9.1 Physical Basics
9.2 Recording Techniques 9.3 Image Processing
9.4 Thematic Classification 9.5 Summary
9 Remote Sensing
hBp://saturn.unibe.ch/.../Fotogrammetrie-‐Bildflug.pdf
• A geographic information system (GIS) is a computer hardware and software system designed to
– Collect – Manage – Analyze – Display
geographically referenced data (geospatial; spatial)
• It is a specialized information system consisting of a (spatial) database and a (special) database system
9 Remote Sensing
Visualiza(on, Cartography
Spa(al Data Management Collec(on of
Spa(al Data
Analysis, Modelling
Func$onal Components Structural Components
• Recording on site
– Terrestrial survey techniques
• Global navigation satellite systems (e.g. GPS)
• Very long baseline interferometer (VLBI)
• Theodolite: measuring both horizontal and vertical angles optically
• Total station: electronic theodolite (transit) integrated with an electronic distance meter
– Hydrographic survey
• Aerial survey and survey by remote sensing
9 Remote Sensing
hBp://i00.i.aliimg.com/
• Remote sensing is the acquisition of information of an object or
phenomenon by the use of device(s)
that are not in
physical or intimate contact with the
object
→ indirect observation technique
9 Remote Sensing
hBp://www.etsu.edu/cas/geosciences/
– That uses the electromagnetic radiation
which is emitted by the observed object
– That carries receiving devices on aircraft or spacecraft
– That serves for the observation of the surface of the earth including all objects thereon, the oceans or the atmosphere
9 Remote Sensing
hBp://www.aero-‐news.net/
• Photogrammetry
– Greek: photo - grammetry = image-measurement – Acquisition and analysis of images to determine the
properties, form and position of arbitrary objects – Remote sensing is the acquisition of
physical properties of objects whereas photogrammetry is the reconstruction
of their geometric form
based on this data
9 Remote Sensing
hBp://www.gisdevelopment.net/…/mm063d_155.htm www.maps.google.de
• System characteristics
– Recording techniques
• Radiometric resolution
• Geometric resolution
– Platform
• Kind of platform
• Altitude
• Orbit
• Period
– Mission
• Temporal coverage
• Spatial coverage
9 Remote Sensing
hBp://www.wdr.de/tv/quarks/
hBp://www.dlr.de/
hBp://www.chip.de/
hBp://www.mari$me-‐technik.de/
• Electromagnetic waves as information carrier
– Straight propagation with the speed of light – Speed of light = wavelength x frequency
– Longer wavelength, lesser energy → more difficult to sense
9.1 Physical Basics
electrical field distance
magne$c field M
E
c
speed of light
ν: frequency
λ: wavelength
number of cycles that passes a certain point per second
hBp://www.fe-‐lexikon.info/images/
Elektromagne$scheWelle.jpg
• Electromagnetic spectrum
– The electromagnetic
spectrum is the range of all possible frequencies of electromagnetic
radiation
9.1 Physical Basics
hBp://en.wikipedia.org/
• Behavior of electromagnetic waves at interfaces
9.1 Physical Basics
Reflec$on
Emission Absorp$on
Transmission
ScaBering
Transmission + Reflec$on + Absorp$on = 1
9.1 Physical Basics
[AS14]
solar radia$on
sensor
received signal
scaBered light atmospheric absorp$on
and scaBering sky radia$on
reflec$on at the surface scaBering at the surface
absorp$on and reflec$on in the water (suspended
par$cles)
reflection at the ground
water depth
– The albedo (lat. albedo = "whiteness"), reflectivity
• The extent to which an object diffusely reflects light from the sun
9.1 Physical Basics
– Albedo depends on wavelength
• There is a strong difference between visual and infrared albedos of natural materials
9.1 Physical Basics
[AS14]
• The sun is the most important source of electromagnetic radiation
