satellite image characteristics what to consider when selecting satellite imagery
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Satellite Image Satellite Image CharacteristicsCharacteristics
What to consider when What to consider when selecting satellite imageryselecting satellite imagery
Image CharacteristicsImage Characteristics
Spatial resolutionSpatial resolution Spectral characteristicsSpectral characteristics Temporal Temporal
characteristicscharacteristics Sensor sensitivitySensor sensitivity Program HistoryProgram History Image surface areaImage surface area Multi-angle capabilityMulti-angle capability Tasking Tasking Price and licensingPrice and licensing Browse optionsBrowse options Processing OptionsProcessing Options
4 sensor characteristics that 4 sensor characteristics that affect what can be seen in an affect what can be seen in an
imageimage
Spatial resolutionSpatial resolution Spectral characteristicsSpectral characteristics Temporal characteristicsTemporal characteristics Sensor sensitivitySensor sensitivity
Source: NASA
4 sensor characteristics that 4 sensor characteristics that affect what can be seen in an affect what can be seen in an
imageimage
Spatial resolutionSpatial resolution Spectral characteristicsSpectral characteristics Temporal characteristicsTemporal characteristics Sensitivity of the sensorSensitivity of the sensor
Often called the 4 types of Often called the 4 types of resolutionresolution
Spatial resolutionSpatial resolution Spectral resolutionSpectral resolution Temporal resolutionTemporal resolution Radiometric resolutionRadiometric resolution
Spatial resolutionSpatial resolution
Often referred to simply Often referred to simply as resolutionas resolution
Size of an image pixel Size of an image pixel in ground dimensionsin ground dimensions
Usually represented by Usually represented by the length of one side of the length of one side of a square (i.e., 30m a square (i.e., 30m resolution)resolution)
Spatial Resolution AdviceSpatial Resolution Advice Moving from detection => Moving from detection =>
identification => analysis identification => analysis requires finer resolutionrequires finer resolution
Rule of thumb – select Rule of thumb – select resolution ~1/10resolution ~1/10thth the size of the size of the feature you want to the feature you want to examineexamine
Rule of thumb is not very useful Rule of thumb is not very useful – strongly linked to feature – strongly linked to feature characteristics (contrast, characteristics (contrast, location, shape…)location, shape…)
Get advice from others – Get advice from others – experience is invaluableexperience is invaluable
High contrast between features High contrast between features allows detection of sub-pixel allows detection of sub-pixel sized featuressized features
Spectral characteristicsSpectral characteristics
BandwidthBandwidth Range of wavelengths (colors) detected by a particular Range of wavelengths (colors) detected by a particular
bandband Band placementBand placement
The portion of the electromagnetic spectrum detected The portion of the electromagnetic spectrum detected by a particular bandby a particular band
Defined by the low and high wavelengths of the range Defined by the low and high wavelengths of the range or the by center of the rangeor the by center of the range
Number of bandsNumber of bands The number of bands imaged by the sensorThe number of bands imaged by the sensor Often grouped as panchromatic (single band), Often grouped as panchromatic (single band),
multispectral (more than one band), or hyperspectral multispectral (more than one band), or hyperspectral (usually over 100 bands)(usually over 100 bands)
Spectral resolution definition is inconsistent and Spectral resolution definition is inconsistent and varies to include some or all of the above varies to include some or all of the above characteristicscharacteristics
Bandwidth – width of rectangles
Band placement – location of the rectangle along the x-axis
Number of bands – number of rectangles
Spectral characteristics for Landsat ETM+
Hyperspectral sensors provide more detailed spectral information
Temporal characteristicsTemporal characteristics
The temporal frequency or minimum time a The temporal frequency or minimum time a particular feature can be recorded twiceparticular feature can be recorded twice Repeat frequency can be shorter than the overpass Repeat frequency can be shorter than the overpass
frequency of the platform if the sensor is pointablefrequency of the platform if the sensor is pointable Just because the satellite is flying over a feature doesn’t Just because the satellite is flying over a feature doesn’t
