1. Satellite Image Characteristics
What to consider when selecting satellite imagery

2. Image Characteristics
Spatial resolution
Spectral characteristics
Temporal characteristics
Sensor sensitivity
Program History
Image surface area
Multi-angle capability
Tasking
Price and licensing
Browse options
Processing Options

3. 4 sensor characteristics that affect what can be seen in an image
Spatial resolution
Spectral characteristics
Temporal characteristics
Sensor sensitivity
Source: NASA

4. 4 sensor characteristics that affect what can be seen in an image
Spatial resolution
Spectral characteristics
Temporal characteristics
Sensitivity of the sensor

5. Often called the 4 types of resolution
Spatial resolution
Spectral resolution
Temporal resolution
Radiometric resolution

6. Spatial resolution Often referred to simply as resolution
Size of an image pixel in ground dimensions
Usually represented by the length of one side of a square (i.e., 30m resolution)

8. Spatial Resolution Advice
Moving from detection => identification => analysis requires finer resolution
Rule of thumb – select resolution ~1/10th the size of the feature you want to examine
Rule of thumb is not very useful – strongly linked to feature characteristics (contrast, location, shape…)
Get advice from others – experience is invaluable
High contrast between features allows detection of sub-pixel sized features

9. Spectral characteristics
Bandwidth
Range of wavelengths (colors) detected by a particular band
Band placement
The portion of the electromagnetic spectrum detected by a particular band
Defined by the low and high wavelengths of the range or the by center of the range
Number of bands
The number of bands imaged by the sensor
Often grouped as panchromatic (single band), multispectral (more than one band), or hyperspectral (usually over 100 bands)
Spectral resolution definition is inconsistent and varies to include some or all of the above characteristics

10. Spectral characteristics for Landsat ETM+
Bandwidth – width of rectangles
Band placement – location of the rectangle along the x-axis
Number of bands – number of rectangles

11. Hyperspectral sensors provide more detailed spectral information

12. Temporal characteristics
The temporal frequency or minimum time a particular feature can be recorded twice
Repeat frequency can be shorter than the overpass frequency of the platform if the sensor is pointable
Just because the satellite is flying over a feature doesn’t mean that feature will be imaged
Short repeat interval increase chances of acquiring a “clean” image
The specific time and date a feature is imaged
Note: Most satellite orbits do not pass over the poles so these data can not always be acquired

13. Northern Madagascar seasonal change
July
November

14. Northeastern Madagascar deforestation
1993
2000

15. Sensitivity of the sensor
Dynamic range of the sensor
Upper and lower limits of the intensity of the signal (reflected radiation) a sensor can measure
Some sensors have a high and low gain setting
Range of values that can represent the value of a pixel (quantization)
Often defined as the bit depth
Common DN ranges are (8-bit) and (10-bit)

16. 8 bits (0-255)
6 bits (0–63)
3 bits (0-7)
1 bit (0-1)

17. Program History When the program started How long the program ran
Will the program continue
Are historic images still available

18. Image Surface Area
The area on the ground covered by an image is called the footprint
Larger footprint increases chances that your study area falls on fewer images
Larger footprint usually reduces problems associated with mosaicing multiple images (color matching, geometric matching, different seasons…)

19. Multi-Angle Options Some sensors are pointable
Improves the ability to target specific areas instead of systematic collection
Can be used to collect stereo pairs for creation of DEMs
Off-angle viewing distorts the image although this is compensated for during processing
Effectively shortens repeat interval

20. Tasking
Many satellite remote sensing systems can be tasked to acquire imagery on a specific date or time period
Tasking can be expensive
Read the small print of the agreement to understand how “acceptable” (cloud cover, time frame, number of tries…) is defined

21. Price and Licensing Image prices range from free to >$100/km2
Some imagery can be distributed free of charge
Many commercial companies only license their imagery so a user never “owns” the data
Licensing agreements define how the imagery can be used and distributed – read them carefully before you buy

22. Browse Options
Most image providers have powerful browse capabilities with tutorials
Some systems have historic imagery scattered around the world at ground receiving stations
Regional centers sometimes have browse capabilities for a variety of remotely sensed imagery
Researching available imagery can be time consuming if you are interested in multiple image types

23. Processing Options Can be a bit confusing
Most vendors provide a variety of radiometric (adjusting the “colors”) and geometric (adjusting the image so it matches a particular orientation or a specific map projection) options
The simplest processing uses on-board calibration and orbital models
Improved processing involves off-the-shelf ancillary data such as a DEM or user supplied data to improve geometry
With appropriate software, skills, and data users can do their own radiometric and geometric processing
Even with pre-processed imagery a user may have to do additional processing if the imagery is to be combined with other spatial data