• With the exception of objects at absolute zero, all objects emit electromagnetic radiation
– The higher the temperature,
the shorter the wavelength
of maximum emission
9.1 Physical Basics
www.eduspace.esa.int/eduspac e/.../images/03.jpg
• Blackbody
– Hypothetical source of energy that behaves in an idealized manner
– It absorbs all incident radiation, none is reflected – It emits energy with perfect efficiency
– Its effectiveness as a radiator of energy varies only as temperature varies
9.1 Physical Basics
hBp://mynasadata.larc.nasa.gov/images/BB_illustra$on2.jpg
• Emissivity
– The ratio between the emitance of a given object and that of
blackbody at the same temperature
– Useful measure of the
effectiveness of objects as radiators
– Kirchhoff‘s law: At thermal
equilibrium, the emissivity of a body (or surface) equals its
absorptivity
9.1 Physical Basics
surface emissivity (8-‐14 μm) blackbody 1
water,
depending on pollu$on
0,973-‐0,979
water with
oil film 0,96-‐0,979
snow 0,99
grass,
dense, short 0,92-‐0,97 Sands,
depending on water moisture
0,88-‐0,985
• Atmospheric window(s)
– Portion(s) of the electromagnetic spectrum that can be transmitted through the atmosphere
9.1 Physical Basics
hBp://www.geographie.ruhr-‐uni-‐bochum.de/agklima/
– Ultraviolet 0.01 - 0.4 μm
• Reflected solar radiation
• Because of atmospheric absorption it can only be used on aircrafts flying at low altitude
• Main application: oil contamination detection in water
– Visible light 0.4 - 0.7 μm
• Reflected solar radiation
• Atmospheric influences particularly on blue and green light
• Several applications, e.g. land use mapping
9.1 Physical Basics
hBp://www.samtgemeinde-‐nord-‐elm.de/
– Near infrared 0.7 - 3 μm
• Reflected solar radiation
• Nearly no atmospheric influences
• Main application: Classification of vegetation, forest health survey (healthy green plants strongly reflect near infrared radiation),
classification of water (expanses of water seem dark as they absorb all)
9.1 Physical Basics
hBp://www.forestwatch.sr.unh.edu/
– Far infrared (thermal energy) 3 - 1000 μm (usually : 8 - 14 μm)
• Radiation emitted by the earth
• Nearly no atmospheric influences (but clouds are
impermeable, CO2 as well: greenhouse effect is measurable!)
• Applicable day and night
• Measurements beneath the surface to some extent
(pipelines and leaks...)
• Applications for which the temperature and its change are important, e.g. sea
temperature, thermal properties of stone, tectonics
9.1 Physical Basics
hBp://www.qualitas1998.net/paul/
– Passive microwaves 1 - 300 mm
• Emitted radiation
• Nearly no atmospheric influences (capable to measure through clouds)
• Measurements beneath the surface to some extent
• Complex signal difficult to interpret
• Low ground resolution (weak signal)
• Disadvantageous signal-to-noise ratio → noisy images
• Main applications: Meteorology (temperature profiles of the atmosphere) and oceanography (ice observation)
9.1 Physical Basics
hBp://nsidc.org/cryosphere/
– Active microwaves (radar) 1 - 300 mm
• Reflected, transmitted microwave radiation
• Nearly no atmospheric influences (except reaction on water drops)
• Applicable day and night
• Measurements beneath the surface to some extent
• Polarization effects
• Higher ground resolution as passive microwaves
• Complex signal
• Doppler effect allows detection of
moving objects (military applications), sea pollution
9.1 Physical Basics
hBp://www.weBeronline.de/
• Orbits
– Altitude, orbital period, – Apogee/perigee
• Greatest/least distance from the earth
– Inclination
• Angular distance of the orbital plane from the equator
9.1 Physical Basics
v orbital speed
R Earth‘s radius= 6 370 km
g0 gravita$onal accelera$on on the Earth‘s surface = 9,81 m/s2
