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Geog319RmSns > Lidar > Flashcards

Lidar Flashcards

1
Q

LiDAR stands for?

Also known as?

A

Light Detection And Ranging

  • Aerial Laser Scanning (ALS)
  • Laser Altimetry
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2
Q

What are the 3 main units of LiDAR System components?

A
  • Laser and deflection unit (Scanning mechanism)
  • Ranging Unit (Recorder)
  • DGPS and INS (positioning)
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3
Q

What is LiDAR, basic how it works

A
  • Active instrument, like radar
  • Transmits laser pulses and receives returned laser signal
  • Measures distance to target via travel time of signal
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4
Q

What is LiDAR, when developed

A
  • Advent of GPS and INS platform positioning technology enabled accurate geo-location of return signal and development of LiDAR beginning in early 1990’s
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5
Q

INS

A
  • Inertial Navigation System
  • Accelerometers and gyroscopes are used to track position and orientation of device
  • Changes in velocity and orientation of remote sensing platform are detected
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6
Q

What data does lidar produce?

A
  • Positional x,y
  • Elevation z
  • Intensity sometimes used
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7
Q

What is used as a lidar platform?

A
  • Aerial platforms
  • Lidar in space technology experiment (LITE) in 1994 and Shuttle Laser Altimeter 1 and 2 (SLA-01 and 02) missions in 1996 and 1997
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8
Q

LITE?

A

Lidar In Space Technology Experiment (1994)
- 1st highly detailed view of vertical structure of cloud and aerosol from surface through middle atm (new application discovered, not just for ground)

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9
Q

LITE Goals

A
  • Mostly to prove tech and use for shuttles
  • Goals: Validate/explore key lidar tech for space borne applications, gain operational experience to develop future systems on satellite platforms
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10
Q

LITE Mission

A
  • Mission: Operated 53 hours, collected 40GB for 1.4million km of ground
  • Instruments on top of shuttle, flipped shuttle upside down to point towards Earth surface to get data
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11
Q

Shuttle Laser Altimeter Earth Science Applications

A
  • Oceanography, wave states
  • Hazards, coastal erosion
  • Geomorphology, drainage evolution
  • Geodynamics, regional tilts
  • Hydrology, lake levels
  • Seismicity, fault scarps
  • Volcanology, eruption volumes
  • Ecology, tree height
  • Climatology, cloud top heights
  • Tectonics, mountain relief
  • Glaciology, glacier dynamics
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12
Q

ICESat

A
  • NASA launched mission in 2003 to understand atm and climate change on polar masses
  • The Ice, Clouds and Land Elevation Satellite
  • Measure ice sheet elevation and change over time, height profiles of clouds and aerosols, land elevations and veg cover, approx. sea ice thickness
  • Ended in 2010
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13
Q

Applications of lidar

A
  • Digital terrain modelling (DTM)
  • faults and uplift
  • forestry
  • oceanography
  • natural hazards, floods
  • man made structure mapping
  • oil and gas exploration
  • natural resource management
  • mapping of linear structures
  • glacier (ice sheet) movement
  • atmosphere
  • often combined with other sources to improve estimation and classification
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14
Q

Laser light forms basis of lidar, pulses of light are?

A
  • beamed towards target (Earth) several times per second
  • Reflected light returns to sensor is measured
  • Lasers focused, coherent beams of light energy w/ little divergence
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15
Q

Single returns are recorded when?

A
  • Pulse strikes solid object like building or rock
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16
Q

Multiple returns are recoded when?

A
  • Pulse strikes vegetation canopy, and some light travels past canopy top and returns come from leaves, stems, trunks and underlying ground
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17
Q

Pulse Repetition Frequency, prf

A
  • Number of pulses per second emitted by a lidar
  • Advent 1990’s = 2000-25000
  • 2000’s = 50000+ w/ TB of data
  • Current = 250000+
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18
Q

Lidar point cloud

A
  • Data before processing like classification
  • 3D point cloud of single and multiple returns
  • Used to create DEM (DSM and DTM) or infer property of target
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19
Q

DEM

A
  • Digital Elevation Model
  • File or database containing elevation points over a contiguous area
  • Subdivided into DSM and DTM
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20
Q

DSM

A
  • Digital Surface Models

- Contain elevation inför about all features in the landscape, such as vegetation, buildings, and other structures

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21
Q

DTM

A
  • Digital Terrain Models

- Elevation info about bare-Earth surface w/o presence of veg or man-made structures

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22
Q

What are techniques for creating DEM’s?