mean that feature will be imagedmean that feature will be imaged Short repeat interval increase chances of acquiring a Short repeat interval increase chances of acquiring a
“clean” image“clean” image The specific time and date a feature is imagedThe specific time and date a feature is imaged
Note: Most satellite orbits do not pass over the Note: Most satellite orbits do not pass over the poles so these data can not always be acquiredpoles so these data can not always be acquired
Northern Madagascar seasonal change
July November
Northeastern Madagascar deforestation
1993 2000
Sensitivity of the sensorSensitivity of the sensor
Dynamic range of the sensorDynamic range of the sensor Upper and lower limits of the intensity of the Upper and lower limits of the intensity of the
signal (reflected radiation) a sensor can signal (reflected radiation) a sensor can measure measure
Some sensors have a high and low gain settingSome sensors have a high and low gain setting Range of values that can represent the Range of values that can represent the
value of a pixel (quantization)value of a pixel (quantization) Often defined as the bit depthOften defined as the bit depth Common DN ranges are 0-255 (8-bit) and 0-Common DN ranges are 0-255 (8-bit) and 0-
1023 (10-bit)1023 (10-bit)
8 bits (0-255) 6 bits (0–63)
3 bits (0-7)
1 bit (0-1)
Program HistoryProgram History
When the program startedWhen the program started How long the program ranHow long the program ran Will the program continueWill the program continue Are historic images still availableAre historic images still available
Image Surface AreaImage Surface Area
The area on the ground covered by an The area on the ground covered by an image is called the footprintimage is called the footprint
Larger footprint increases chances that Larger footprint increases chances that your study area falls on fewer imagesyour study area falls on fewer images
Larger footprint usually reduces problems Larger footprint usually reduces problems associated with mosaicing multiple images associated with mosaicing multiple images (color matching, geometric matching, (color matching, geometric matching, different seasons…)different seasons…)
Multi-Angle OptionsMulti-Angle Options
Some sensors are pointableSome sensors are pointable Improves the ability to target specific areas Improves the ability to target specific areas
instead of systematic collectioninstead of systematic collection Can be used to collect stereo pairs for Can be used to collect stereo pairs for
creation of DEMscreation of DEMs Off-angle viewing distorts the image Off-angle viewing distorts the image
although this is compensated for during although this is compensated for during processingprocessing
Effectively shortens repeat intervalEffectively shortens repeat interval
TaskingTasking
Many satellite remote sensing systems can Many satellite remote sensing systems can be tasked to acquire imagery on a specific be tasked to acquire imagery on a specific date or time perioddate or time period
Tasking can be expensiveTasking can be expensive Read the small print of the agreement to Read the small print of the agreement to
understand how “acceptable” (cloud understand how “acceptable” (cloud cover, time frame, number of tries…) is cover, time frame, number of tries…) is defineddefined
Price and LicensingPrice and Licensing
Image prices range from free to >$100/kmImage prices range from free to >$100/km22 Some imagery can be distributed free of Some imagery can be distributed free of
chargecharge Many commercial companies only license Many commercial companies only license
their imagery so a user never “owns” the their imagery so a user never “owns” the datadata
Licensing agreements define how the Licensing agreements define how the imagery can be used and distributed – read imagery can be used and distributed – read them carefully before you buythem carefully before you buy
Browse OptionsBrowse Options
Most image providers have powerful Most image providers have powerful browse capabilities with tutorialsbrowse capabilities with tutorials
Some systems have historic imagery Some systems have historic imagery scattered around the world at ground scattered around the world at ground receiving stationsreceiving stations
Regional centers sometimes have browse Regional centers sometimes have browse capabilities for a variety of remotely capabilities for a variety of remotely sensed imagerysensed imagery
Researching available imagery can be Researching available imagery can be time consuming if you are interested in time consuming if you are interested in multiple image typesmultiple image types