r radius of the satellite orbit
www.satellitentracking.de/txt/ 04_satellitenbahnen.html
– Low Earth Orbit (LEO)
• Heights between 200 and 600 km
• Manned space stations: low inclination and heights above 400 km
• Satellites with biological or material experiments and astronomical satellites
• Spy satellites 90° inclination , perigee 200-250 km, apogee 600-900km
– Medium Earth Orbits (MEO)
• All orbits above 1000 km up to 36000 km
• Navigation satellite systems (GPS, Glonass)
• Small communication satellites
9.1 Physical Basics
hBp://www.tobedetermined.org/
– Geosynchronous/geostationary Orbit (GSO)
• Orbit height approximately 35786 km, 0° inclination
• Period is equal to the Earth's rotational period → It
maintains the same position relative to the Earth's surface
• Television satellites, weather satellites
– Sun Synchronous Orbit (SSO) or Polar Earth Orbit (PEO)
• Orbit height between 700 and 1000 km, inclination approximately 90°
• Orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time so the
surface illumination angle will be nearly the same every time
• Earth observation satellites
9.1 Physical Basics
hBp://cimss.ssec.wisc.edu/sage/
– To scale
representation of the Earth, LEO, and MEO
9.1 Physical Basics
the Earth LEO
MEO
• Passive systems: photography, scanner (optomechanical, optoelectronical)
• Active systems: radar sensors
9.2 Recording Techniques
reflected solar radia$on thermal
radia$on reflected ar$ficial
radia$on
R R
T/R
passive systems ac$ve systems
• Passive technique
• VIS and NIR (400-1000 nm)
• Analog storage medium
• Common types of films
– Black and white/panchromatic:
• Highest geometric resolution
– Infrared
• Unusual representation
• Contrastier
• Distinction between coniferous and deciduous forests
• Surfaces of water easier to identify
9.2 Photographic Systems
[AS14]
– Color/chromatic:
• Worse geometric resolution as black and white, better thematic interpretability
– Color infrared films:
• The blue-sensitive layer is replaced by an emulsion sensitive to a portion of the near infrared region
• Good thematic interpretability (vegetation)
9.2 Photographic Systems
[AS14]
• Example: Cosmos with KVR 1000 Camera
– Russian spy satellite
– Polar, sun-synchronous – Altitude 200km
– Ground resolution 2m – Black and white film – Durability 45 days – Missions 1981–2000
9.2 Photographic Systems
hBp://www.spo$mage.fr/web/en/186-‐kvr-‐1000.php
• Example: digital
aerial orthophotos of Braunschweig
– Central projection – Planimetrically
corrected
– 30. March 2014
9.2 Photographic Systems
hBps://www.braunschweig.de/
• Disadvantages
– Difficult radiometric calibration – Low spectral bandwidth
– Analog data
• Advantages
– Relatively cheap – High resolution
– "Spontaneous"
recording of areas
9.2 Photographic Systems
hBp://saturn.unibe.ch/.../Fotogrammetrie-‐Bildflug.pdf
• Optomechanical scanner
• A rotating 45 degree scan
mirror continuously scans the Earth beneath the
platform perpendicular to the direction of flight
• The system collects data one pixel at a time sequentially
• A scan line (mirror rotation) is equivalent to the image swath
• The forward motion of the platform used to acquire a scene with sequential scan lines
9.2 Whisk Broom Scanner
hBp://www.mikroelektronik.fraunhofer.de/
9.2 Whisk Broom Scanner
scan direc(on
aperture angle
al(tude
sensor plaHorm
flight direc(on
a: geometric resolu$on > ground segment s: swath width
instantaneous field of view IFOV: pixel
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
motor
rota$ng mirror
radia$on
op$cal system telescope
beam spliBer dispersion prism
photodetectors beam spliBer
interference grid
electronics
amplifier, converter
streamer
magne$c tape HDDT, CCT
• Radiation imaging
