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