Processing OptionsProcessing Options
Can be a bit confusingCan be a bit confusing Most vendors provide a variety of radiometric (adjusting Most vendors provide a variety of radiometric (adjusting
the “colors”) and geometric (adjusting the image so it the “colors”) and geometric (adjusting the image so it matches a particular orientation or a specific map matches a particular orientation or a specific map projection) optionsprojection) options
The simplest processing uses on-board calibration and The simplest processing uses on-board calibration and orbital modelsorbital models
Improved processing involves off-the-shelf ancillary data Improved processing involves off-the-shelf ancillary data such as a DEM or user supplied data to improve geometrysuch as a DEM or user supplied data to improve geometry
With appropriate software, skills, and data users can do With appropriate software, skills, and data users can do their own radiometric and geometric processingtheir own radiometric and geometric processing
Even with pre-processed imagery a user may have to do Even with pre-processed imagery a user may have to do additional processing if the imagery is to be combined additional processing if the imagery is to be combined with other spatial datawith other spatial data
Types of satellite imageryTypes of satellite imagery Passive sensors (use sunlight for energy Passive sensors (use sunlight for energy
source)source) Low resolution (>100 meter resolution)Low resolution (>100 meter resolution)
MODIS, AVHRR, SPOT VegetationMODIS, AVHRR, SPOT Vegetation Moderate resolution (15 – 100 meter resolution)Moderate resolution (15 – 100 meter resolution)
Landsat TM/ETM+, SPOT, ASTER, IRSLandsat TM/ETM+, SPOT, ASTER, IRS High resolution (<15 meter resolution)High resolution (<15 meter resolution)
IKONOS, Quickbird, OrbViewIRS, SPOT, CoronaIKONOS, Quickbird, OrbViewIRS, SPOT, Corona
Active sensors (generate their own energy)Active sensors (generate their own energy) RadarRadar
Radarsat, ERS, Envisat, Space ShuttleRadarsat, ERS, Envisat, Space Shuttle Lidar Lidar
Mostly airborne platforms for now Mostly airborne platforms for now ICESat is only satellite lidar platformICESat is only satellite lidar platform
MODIS (500m) – Composited using imagery acquired from June – September 2001
Landsat ETM+ (30m) - 2 April Landsat ETM+ (30m) - 2 April 20022002
ASTER (15m) - 8 November ASTER (15m) - 8 November 20032003
CORONA (5m) – 4 March 1967CORONA (5m) – 4 March 1967
IKONOS Pan merge (1m) – 29 April IKONOS Pan merge (1m) – 29 April 20022002
IKONOS zoomedIKONOS zoomed
Active systemsActive systems
Active sensors provide their own Active sensors provide their own illumination source whereas passive illumination source whereas passive sensors, such as Landsat, use the sun for sensors, such as Landsat, use the sun for illumination.illumination.
Radar and lidar are the two most popular Radar and lidar are the two most popular active remote sensing systems.active remote sensing systems.
Active sensors can provide direct Active sensors can provide direct measurements of vegetation structure.measurements of vegetation structure.
Currently a lot of research on developing Currently a lot of research on developing active systems and using active sensors active systems and using active sensors for ecological studies.for ecological studies.
LIDARLIDAR LIDAR = LIght Detection And RangingLIDAR = LIght Detection And Ranging Measures the distance between the sensor and a Measures the distance between the sensor and a
target and strength of returntarget and strength of return Wavelengths from the blue through near-infrared Wavelengths from the blue through near-infrared
are usedare used Lidar can be deployed as fixed or scanningsystemsLidar can be deployed as fixed or scanningsystems Best known for acquiring DEM informationBest known for acquiring DEM information Can be used to measure vegetation height and Can be used to measure vegetation height and
structurestructure Measures discrete portions of the returned signal Measures discrete portions of the returned signal
and more sophisticated systems can measure the and more sophisticated systems can measure the entire waveform of the returned signalentire waveform of the returned signal
Most systems are for airborne platforms although Most systems are for airborne platforms although NASA’s ICESat is a satellite sensor has been used to NASA’s ICESat is a satellite sensor has been used to measure tree height and other vegetation measure tree height and other vegetation characteristics. characteristics.