– Mirror rotates around an axis parallel to the flight direction – The radiation is split into its various wavelengths and focused
onto detectors
– Stored on magnetic tape (HDDT, CCT), remote data transmission
9.2 Whisk Broom Scanner
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
• Advantages
– Precise spectral and radiometric measurements
– Wide total field of view – Digital data, remote data
transmission
• Disadvantages
– Relatively short dwell-time – S-bend
– Panoramic distortion
– Low SNR → limited radiometric resolution
9.2 Whisk Broom Scanner
hBp://landsat.gsfc.nasa.gov/images/archive/c0005.html
• Landsat
– American satellite series
• Landsat 1: 1972-1978
• Landsat 2: 1975-1981
• Landsat 3: 1978-1983
• Landsat 4: 1982-1993
• Landsat 5: 1984-2013
• Landsat 6: 1993 failure
• Landsat 7: since1999
• Landsat 8: since 2013
9.2 Whisk Broom Scanner
hBp://de.wikipedia.org/wiki/Landsat
1-‐3
6, 7 4, 5
– Orbit
• Near polar, sun synchronous
• Altitude: 907-913 km (Landsat 1-3), 705 km (Landsat 4-7 )
• Inclination: 99.2° (Landsat 1-3),
98.2° (Landsat 4-7)
• Orbital period:
approximately 100 minutes
→ 14 circulations per day
• Provide complete coverage
of the Earth every 18
(Landsat 1-3) respectively 16
days (Landsat 4-7)
9.2 Whisk Broom Scanner
ground trace for Landsat1-‐3 for one day [AS14]
LANDSAT 4,5 (1-‐3) LANDSAT 4,5 LANDSAT 7 sensor Mul$spectral Scanner
(MSS) Thema$c Mapper (TM) Enhanced Thema$c
Mapper Plus (ETM+) pixel size 79 x 79 m² 30 x 30 m² 30 x 30 m²
spectral
channels 1 (4) 0,50 -‐ 0,60 µm, green
2 (5) 0,60 -‐ 0,70 µm, red
3 (6) 0,70 -‐ 0,80 µm, near infrared
4 (7) 0,80 -‐ 1,10 µm, near infrared
1 0,45 -‐ 0,52 µm, blue-‐
green
2 0,52 -‐ 0,60 µm, green 3 0,63 -‐ 0,69 µm, red 4 0,76 -‐ 0,90 µm, near infrared
5 1,55 -‐ 1,73 µm, mid infrared
7 2,08 -‐ 2,35 µm , mid infrared
1 0,45 -‐ 0,52 µm, blue-‐
green
2 0,52 -‐ 0,60 µm, green 3 0,63 -‐ 0,69 µm, red 4 0,76 -‐ 0,90 µm, near infrared
5 1,55 -‐ 1,73 µm, mid infrared
7 2,08 -‐ 2,35 µm , mid infrared
thermal channel 6 10,4 -‐ 12,5 µm
(120 x 120 m²) 6 10,4 -‐ 12,5 µm (60 x 60 m²)
panchroma(c channel 8 0,52 -‐ 0,90 µm
(15 x 15 m²)
9.2 Whisk Broom Scanner
– Typical combination of channels
9.2 Whisk Broom Scanner
0,5-‐0,6 μm 0,8-‐0,9 μm
false colour composite
0,6-‐0,7 μm
true colour composite
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
infrared red
green
9.2 Whisk Broom Scanner
hBp://landsat.gsfc.nasa.gov/images/lg_jpg/f0012_77-‐89-‐06.jpg
• Optoelectronical scanner
• Employs a linear array of solid semi- conductive elements to acquire one entire line of spectral data
simultaneously
• Scan lines perpendicular to the direction of flight
• Forward motion of the platform to acquire a sequence of imaged lines to map a scene
• CCDs (charge coupled device) to serialize parallel analog signals
9.2 Push Broom Scanner
hBp://www.fotos.docoer-‐dig.de/
9.2 Push Broom Scanner
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
scan direc(on
: aperture angle
al(tude
sensor plaHorm
flight direc(on
a: geometric resolu$on > ground segment s: swath width
focal distance
lens
aperture angle sample mirror
CCD sensors
op$cal system radia$on
• Radiation imaging
– Tilted mirror, sometimes fixed sometimes tiltable
– CCD image sensors in the image plane of the lens: line scan camera
– Data storage in parallel memory chips, remote data transmission
9.2 Push Broom Scanner
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
• Spot (Systeme Probatoire d'Oberservation de la Terre) – French satellite series
• Spot-1: 1986-1990
• Spot-2: 1990-2009
• Spot-3: 1993-1997
• Spot-4: since 1998
• Spot-5: since 2002
• Spot 6: since 2012
• Spot 7: since 2014
– Two identical parallel sensors that can be operated independently
9.2 Push Broom Scanner
hBp://www.uni-‐potsdam.de/...