A
  • In situ surveying (costly and time consuming)
  • Interferometric SAR (InSAR) (High res from space but veg and steep topo lead to error)
  • Photogrammetry (accurate, timely, relatively affordable but inferred and less accurate than Lidar)
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23
Q

Discrete Lidar system

A
  • records x,y,z and intensity
  • z data from ‘pulse ranging principle
  • intensity from amplitude of returned signal
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24
Q

Pulse ranging principle

A
  • Distance/range determined by the timing of pulses from and to the Lidar
  • Range = speed of light x (time returned - time emitted)/2
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25
Q

Early discrete lidar systems

A
  • Only captured single returns

- Year 2000 captured 3-5 returns per pulse

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26
Q

What is the result of more returns per pulse?

A
  • More returns increases size of dataset

- Most missions use 3 returns to balance detail with data size

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27
Q

Full waveform Lidar system

A
  • Records entire waveform of return laser pulse as function of time
  • Mainly research purposes (veg density, wildlife habitat mapping,) b/c data volume very high and processing difficult
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28
Q

What are the 2 types of Lidar sensors? How are they the same/different?

A
  • Profiling
  • Imaging
  • Measurement same for both types
  • Differ in how swath is collected
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29
Q

Profiling Lidar

A
  • Pulses aimed directly beneath platform at NADIR
  • Like single beam
  • Echo profile along flight path of sensor
  • Can get canopy height
30
Q

How can canopy height be determined from Lidar?

A
  • Canopy topography, 1st return canopy, 2nd topography

- Subtract 2nd from 1st to get canopy height

31
Q

Imaging Lidar

A
  • Scanner directs pulses over a swath beneath platform as it travels
  • Extended echo profile beyond flight path of sensor
32
Q

Imaging Lidar

A
  • Scanner directs pulses over a swath beneath platform as it travels
  • Extended echo profile beyond flight path of sensor
  • Ground coverage has ‘saw-tooth’
33
Q

Why does imaging lidar produce a ‘saw-tooth’ pattern?

A
  • Because platform moves forward along flight path as swath moves around
  • B/c oscillating rotating mirror
34
Q

What are the different types of scanning mechanisms for imaging lidar and what swath patterns do they create?

A
  • Oscillating mirror, z-shaped/sinusoidal (saw-tooth)
  • Rotating polygon, parallel lines in diagonal
  • Nutating mirror/Palmer scan, Elliptical
  • Fiber switch, parallel lines (still experimental)
35
Q

What is the most common scanning mechanism of imaging lidar?

A
  • Oscillating mirror

- ‘Saw-tooth’ pattern

36
Q

Practical limitations of space borne Lidar means that it is what type of sensor?

A
  • Profiling type (not imaging)

- Returns from directly below platform

37
Q

Lidar Footprint for Imaging Lidar

A
  • Approx. circular on ground, instantaneous footprint
  • Fp = [height of aircraft/(cos2 of scan angle under investigation)] * gamma
  • Where gamma = divergence of laser beam (in radians?)
38
Q

Calculating Lidar footprint for Profiling Lidar?

A
  • Diameter of illuminated area approx. = height above ground * Gamma
  • Where gamma is divergence of laser
  • Simple b/c eliminate angular b/c looking straight down
39
Q

What is the divergence of laser beam when calculating Lidar footprint?

A
  • Divergence angle in radians from vertical to edge of beam
40
Q

What are the typical footprint sizes for airborne Lidar and space spaceborne?

A
  • Airborne = 0.2-0.9m

- Spaceborne = 70m

41
Q

What is the swath pattern of Lidar determined by?

A
  • Lidar sensor type (profiling, imaging)

- Point Density

42
Q

What kind of swath pattern does a profiling lidar produce?

A
  • Pulses aimed directly beneath platform at Nadir
43
Q

What kind of swath pattern does an imaging lidar produce? What happens when a platform increases in height above ground?

A
  • Pulses aimed over a swath beneath platform
  • Swath width is function of scan angle and flying height
  • Swath = 2height of sensor above groundtan(sensor scan angle/2)
  • Fly higher increases swath but decreases resolution
44
Q

What is the relationship of point density and Lidar swath pattern?

A
  • Refers to spacing of hits along a profile (profiling lidar) or within a swath (imaging)
  • Determined by flying height, platform velocity, field of view, prf
45
Q

What is in a Lidar dataset?

A
  • Irregularly spaced hits in x,y dimension, processed data is smoothed
  • 3D point clouds in z dimension, just location, not intensity
46
Q

What does data processing of Lidar require?

A
  • Most software is proprietary or university product
  • Dataset of x,y,z hits can be processed using GIS
  • Processing requires user knowledge of target characteristics (can be augmented by optical data)
47
Q

What are the 3 main data processing steps of Lidar?

A
  • Filtering
  • Classification
  • Interpolation (and display)
48
Q

Lidar processing: Filtering

A
  • Removal of unwanted data
  • May be only main processing step required
  • Removal of partial returns (secondary) and keep first and last returns
  • Requires filtering algorithm
49
Q

First returns used for?

A
  • DSM (surface above ground, i.e. canopy etc.)
50
Q

Last returns used for?