From: Lefsky, M.A., W.B. Cohen, G.G. Parker and D.J. Harding. 2002. Lidar remote sensing for ecosystem studies. Bioscience 52(1) 19-30.
RADARRADAR RADAR = RAdio Detection And RangingRADAR = RAdio Detection And Ranging Radar systems measure the strength of the backscatter Radar systems measure the strength of the backscatter
(portion of energy received by the radar antenna) and the (portion of energy received by the radar antenna) and the time delay between when the signal was emitted and when time delay between when the signal was emitted and when it was receivedit was received
Radio wavelengths are much longer than visible or infrared Radio wavelengths are much longer than visible or infrared wavelengths and because of that they are penetrate wavelengths and because of that they are penetrate vegetation canopies and the longer wavelength radar vegetation canopies and the longer wavelength radar systems can penetrate the soil surfacesystems can penetrate the soil surface
The way a radar signal interacts with depends on the The way a radar signal interacts with depends on the object’s size, shape, surface roughness, the angle of the object’s size, shape, surface roughness, the angle of the incident microwave energy, and dielectric constantincident microwave energy, and dielectric constant
Radar systems can emit and receive vertically and Radar systems can emit and receive vertically and horizontally polarized radio waveshorizontally polarized radio waves
Radar systems are flown on aircraft and satelliteRadar systems are flown on aircraft and satellite Some microwave systems are passive instruments that Some microwave systems are passive instruments that
measure emitted radio waves. These systems are used to measure emitted radio waves. These systems are used to measure soil moisturemeasure soil moisture
Radar is used to map land cover, measure vegetation Radar is used to map land cover, measure vegetation structure, and create DEMsstructure, and create DEMs
Processing and interpreting radar imagery requires special Processing and interpreting radar imagery requires special trainingtraining
What if “traditional” imagery is What if “traditional” imagery is not available?not available?
Look for less common data sets such as:Look for less common data sets such as: Aerial photographyAerial photography Corona imageryCorona imagery Airborne imageryAirborne imagery
AIRSARAIRSAR AVIRISAVIRIS
Research satellites/sensorsResearch satellites/sensors HyperionHyperion Advanced Land ImagerAdvanced Land Imager
Astronaut photographyAstronaut photography RADARRADAR LIDARLIDAR
Modify analysis methods and/or study areaModify analysis methods and/or study area Modify project objectivesModify project objectives
Andros, BahamasAstronaut photo
Andros, BahamasAstronaut photo
Landsat Scan Line Correction Landsat Scan Line Correction ErrorError
Broken on May 2003Broken on May 2003 Error causes gaps in Error causes gaps in
the images that start the images that start at the center and at the center and become wider at the become wider at the edgesedges
Data is still being Data is still being obtained and sold for obtained and sold for $300 per image (half $300 per image (half the normal price)the normal price)
Landsat Scan Line Correction Landsat Scan Line Correction ErrorError
Data offered with “gaps filled” from older dataData offered with “gaps filled” from older data
SLC-Off April 2004 Image April 2003 Image
SLC-Off “Gap-Filled” Image
Kruger National Park – Republic of South Africa
Landsat ETM+ (left) and ALI (right)
AIRSAR - Howland forest, Maine
AVIRIS - Howland forest, Maine
So - How do you decide what to So - How do you decide what to use?use?
