly to the flat target Otherwise a footprint is an ellipse or an irregular shape that follows the ground surface 20 10

Various methods how to distribute laser range measurements Mirror system Pattern on the ground Nutating mirror, Palmer scan Oscillating mirror Rotating polygon 21 Various methods how to distribute laser range measurements An optical fiber scanner, light is directed to the desired directions using optical fibers Along flying track point density is high, but accross track the system leaves gaps TopoSys 22 11

The speckle effect When monochromatic light reflects from materials (which cause a diffuse reflection), speckle becomes visible Granular appearance of laser footprint The reason is the phase difference of stationary interference patterns Causes noise but also gives information about the surface structure The speckle pattern of a green laser pointer (image https://en.wikipedia.org/wiki/speckle_pattern) 23 Devices Optech Leica Riegl TopoSys 24 12

Laser pulses and echoes In this case, echo is considered to be the returned light pulse First pulse/echo Middle pulses/echoes Last pulse/echo Full waveform 25 First and last pulse? First and last pulses http://www.aamhatch.com.au/resources/pdf/publications/news/scanhoriz14.pdf 26 13

How well a laser scanner can separate objects in depth (within a footprint)? The length of the light pulse Δτ defines how small distances Δρ are separable in depth Δρ = cδτ 2 Backscattered pulse parts cannot be easily separated For example, if a light pulse is 10 ns long, it is possible to separate objects that have at least 1.5 m distance between them 27 Full waveform Example from a corn field source: Hug et al. 2006 28 14

Full waveform One echo from the ground U. Pyysalo Two echoes are coming from vegetation and one from the ground 29 Full waveform Rönnholm First pulse Rönnholm Full waveform 30 15

Why doesn t everybody use full waveform? A full waveform data collection significantly increases the amount of data It is difficult to handle large data sets Typically, first and last pulses (and sometimes one or two pulses in between) are collected -> easier to handle data Full waveform will increase its popularity when commercial software provide more efficient tools to handle it 31 Terrain affects to the shape of waveform Waveform is affected by: Partial obstacles Terrain slopeness Terrain material (asphalt, grass etc.) 32 16

Wavelength of laser Typically varies between 500 1550 nm depending on device and the final application for what it has been designed. 1 μm = 0.001 mm 1 nm = 0.001 μm 33 The influence of wavelength Underwater Laser Scanning Blue and green wavelength areas Penetrate in water up to ca. 50 m depth (if water is not clear, the results are much worse) Unfortunately, waters in Finland are not clear Red and infrared wavelengths are not penetrating easily into the water Usually, some observations come from the water surface This can be utilized if only the water depth is important The reflectivity of an object surface is sometimes dependent on the wavelength Preferably, the reflectivity properties are pre-known and the wavelength of the instrument is chosen accordingly 34 17

Optech 35 Intensity Intensity is the amount of returning light echo (power) at the trigger point Intensity values are affected e.g. by: Surface reflectance Shape of objects Color of objects Location and attitude of objects Texture of objects Special properties of objects (e.g. retro targets) Lighting conditions: direct or indirect (background) light 36 18

Selection of a trigger point can vary Different methods: Amplitude of an echo Constant threshold Center of mass Maximum Zero of second derivative time/distance Constant distance from the beginning of pulse 37 The result of range measurement depends on the selected trigger method In many cases, echo extraction method is predefined and end-user have no access to that information Different methods provide slightly different range results To get the best result, you should change the triggering style according to the terrain/target 38 19

Visualization using intensity 39 Visualization using intensity 40 20

Calibration of intensity Because intensity values vary according to conditions, they must be calibrated before, e.g., automatic interpretation Range calibration Bidirectional reflectance distribution function (BRDF) calibration Calibration with the help of reference targets 41 Range calibration The range effect of intensity depends on how much the target fills the laser footprint of a single laser beam Intensity weakens proportional to R 2 if a homogenous target fills the full footprint R 3 if a target is linear (e.g. wire) R 4 if a target is an individual large scatterer 42 21

BRDF (Bidirectional reflectance distribution function ) 2 Ps 1 BRDF = Pi Ω cosθs P s = Scattered power P i = Incident power Ω = Scattering solid angle Θ s = Angle between scattering direction and surface normal BRDF describes how a surface reflects light Light Θ s Ω Detector 43 Automatic gain control Some laser scanners apply an automatic gain control to data Received energy (intensity) is fit to the range of the receiver during the flight Because data has unpredictable intensity variations, calibration becomes difficult To some extent the effect of the automatic gain control can be removed using calibration targets I = a + a I a I AGC off 1 2 on + 3 AGC =automatic gain control value, I on =intensity value when AGC is on, a i are to be solved on 44 22

