What is Remote Sensing?

1. What is Remote Sensing?

2. Defining Remote Sensing Remote sensing = collection of data about features or phenomena of the earth surface (and near surface) without being in direct contact • Lack of contact with features or phenomena • Sensors utilize electromagnetic radiation (EMR) • Collection of data • Analysis of data collected

3. Sensing • Data are collected by sensor • Passive – collection of reflected or emitted electromagnetic radiation • Active – Generates signal and collects backscatter from interaction with terrain • Imaging & Non-Imaging • Photographic vs. Non-Photographic

4. Distance – How remote is remote? • Platforms for sensors operate at multiple levels • Cranes • Balloons • Aircraft • Satellite • Permit near-surface to global scale data collection

5. Remote sensing: the collection of information about an object without being in direct physical contact with the object.

6. Remote Sensing vs. Aerial Photography • Remote sensing is performed using a variety of sensors and platforms that may operate in multiple parts of the EMR spectrum • Aerial photography is performed using film-based cameras that sense only in UV, visible, and NIR spectrum and are operated on aircraft • Aerial photography is a subset of remote sensing

7. Image vs. Air Photo Interpretation • Images can be produced by most remote sensing systems • Air photos are produced by aerial photographic camera systems • Air photo interpretation is a subset of image interpretation

8. Science vs. Art • Science • RS is a tool for scientific analyses • Draws on multiple scientific disciplines • Probabilistic • Art • Interpretation combines scientific knowledge with experience and knowledge of the world • Interpretation skill is primarily learned through practice

9. Simplified Information Flow • Passive systems – detect naturally upwelling radiation • Flow: Source  Surface  Sensor • Source, the sun, illuminates surface • Surface reflects/emits radiation • Sensor detects reflected radiation within its field of view (FOV). • Photographic systems – detected radiation exposes film • Non-photographic – detected radiation generates electrical signal • Interpretation – manual or machine

10. Complexities in Information Flow • Variation in illumination • Sun angle • Clouds • Aerosol concentrations, other scattering • Variation in surface properties/coverage • Soil moisture, vegetation growth/conditions, surface roughness affect reflectance properties • Reflectance properties are dependent on solar angles, ratio of diffuse and direct, viewing angle

11. Complexities of Information Flow (cont.) • Sensor/platform variation • Attitude • Altitude • Orbit • Film/wavelength sensitivities • Calibration or Optics • Processing/interpretation variation • Film or digital processing • Repeatability of interpretation results

12. In situ vs. Remote Sensing • Both attempt to observe/measure phenomena • In situ • Physical contact • Instruments for direct measure • May be source of error • Interaction with phenomena • Sampling method • Ground reference vs. “ground truth”

13. In situ or remote sensing? GroundMeasurement In Support of Remote Sensing Measurement Ground spectroradiometer measurement of soybeans Ground ceptometer leaf-area-index (LAI) measurement

14. Advantages of Remote Sensing • Different perspective • Obtain data for large areas • In single acquisition – efficient • Synoptic • Systematic • Obtain data for inaccessible areas • No effect/interaction with phenomena of interest

15. Disadvantages of Remote Sensing • Accuracy and Consistency • Artifacts • Noise • Generalization • Processing • Scale-related effects • High initial outlays for equipment and training

16. Data Collection - Sensors • Cameras (film based) • Metric, Strip, Panoramic, Multi-spectral • Video Systems • Video cameras, Return Beam Vidicon • Imaging Radiometers • Digital frame, Scanners, Pushbroom, Hyperspectral • Passive Microwave • Radar • Operational vs. State-of-the-art

17. Data Collection - Imagery • Panchromatic (monochrome or B&W) – sensitive across broad visible wavelengths • Color – may provide added discrimination • Color film • Color composites • Thermal – in region 3 microns to 1 mm, sensitive to temperature • Microwave – all weather capability

18. Three-way Interaction Model Between the Mapping Sciences as Used in the Physical, Biological, and Social Sciences Jensen, 2000

19. Art vs. Science • Image interpretation is not exact science • Interpretations tend to be probabilistic not exact • Successful interpretation depends on • Training and experience • Systematic and disciplined approach using knowledge of remote sensing, application area and location • Inherent talents

20. Image Interpretation - Defined Act of examining images for the purpose of identifying and measuring objects and phenomena, and judging their significance

21. Image Interpretation (II) Tasks • In order of increasing sophistication... • Detection • Identification • Measurement • Problem-Solving • Not necessarily performed sequentially or in all cases