Compare options using tables like the one Compare options using tables like the one at:at:http://homepage.mac.com/alexandreleroux/http://homepage.mac.com/alexandreleroux/
arsist/arsist/ Image availability and cost are the two Image availability and cost are the two
primary limiting factorsprimary limiting factors Easy to get lost in the details – use Easy to get lost in the details – use
common sensecommon sense Get advice from someone with experienceGet advice from someone with experience
Sources for dataSources for data Global Land Cover Facility (GLCF) Global Land Cover Facility (GLCF)
http://http://landcover.orglandcover.org USGS EarthExplorerUSGS EarthExplorer
http://edcsns17.cr.usgs.gov/EarthExplorer/http://edcsns17.cr.usgs.gov/EarthExplorer/ USGS Global Visualization ViewerUSGS Global Visualization Viewer
http://http://glovis.usgs.govglovis.usgs.gov// EROS Data GatewayEROS Data Gateway
http://http://edcimswww.cr.usgs.gov/pub/imswelcome/plain.htmledcimswww.cr.usgs.gov/pub/imswelcome/plain.html USGS National Map Seamless Data Distribution SystemUSGS National Map Seamless Data Distribution System
http://http://seamless.usgs.govseamless.usgs.gov// Astronaut PhotographyAstronaut Photography
http://http://eol.jsc.nasa.gov/default.htmeol.jsc.nasa.gov/default.htm MrSid GeoCover Landsat TM imagesMrSid GeoCover Landsat TM images
https://https://zulu.ssc.nasa.gov/mrsidzulu.ssc.nasa.gov/mrsid// TerraserverTerraserver
http://terraserver-usa.comhttp://terraserver-usa.com Astronaut PhotographyAstronaut Photography
http://http://eol.jsc.nasa.gov/default.htmeol.jsc.nasa.gov/default.htm NGA Raster RoamNGA Raster Roam
http://geoengine.nima.mil/geospatial/SW_TOOLS/NIMAMUSE/webinter/rast_roam.htmlhttp://geoengine.nima.mil/geospatial/SW_TOOLS/NIMAMUSE/webinter/rast_roam.html GeoData.govGeoData.gov
http://www.geodata.gov/goshttp://www.geodata.gov/gos
More sources for dataMore sources for data
Tropical Rain Forest Information Center (TRFIC)Tropical Rain Forest Information Center (TRFIC) TRIFC - http://bsrsi.msu.edu/trfic/home.htmlTRIFC - http://bsrsi.msu.edu/trfic/home.html
Landsat.orgLandsat.org Landsat.org - http://www.landsat.org Landsat.org - http://www.landsat.org
African Data Dissemination ServiceAfrican Data Dissemination Service http://edcsnw4.cr.usgs.gov/adds/index.phphttp://edcsnw4.cr.usgs.gov/adds/index.php
Centre for Remote Imaging, Sensing and Processing (CRISP)Centre for Remote Imaging, Sensing and Processing (CRISP) http://www.crisp.nus.edu.sg/crisp.htmlhttp://www.crisp.nus.edu.sg/crisp.html
ESA Earth Observation Earthnet OnlineESA Earth Observation Earthnet Online http://earth.esa.int/http://earth.esa.int/
SPOT Vegetation productsSPOT Vegetation products http://free.vgt.vito.be/http://free.vgt.vito.be/
Free data for CanadaFree data for Canada http://geogratis.cgdi.gc.ca/clf/enhttp://geogratis.cgdi.gc.ca/clf/en
Shuttle Radar Topography Mission (SRTM)Shuttle Radar Topography Mission (SRTM) ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/ and GLCF ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/ and GLCF
ASTER Protected Area ArchiveASTER Protected Area Archive http://asterweb.jpl.nasa.gov/APAA/default.htm http://asterweb.jpl.nasa.gov/APAA/default.htm
Wim Bakker’s list of data directories and inventoriesWim Bakker’s list of data directories and inventories http://www.itc.nl/~bakker/invdir.html http://www.itc.nl/~bakker/invdir.html