A
  • DTM
51
Q

What is the difference between the first and last returns?

A
  • Height distribution model (canopy height etc)
52
Q

Lidar filtering algorithms, deriving CHM

A
  • Deriving canopy height model (CHM) often of importance for forest resource management as it is indicator of other variables such as timber volume, biomass, carbon sequestration
  • Discrete return of Lidar data of forest can be filtered to derive CHM
  • Vegetation removal filter
53
Q

Lidar Processing: Classification goals

A
  • Find a specific structure from data after filtering
  • Assigning proper class labels to filtered data
  • Achieved by applying rules or statistical models that separate classes
54
Q

Classification using un-filtered data

A
  • Possible when point cloud data contains info that aids classification process
  • Ex. differentiating buildings from canopy of similar height when both are first returns to Lidar, veg will also have partial returns and multiples
55
Q

Interpolation

A
  • Creates smooth, continuous dataset from discrete objects like Lidar points
  • Irregularly spaced hits re-projected to form a regular image-like array
  • Different interpolation methods exist, choose one that represents the surface (DEM)
56
Q

Interpolation methods differ in terms of?

A
  • Ease of use
  • Mathematical complexity
  • Computational expense
57
Q

What are the common methods of interpolation algorithms?

A
  • Interpolation rasterizes based on 2 common methods:
  • Inverse Distance Weighting (IDW)
  • Spline
  • Others based on natural neighbours
58
Q

IDW

A
  • Inverse Distance Weighted
  • Weights assigned to known values w/in neighbourhood according to distance away
  • Weight based on 1/distance of point squared (i.e. inverse distance)
  • Power is additional option
  • New value based on sum of weights*points/sum of weights in neighbourhood search radius
59
Q

What are the benefits and drawbacks of IDW?

A
  • Benefit: useful for locationally dependent variables

- Drawbacks: trends not well accounted for, ‘duck-egg’ pattern

60
Q

Why are trends not accounted for in IDW?

A
  • Weighted means, spaces btwn points trend towards mean

- Causes pits where the should be peaks and vice versa

61
Q

What is the ‘duck-egg’ pattern in IDW?

A
  • Solitary points have values that differ greatly from their surroundings
62
Q

Spline

A
  • Interpolation method
  • Smooth curves locally fitted to a set of data points using piece-wise polynomial functions
  • Piece-wise uses a few points at a time
  • Smoothing around corners, e.g. peaks and valleys in vertical dimension
63
Q

What are the benefits and drawbacks of Spline?

A
  • Benefit: quick and efficient DEM creation, smooth topography and aesthetic appearance, small scale features retained
  • Drawback: uncharacteristically smooth surface
64
Q

Interpolated data display

A
  • Elevation raster
  • Slope (Rate of elevation change in neighbourhood is assigned a colour
  • Aspect (Downslope direction assigned a colour)
  • Hillshade (shaded relief by assuming a source of illumination giving orientation
65
Q

Applications of Lidar are dependent on what? Examples?

A
  • Wavelength
  • Atmospheric: clouds and aerosols at 532nm
  • Bathymetry: water body penetration to approx. 50m at 532nm
  • Vegetation and surface: 1064nm
66
Q

Differential Measurement Lidar? Example?

A
  • Application where Lidar measured at 2 different wavelengths btwn surfaces
  • Bathymetry: 1064nm reflects water surface, 532 reflects from ocean floor, Time difference = bottom profile
67
Q

Differential Absorption Lidar (DIAL)

A
  • Ratio of intensity of return signals at 2 wavelengths, as a function of range, used to infer atm properties
  • Ex. aerosol concentration, 532nm partially absorbed by aerosols, 1064nm not absorbed
68
Q

Lidar and surface elevation change applications

A
  • Thinning of ice sheets
  • NASA’s Airborne Topographic Mapper (ATM)
  • IceSAT
69
Q

ATM, wavelength, footprint

A
  • NASA’s Airborne Topographic Mapper
  • 5000Hz Swath Lidar at 532nm
  • 1m footprint
  • Measures small scale topo changes to approx. 10m accuracy
  • Flown over Greenland btwn 1997-2003
70
Q

IceSat, sensor type, wavelength, spacing, goals

A
  • Ice, Cloud and Land Elevation Satellite
  • GLAS instrument
  • Geoscience Laser Altimeter
  • Wavelengths 1064nm and 532nm
  • 70m spacing
  • Goals: surface elevation data, Cloud properties
71
Q

Calipso Mission, wavelength, goal

A
  • Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation
  • 1064nm and 532nm
  • Goals: High resolution vertical profiles of aerosols and clouds
72
Q

What is necessary to improve Lidar classification?

A
  • Fusion with multi and hyper spectral optical data
  • Merged more closely w optical to improve interpretation
  • Focus of much research

Geog319RmSns (15 decks)

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