/febasis/febasis06_04-‐1206.pdf
hBp://www.fe-‐lexikon.info/images/Spot5.jpg
1-‐3
4
5
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
angled
view nadir-‐
looking
– Pivoting of the sensors can be employed for stereoscopy and also for a higher repeat circle – Sensors are operated from the ground stations
9.2 Push Broom Scanner
hBp://www.terraengine.com/Dgroundsta$on.cfm
– Orbit
• Sun synchronous
• Altitude: 822 km
• Inclination 98,7°
• Orbital period 101,4 min
→ approximately 14 circulations per day
9.2 Push Broom Scanner
SPOT 1-‐3 SPOT 4 SPOT 5
sensor HRV (Instrument Haute
Résolu$on Visible) HRVIR (High Resolu$on
Visible and Infrared) HRG (High Resolu-‐
$on Geometric) geometric
resolu(on
20 m (XS), 10 m (PN)
20 m (XS), 10 m (P)
10 m (VIS, NIR), 2,5/5 m (PAN), 20 m (MIR) radiometric
resolu(on 0,5-‐0,9 μm:
3 VIS, 1 NIR 0,5-‐1,75 μm:
3 VIS, 1 NIR, 1 MIR 0,45-‐1,75 μm:
2 VIS, 2 NIR, 1 MIR
hBp://spot5.cnes.fr/.../35.htm
9.2 Push Broom Scanner
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
Spot-1 HRV P-Modus
San Diego(USA),
panchromatic, resolution 20 m
Spot-1 HRV XS-Modus
Detroit(USA), false colour composite, resolution 30 m
Spot-5 HRG XS-Modus: stereo
9.2 Push Broom Scanner
hBp://www.uni-‐potsdam.de/.../febasis/febasis06_04-‐1206.pdf
Dead sea (Jordan), panchromatic, 11/2002 resolution 2,5 m
• Radio Detection And Ranging
• Principle:
– Transmitting radar
pulses (microwaves)
and recording the
reflected radiation
→ active
– The transit time
and the strength
of the reflected
signal is measured
9.2 Radar
[LKC15]
• Nadir:
– The local vertical direction pointing in the direction of the force of gravity at that location
• Range:
– Line of sight
• Azimuth:
– Direction of flight
9.2 Radar
hBp://ladamer.org/Feut/studium/fe1/FE1-‐06-‐Radar.pdf
• Recording parameters
– Polarization
• Direction of the electric field which is
perpendicular to the direction of
propagation in the transmitted radar
signal (H = horizontal, V = vertical)
→ 4 possibilities: HH, VV, HV, VH
– Depression angle θd – Pulse length
– Wavelength is divided into 5 bands
9.2 Radar
hBp://ladamer.org/.../FE1-‐06-‐Radar.pdf
K-‐band X-‐band C-‐band L-‐band P-‐band 0,7-‐1 cm 2,4-‐4,5 cm 4,5-‐7,5 cm 15-‐30 cm 77-‐136 cm
B GR2
GR1
R2 R1
A
A β B
• Azimuth resolution AR depends on beam width ( β ) and the ground range distance (GR)
→ Azimuth resolution is better in the near range
9.2 Radar
[LKC15]
and L: antenna length
λ: wavelength where
• Ground range resolution (GRR) depends on the pulse length ( τ ) and the depression angle ( θ )
– Distinction between
A and B only possible
if the pulse passed A
completely before
reaching B
→ Better ground range resolution in the far range
9.2 Radar
Pulse length τ
Front of return wave from A
Front of return wave from B
A B τ
2
<
Rear of
outgoing wave
[LKC15]
• In order to improve the resolution
– Ground range
• Decrease pulse length
– Azimuth
• Decrease wavelength
• Increase antenna length
• The azimuth resolution
is unacceptably coarse
for systems operating at satellite altitudes
9.2 Radar
hBp://ladamer.org/Feut/studium/fe1/FE1-‐06-‐Radar.pdf
• Synthetic aperture radar (SAR)
– Scene is illuminated over an interval of time → history of reflections – The further an object
the longer the time it is illuminated
– As changes in frequency are systematic separate components of the
reflected signal can be assigned to their correct position
9.2 Radar
hBp://ladamer.org/Feut/studium/fe1/FE1-‐06-‐Radar.pdf
• Doppler-effect
– Approaching → increase in frequency
– Receding → decrease in frequency
• Physical antenna as small as possible
• Azimuth resolution
independent of GR and λ
9.2 Radar
hBp://ladamer.org/Feut/studium/fe1/FE1-‐06-‐Radar.pdf
synthe$c aperture
radar pulse with frequency v2
frequency v2
object
v1 – v2 > 0 v3 – v2 < 0
• Comparison of the resolution between systems with real (a) and synthetic (b) aperture
9.2 Radar
hBp://ladamer.org/Feut/studium/fe1/FE1-‐06-‐Radar.pdf
• Interactions between radar signals and materials very complex as it depends on:
– Wavelength
– Incidence angle
– Electrical properties – Moisture
– Surface property
9.2 Radar
hBp://www.meteo.physik.uni-‐muenchen.de/.../fe_boden_micro.html
• Penetration depths of microwaves
– Increases with decreasing wavelength – Decreases with increasing
conductivity, which is also influenced by moisture
– Is higher for smoother
surfaces
9.2 Radar
vegeta(on
dry alluvium
glacier
[AS14]
• Problem-oriented quantitative analysis of radar images is difficult as it relies mostly on hardly comprehensible interdependencies