24. Types of satellite imagery
Passive sensors (use sunlight for energy source)
Low resolution (>100 meter resolution)
MODIS, AVHRR, SPOT Vegetation
Moderate resolution (15 – 100 meter resolution)
Landsat TM/ETM+, SPOT, ASTER, IRS
High resolution (<15 meter resolution)
IKONOS, Quickbird, OrbViewIRS, SPOT, Corona
Active sensors (generate their own energy)
Radar
Radarsat, ERS, Envisat, Space Shuttle
Lidar
Mostly airborne platforms for now
ICESat is only satellite lidar platform

25. MODIS (500m) – Composited using imagery acquired from June – September 2001

26. Landsat ETM+ (30m) - 2 April 2002

27. ASTER (15m) - 8 November 2003

28. CORONA (5m) – 4 March 1967

29. IKONOS Pan merge (1m) – 29 April 2002

30. IKONOS zoomed

31. Active systems
Active sensors provide their own illumination source whereas passive sensors, such as Landsat, use the sun for illumination.
Radar and lidar are the two most popular active remote sensing systems.
Active sensors can provide direct measurements of vegetation structure.
Currently a lot of research on developing active systems and using active sensors for ecological studies.

32. LIDAR LIDAR = LIght Detection And Ranging
Measures the distance between the sensor and a target and strength of return
Wavelengths from the blue through near-infrared are used
Lidar can be deployed as fixed or scanningsystems
Best known for acquiring DEM information
Can be used to measure vegetation height and structure
Measures discrete portions of the returned signal and more sophisticated systems can measure the entire waveform of the returned signal
Most systems are for airborne platforms although NASA’s ICESat is a satellite sensor has been used to measure tree height and other vegetation characteristics.

33. From: Lefsky, M.A., W.B. Cohen, G.G. Parker and D.J. Harding Lidar remote sensing for ecosystem studies. Bioscience 52(1)

34. RADAR RADAR = RAdio Detection And Ranging
Radar systems measure the strength of the backscatter (portion of energy received by the radar antenna) and the time delay between when the signal was emitted and when it was received
Radio wavelengths are much longer than visible or infrared wavelengths and because of that they are penetrate vegetation canopies and the longer wavelength radar systems can penetrate the soil surface
The way a radar signal interacts with depends on the object’s size, shape, surface roughness, the angle of the incident microwave energy, and dielectric constant
Radar systems can emit and receive vertically and horizontally polarized radio waves
Radar systems are flown on aircraft and satellite
Some microwave systems are passive instruments that measure emitted radio waves. These systems are used to measure soil moisture
Radar is used to map land cover, measure vegetation structure, and create DEMs
Processing and interpreting radar imagery requires special training

36. What if “traditional” imagery is not available?
Look for less common data sets such as:
Aerial photography
Corona imagery
Airborne imagery
AIRSAR
AVIRIS
Research satellites/sensors
Hyperion
Advanced Land Imager
Astronaut photography
RADAR
LIDAR
Modify analysis methods and/or study area
Modify project objectives

37. Andros, Bahamas
Astronaut photo

38. Andros, Bahamas
Astronaut photo

39. Landsat Scan Line Correction Error
Broken on May 2003
Error causes gaps in the images that start at the center and become wider at the edges
Data is still being obtained and sold for $300 per image (half the normal price)

40. Landsat Scan Line Correction Error
Data offered with “gaps filled” from older data
SLC-Off April 2004 Image
April 2003 Image
SLC-Off “Gap-Filled” Image

41. Kruger National Park – Republic of South Africa
Landsat ETM+ (left) and ALI (right)

42. AIRSAR - Howland forest, Maine

43. AVIRIS - Howland forest, Maine

44. So - How do you decide what to use?
Compare options using tables like the one at:
Image availability and cost are the two primary limiting factors
Easy to get lost in the details – use common sense
Get advice from someone with experience

45. Sources for data Global Land Cover Facility (GLCF)
USGS EarthExplorer
USGS Global Visualization Viewer
EROS Data Gateway
USGS National Map Seamless Data Distribution System
Astronaut Photography
MrSid GeoCover Landsat TM images
Terraserver
NGA Raster Roam
GeoData.gov

46. More sources for data Tropical Rain Forest Information Center (TRFIC)
TRIFC -
Landsat.org
Landsat.org -
African Data Dissemination Service
Centre for Remote Imaging, Sensing and Processing (CRISP)
ESA Earth Observation Earthnet Online
SPOT Vegetation products
Free data for Canada
Shuttle Radar Topography Mission (SRTM)
ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/ and GLCF
ASTER Protected Area Archive
Wim Bakker’s list of data directories and inventories