Before intensity calibration After intensity calibration Martina Bednjanec 45 Calibration by using reference targets Reference targets with a known reflectance Standard targets: Spectralon by Labsphere Inc (close to a Lambertian surface) Natural materials, e.g., sand or gravel placed on the scene Natural materials that can be found in the field (e.g. football field) True surface reflectance from the intensity observation I r measurement = I measurement standard standard BRDF correction can be combined with a reference target calibration r 46 23

Examples from the Finnish Geospatial Research Center Intensity calibration tarps Gravel calibration field Hannu Hyyppä 47 Multispectral laser scanning Optech Titan is the first commercial multispectral airborne laser scanner three active laser beams with independent wavelengths of 532 nm, 1064 nm, and 1550 nm Optech 48 24

UAV laser scanning (Unmanned aerial vehicle) UAV laser scanners improve flexibility of measurements Flying regulations usually prevent mapping of large areas (in Finland you must have visual contact to drone) Operating costs are lower than with helicopters or airplanes 49 Applications of airborne laser scanning Producing elevation models Forest inventory Mapping of linear features (power lines, roads, railroads etc.) City mapping, 3D models Flood mapping Change detection Mass calculations Basis for several planning tasks Etc. 50 25

Laser scanning process in practice Flight planning and preparations Define the point density Define the flying altitude Choose the laser beam divergence Decide the width of laser scanning strips Define the amount of overlap for laser scanning strips/swaths Timing Good weather Leafs-off or leafs-on? 51 What does frequency mean? Frequency (f ) is the number of occurrences of a repeating event per unit time (wikipedia) If the time period of regularily repeated phenomena (T ) in known, the frequency is 1 f = T 1 Unit is hertz (Hz) Hz = s In other words, one Hz means that regularily repeated phenomena happens once a second 52 26

Flying altitude and width of laser scanning strip θ strip width = 2H tan 2 For example, to get the laser scanning strip of 400 m width (at ground), what should the flying height be? The opening angle of the scanner is ± 25 degrees. strip width = H 2 tan θ 2 400m = 428,90m 50 2 tan 2 θ Η 53 Flying altitude and width of laser scanning strip The equation θ strip width = 2H tan 2 is usable also with Palmer scanners (even if its footprint is elliptical), if the angle is measured directly downwards and perpendicularly to the flying direction 54 27

How many points (N) we get within one laser scanning sweep? PRF=pulse repetition frequency f scanner =scanning frequency N = PRF f scanner For example, PRF=70 khz and scanning frequency is 80 Hz. How many points we get during one laser scanning sweep? N = 70000Hz 80Hz = 875 points 55 Point density in the across flight line direction dx across = strip width N If we use values from previous examples, we find that distances between points in across flightline direction are 400m dx across = = 0. 46m 875 56 28

Point density in the across flight line direction Scanner can produce Z-shaped footprint (oscillating mirror). It can vary, if scanning frequency means one sweep (half of Z- shape) or the situation when mirror is back at the starting position (complete Z-shape) If scanning frequency means complete Z-shape, the result from previous slide must be multiplied by two Note that the point distribution along the Z-shape is not necessarily even (typically point cloud is denser at the sides of laser strips) 57 Point density in the across flight line direction For optical fiber scanners, the across flightline point density is θ dx across = H N 1 For Palmer scanners we can calculate approximation dx across strip width = π N And more accurately.4429h 2 dx across = tan (2SN) + tan N 4 2 (1.41SN) SN=inclination angle of mirror 58 29

Point density in the along flight line direction Independent of the flying height Depends on the speed of an aircraft (v) and scanning frequency (f scanner ) v dx = along scanner For example, if the speed of an aircraft is 75 m/s (270 km/h) and scanning frequency is 80 Hz f m 75 dx s along = 0. 94m 80Hz = 59 Distance between flying paths What is a proper distance of flying paths to get desired overlap (s overlap ) of scanning strips s overlap scanning strip distance = scanning strip width 1 100 For example, if 20% overlap is wanted when the strip width is 400: 20 distance between scanning strips = 400m 1 = 320m 100 60 30

Laser scanning process in practice Preparation of GPS equipments Terrestrial GPS reference station (dgps) VRS=Virtual (GPS) Reference Station, for example gpsnet.fi/ 61 GPS/GNSS Webserver VRS-network has permanent GPS reference stations and data processing centers GPSNet.fi 62 31

Laser scanning process in practice Laser data acquisition Flying over the target Collecting of GPS, IMU and laser observations 63 Laser scanning process in practice According to observations the 3D ground coordinates are calculated Position and attitude of laser scanner must be know for each range measurement 64 32

Laser scanning process in practice Data quality verification Compare with know ground features Compare overlapping scanning strips (specially with cross strips) Check that data has no unwanted gaps Post-processing of data (next lecture) 65 33