9.2 Radar
C-‐Band L-‐Band P-‐Band
hBp://www.ccrs.nrcan.gc.ca/resource/tutor/gsarcd/pdf/bas_intro_e.pdf
• Sentinel-1
– SAR satellites in the Copernicus program
– Sentinel-1A launched on 3 April 2014, Sentinel-1B on 25 April 2016
– Orbit
• Sun synchronous
• 693 km altitude
• 98.2° inclination
• Orbital period 98.5 min
• Repeat circle 6 days (two satellites)
9.2 Radar
hBp://104.131.251.97/copernicus/
– Mass: 2300 kg (including 130 kg fuel)
– Size: 2.8 m long, 2.5 m wide, 4 m high
with 2×10 m-long solar arrays and a 12 m-long radar antenna
– Solar array average power: 5900 W
– Battery capacity: 324 Ah
– Azimuth resolution:
5, 20, 40 m
– Ground range resolution: 5, 20 m
9.2 Radar
hBp://104.131.251.97/copernicus/
• Image of Ireland (May 2015)
– Blue: strong changes in bodies of water or
agricultural activities within 12 days
– Yellow: urban centers – Green: vegetated fields
and forests – Red and orange: bare
soil and rocks
9.2 Radar
hBps://directory.eoportal.org/
• Nepal earthquake displacement
– Image shows how and where the land uplifted and sank from the 7.8-
magnitude
earthquake that struck
Nepal on 25 April 2015
9.2 Radar
hBp://www.esa.int/
• Map of Greenland ice sheet velocity
– January–March 2015
– About 1200 radar scenes were used
– Colour scale in meters per day
9.2 Radar
hBp://www.esa.int/
• Light Detection and Ranging
• Active sensor
• Laser beams (UV, VIS near IR) to measure
– Distance – Speed
– Chemical composition and concentrations
• Often imprecisely called "laser-radar"
9.2 LIDAR
• Airborne Laserscanning
– The distance between the sensor and the surface to be measured is determined from the runtime of a light pulse
– By deflection of the laser beam and the forward movement of the aircraft a wide strip is scanned
9.2 LIDAR
ellip(cal scanning fibre scanner swiveling mirror
– Parameters
• Sampling rate
• Scan angle
• Scan frequency
• Altitude
• Aircraft speed
– Recorded data
• Position
• Orientation of the aircraft
• Angle of every emitted beam
• Measured distance
9.2 LIDAR
hBps://www.e-‐educa$on.psu.edu/geog481/
Last return (DTM)
Primary return (DOM)
– One laser beam might be reflected at different heights, e.g. in presence of vegetation:
• Primary return: originate from the first objects a lidar pulse encounters, often the upper surface of a vegetation canopy
• Well suited to create a
digital object model (DOM)
9.2 LIDAR
hBp://www.zt-‐stuBgart.de/.../gis_day2004_guelch.pdf
• Secondary returns: lower
vegetation layers and the ground surface
• Last return provides data for a digital terrain model (DTM) if the vegetation is not too dense
9.2 LIDAR
hBp://publik.tuwien.ac.at/files/PubDat_166922.pdf
emiZed
pulse first echo last echo
$me
$me
$me signal
strength
scrup terrain
discrete echo determina(on
full waveform digi(sa(on signal
strength
signal strength
– Coordinates of the reflection points:
• Calculated from the
position and orientation of the sensor (by GPS and INS), the deflection angle of the beam and the distance between sensor and reflection point
– Result: 3D point set along the trajectory
9.2 LIDAR
hBp://www.photo.verm.tu-‐muenchen.de/.../EFE03_Kap23.pdf
– Advantages
• Uniform, dense acquisition of points
• Acquisition of height information for DOM (with vegetation), as well as for DTM (without vegetation)
• Accuracy in height between 50 and 15 cm in position1m
• Fast area-wide acquisition
• Active measuring method, nearly independent of illumination
9.2 LIDAR
hBp://www.zt-‐stuBgart.de/.../gis_day2004_guelch.pdf
– Disadvantages
• Arbitrary points, no structure elements (prominent terrain points, borders)
• Only single points, interpolation necessary
• Relatively noisy
9.2 LIDAR
hBp://www.zt-‐stuBgart.de/{v/{vweb/veranstaltungen/GIS-‐Day/Rueckblick/gis_day2004_guelch.pdf
– Reconstruction of buildings from airborne LIDAR point clouds is still subject of research
• Building polyhedral models by intersecting detected planes
• Bottom-up reconstruction using a given number of building parts
• Top-down statistical
reconstruction of building roofs
9.2 LIDAR
[HBS11]
• Comparison between remotely sensed images and topographic maps
9.3 Image Processing
Proper(es
Remotely sensed image Topographic map Mapping not true to scale,
image scales are only approxima$ons, addi$onal errors if terrain is uneven
Mapping true to scale, only minor changes due to generaliza$on
Mapping not posi$onal accurate,
influenced by sensor alignment, grade, earth curvature, etc.