22. II Tasks - Detection • Lowest order • Presence/absence of object or phenomena • Examples: buildings, water, roads and vegetation

23. II Tasks - Identification • More advanced than detection • Labeling or typing of the object/phenomena • Tends to occur simultaneously with detection • Examples: houses, pond, highway, grass/trees

24. II Tasks - Measurement • Quantification of objects / phenomena • Direct physical measurement from the imagery • Examples • Inventories (count) • Length, area and height of objects

25. II Tasks – Problem Solving • Most complex task • Uses information acquired in first three tasks to put objects in assemblages or associations needed for higher-level identification • With experience, recognition becomes more automatic and tasks become less distinct • Example: residential housing density

26. Interpreter Requirements - Cognition • Concerned with how interpreter derives information from the image data • Varies from individual to individual • Reasons for differences/inconsistencies among interpreters • Cognitive processes are concerned with perceptual evaluation of elements of interpretation and how they are used in interpretation process

29. Resolution & Discrimination Germane to Task • Resolution • The ability of a remote sensing system to distinguish between signals that are radiometricall/spectrally/spatially near or similar • Discrimination • The ability to distinguish an object from its background • Function of spatial, spectral, and radiometric resolution • Germane to task • What is required for the particular assessment/task

30. Resolution • Four components of resolution • Spatial • Spectral • Radiometric • Temporal

31. Spatial Resolution • Indication of how well a sensor records spatial detail • Refers to the size of the smallest possible feature that can be detected as distinct from its surroundings • Aerial Camera: function of of platform altitude and film and optical characteristics • Non-film sensor: function of platform altitude and instantaneous field of view (IFOV)

32. Lower (coarser) spatial resolution Higher (finer) spatial resolution

33. Spatial Resolution Jensen, 2000

34. The width of the specific EMR wavelength band(s) to which sensor is sensitive Broadband Few, relatively broad bands Hyper-spectral Many, relatively narrow bands Spectral Resolution

35. Spectral Resolution Jensen, 2000

36. Airborne Visible Infrared Imaging Spectrometer (AVIRIS) Datacube of Sullivan’s Island Obtained on October 26, 1998 Color-infrared color composite on top of the datacube was created using three of the 224 bands at 10 nm nominal bandwidth. Jensen, 2000

37. Radiometric Resolution • Ability of a sensor to distinguish between objects of similar reflectance • Measured in terms of the number of energy levels discriminated • 2n, where n = number of ‘bits’ (precision level) • Example: 8 bit data = 28 = 256 levels of grey • 256 levels = 0-255 range • 0 = black, 255 = white • Affects ability to measure surface properties

39. 1 - bit 2 - bit 8 - bit

40. Temporal Resolution • The ability to obtain repeat coverage for an area • Timing is critical for some applications • Crop cycles (planting, maximum greenness, harvest) • Catastrophic events • Aircraft • Potentially high • Actually (in practice) lower than satellites • Satellite • Fixed orbit • Systematic collection • Pointable sensors

41. TemporalResolution Landsat Data Acquisition June 1, 2001 June 17, 2002 July 3, 2003 16 days Jensen, 2000

42. Discrimination • Ability to distinguish object from its background and involves basic tasks of image interpretation: • Detection, identification, measurement, and analysis • As complexity of task increases, so do resolution requirements • Key: target to background contrast • Function of all 4 resolution elements • Example: vegetation type, soil/rock type, building type

43. Electromagnetic Radiation (EMR): Properties, Sources and Atmospheric Interactions

44. Sun Sensor Surface EMR as Information Link • Link between surface and sensor

45. Energy Transfer (cont) • Radiation • Energy transferred between objects in the form of electromagnetic waves/particles (light) • Can occur in a vacuum (w/o a medium)

46. EMR Properties • Wave Theory • EMR => continuous wave • Energy transfer through media (vacuum, air, water, etc.) • Properties • Quantum (Particle) Theory • EMR => packets of energy • Photons or quanta • Interaction of energy with matter

47. Wave Theory • Explains energy transfer as a wave • Energy radiates in accordance with basic wave theory • Travels through space at speed of light at 3 x 108 ms-1(meters per second)

48. Electromagnetic Wave Two components or fields E = electrical wave M = magnetic wave http://www.colorado.edu/physics/2000/waves_particles/

49. Quantum Theory • Wave theory doesn’t account for all properties of EMR • Interaction of EMR with matter (atoms) • Absorption • Emission • EMR is transferred in discrete packets (particles) • Photons (quanta)

50. Wavelength & Frequency Near Infrared Green Microwave