Mapping posi$onal accurate, only minor changes due to generaliza$on No parallel projec$on Orthogonal parallel projec$on of the
earth‘ s surface on the map reference plane
9.3 Image Processing
Content
Remotely sensed image Topographic map Communica$ng informa$on in
images Informa$on coded by graphic symbols
Content defined causally by
physical-‐chemical processes Content defined conven$onally, s$pulated map symbols, explained in a legend
High informa$on density, but
irrelevant data included Low informa$on density, but all topographically relevant
Unlimited diversity of forms Limited number of map symbols
Snap shot, contains transient data Contains only topographically stable data Content scale independent, no
selec$on Content scale dependent, reduc$on of
informa$on by generaliza$on
Up to date , short produc$on $me Not up to date, long produc$on $me, problem of revision
9.3 Image Processing
Readability and interpreta(on
Remotely sensed image Topographic map Varying image quality Uniform map quality
No readability, objects have to be
interpreted Objects are directly readable as they are represented by clearly defined symbols
Ambiguous, as interpreta$on depends
on the interpreter Unambiguous independent of the user
Real 3d impression possible, if third
dimension by stereoscopy captured No real 3d impression, third
dimension may only be coded by symbols
Interpreta$on scale dependent,
resolu$on determines if objects can be recognized
Readability scale independent, granted by generaliza$on
9.3 Image Processing
Visual comparison
Remotely sensed image Topographic map
[AS14]
• Geometric errors, distortions
– Inaccurate position and form of objects – Causes
• Recording techniques and system
• Relief
• Platform (instability, motion)
• Radiometric errors
– Faulty pixel values – Causes
• Atmospheric interference
• Topographical effects
• Technical defects (sensors, data transfer)
9.3 Image Processing
hBp://www.fas.org/irp/
• Goals of geometric corrections
– Represent objects in uniform scale and true geometry (system correction)
– Register overlapped images of a scene from different dates and views (image to image registration)
– Register the image to real world map coordinates (image to map registration)
• The planimetrically
corrected image
is called
orthophoto
9.3 Geometric Errors
[AS14]
aerial photo, uncorrected corrected → orthophoto
• Radial displacement
– Causes objects to be displaced outward from the nadir
– Increases with the height of the object and distance from the nadir
– E.g. tops of buildings are
displaced outward relative to the bases
9.3 Geometric Errors in Photographic Systems
9.3 Geometric Errors in Photographic Systems
[SX08]
• Varying scale
– Mapping scale changes with variations in terrain
– The scale of objects closer to the camera is larger than that of objects being further away
– The mapping of a rectangle that covers a terrace is not a rectangle
9.3 Geometric Errors in Photographic Systems
higher
lower Map:
constant scale Aerial photo:
varying scale
terrace
hBp://homepage.univie.ac.at/thomas.engleder/
• Capturing a scene (image) takes a certain time
• During the recording time the earth rotates
eastward, so that the starting point of the last scan line is further west than that of the first line
• Displacement depends on the relative speed of the satellite, the earth rotation, and the size of the image
• Example (Landsat 7):
– 33.8°S (Sidney)
– Image size: 185 km
→ Offset: 10.82 km (~ 6%)
9.3 Geometric Errors in Scanners
pixel satellite
mo(on↓ earth rota(on →
hBp://ladamer.org/Feut/pdf/Kursbegleitung/
• Whiskbroom scanner
– The distance between sensor and terrain increases towards the edges
– Size of scanning spots increases towards the edges
9.3 Geometric Errors in Scanners
[AS14]
scan direc(on flight
direc(on
↑
hBp://ladamer.org/Feut/pdf/Kursbegleitung/
dbv_vl/dbv_vl_kapitel3.pdf
– If the angular speed is constant, the image seems to be increasingly compressed towards the edges
– More elevated surfaces are perpendicular moved away from the flight direction
9.3 Geometric Errors in Scanners
[AS14]
hBp://homepage.univie.ac.at/.../lba_fe_28102004.pdf
• Image geometry depends on the depression angle and the terrain
• Oblique perspective (i.e. side-looking) leads to relief displacement
– The type and degree
of relief displacement in the radar image is a
function of the angle at which the radar
beam hits the ground
9.3 Geometric Errors in Radar Systems
• Foreshortening
– Compression of those features in the scene which are tilted toward the radar
– Foreshortening effects are
reduced with increasing
incident angles
9.3 Geometric Errors in Radar Systems
hBp://www.ccrs.nrcan.gc.ca/.../bas_intro_e.pdf hBp://www.geoinforma$on.net/
• Radar shadow
– Areas not illuminated by the radar
– Caused by either concave or convex relief features if the slope on the opposite side of the antenna is larger than the depression angle
– Typical in high relief
terrain
– Occur in the down-
range direction
– Most prominent
with large incidence
angle illumination
9.3 Geometric Errors in Radar Systems
hBp://www.ccrs.nrcan.gc.ca/resource/tutor/gsarcd/pdf/bas_intro_e.pdf
• Layover
– Occurs when the reflected energy from the upper
portion of a feature is received before the return from its lower
– The top of the feature will be
displaced, or "laid over"
relative to its base
9.3 Geometric Errors in Radar Systems
hBp://www.ccrs.nrcan.gc.ca/.../bas_intro_e.pdf hBp://history.nasa.gov/
• Instability of the platform (aircraft)
9.3 Geometric Errors
change of flight speed
pitching change of
al(tude rolling
yawing
[AS14]
hBp://wdc.dlr.de/data_products/SURFACE/LCC/diplomarbeit_u_gessner_2005.pdf
• Model-based correction algorithm
– Develop a model for a given recording technique and platform that considers all its
inherent causes for distortions
– Parameterize the model to fit the actual conditions under which the image was taken
– Suitable if the kind and cause of the distortion is known, as earth rotation, satellite orbit or positional parameters of the platform
9.3 Geometric Corrections
hBp://www.der-‐schweighofer.at/
• Mathematical function to map the positions of
pixels on the coordinates of the same points in a map
– Independent of the sensor platform – Commonly used
– Uses ground control points (GCPs)
i.e. features visible on the image with
known ground coordinates
9.3 Geometric Corrections
corrected image raw image e
n e = f (c,r) r c
n = f (c,r)
– Assigns to each pixel a new position in the reference grid
– Needs 6 GCPs for two-dimensional second order
polynomials (12 unknowns)
– Involves the following steps:
I. Choice of a suitable function
(mapping)
II. Coordinate transformation III. Resampling (interpolation)
9.3 Geometric Corrections
• Example: image to image geocorrection
– Matching the coordinate systems or column and row systems of two digital images
– One image acting as a reference image and the other as the image to be rectified
• Reverence image
– Satellite imagery from GoogleMaps
• Input image
– Mathematically distorted reference image
9.3 Image Rectification
• Reference image
9.3 Image Rectification
• Mathematical distortions
– Central projection – Change of altitude – Pitching
– Rolling – Yawing
9.3 Image Rectification
• Distorted image
9.3 Image Rectification
• Ground control point (GCP)
– Need to be accurately located on the image, e.g.
highway crossings, building corners
– Should be well distributed on the reference and the distorted image
– Number of necessary GCPs depends on the function used for rectification
– Can be used to determine the quality of the rectification, if more GCPs than needed are defined
9.3 Image Rectification
+ +
+
+
• Reference image with ground
control points
9.3 Image Rectification
• Distorted image with ground
control points
9.3 Image Rectification
• Mapping functions
– Polynomials are often used
• Degree 1 needs 3 GCPs
• Degree 2 needs 6 GCPs
• Degree 3 needs 10 GCPs
9.3 Image Rectification
hBp://en.wikipedia.org/wiki/Polynomial
