Remote Sensing Part 1

Lecture Overview Introduction to electromagnetic waves What is remote sensing? Introduction to satellite imagery 5 resolutions of satellite imagery Image display

Energy and Electromagnetic Waves Processes that produce or release energy emit electromagnetic waves Electromagnetic waves can also come from objects that release energy previously absorbed from elsewhere For Example: The sun is emitting heat, light, and many other types of electromagnetic waves that are produced through the process of fusion You are also emitting electromagnetic waves right now as your body metabolizes calories and emits heat (infrared energy) So, if we could see infrared light, you would appear brighter than the wall Can you think of an example for emitting energy that was previously absorbed?

Electromagnetic Waves Examples of electromagnetic waves: Radio waves Infrared waves Red light Green light Blue Light Ultraviolet rays X rays Gamma rays

Electromagnetic Waves Wavelength () f = frequency: The number of waves passing through a point within a unit time (usually expressed per second) EMR energy moves at the speed of light (c): c = f  Energy () carried by a photon:  = h f [h=Planck constant (6.62610-34 Joules*seconds)] The shorter the wavelength, the higher the frequency, and the more energy a photon carries. For example, ultraviolet rays are shorter than visible light so they have more energy and are more dangerous (sunburns) C stays constant (in a vacuum) so if the frequency (i.e., energy goes up) what must happen to the wavelength?

One micrometer (μm) = one millionth (10-6) of a meter ers) One micrometer (μm) = one millionth (10-6) of a meter

You may also see wavelength in nanometers: One nanometer (nm) = one billionth (10-9) of a meter

What makes objects a certain color? The color of an object is determined by how much energy it reflects from each wavelength in the visible light portion of the EM spectrum. Objects reflect some wavelengths Objects absorb some wavelengths Objects don’t interact with some wavelengths (i.e., the waves pass through the objects largely unaffected) Examples White objects reflect all wavelengths of light Black objects absorb all wavelengths of light Blue objects reflect blue light and absorb red and green light Why is vegetation green?

Spectral Signatures Note red edge As with color, different objects reflect different amounts of energy at other wavelengths

Remote Sensing Remote sensors are devices that sense energy from a remote location (i.e., a device not in physical contact with what it is sensing) Remote sensing is the science of acquiring, processing and interpreting information/data collected by remote sensors The most common application of remote sensing is classification Differentiating land cover classes (e.g., water, pavement, vegetation) Differentiating subtleties within classes (e.g., tree species)

Remote Sensing Passive - detect emitted/reflected energy from other sources (e.g., the sun) Satellite sensors Air photos Cameras Video recorders Active – emit energy and detect reflections Sonar Radar Lidar

Satellite Imagery Digital data is obtained by sensors on satellite platforms.

Imagery Resolutions Spatial resolution Temporal resolution Spectral resolution Radiometric Resolution View angle resolution

Spatial Resolution The area on ground represented by each pixel (i.e., raster cell size) There is a tradeoff between the image footprint (area captured by a single image) and spatial resolution Some sensors have different spatial resolutions for different bands Examples Advanced Very High Resolution Radiometer (AVHRR) ~ 1 km Landsat Multispectral Scanner (MSS) – 79 m Landsat Thematic Mapper (TM) – 30 m IKONOS – 1 m panchromatic /4 m multispectral

Temporal Resolution How often a satellite can obtain imagery of a particular area For a fixed sensor (can only point straight down) this depends on: 1) The number of days between overhead passes at the same location 2) The swath width of the sensor (i.e. the footprint of the images produced) Temporal resolution is higher for sensors that can move/tilt (side-to-side), but views from angles introduce distortion Examples Landsat - 16 days AVHRR - daily IKONOS - 1 to 3 days (sensor can tilt)

Spectral Resolution The specific wavelength intervals in the electromagnetic spectrum captured by each sensor Determined by the number, spacing and width of sampled wavelength bands Higher resolution results in more precision in representation of spectral signatures Hyperspectral imagery has very high spectral resolution (e.g., Aviris data has 220 bands) This makes a very detailed spectral signature, but the data are somewhat rare, expensive, and cumbersome to use

Radiometric Resolution The number of possible data values reported by the sensor (i.e., how sensitive the sensor is to changes in brightness of objects that it views) Range is expressed as a power (2n ) 8-bit resolution has 28 values, or 256 values Range is 0-255 11-bit resolution has 211 values, or 2048 values Range is 0-2048 The value in each pixel is called the Digital Number (DN) Brightness Value (BV)

View Angle Resolution The number of angles at which the ground objects are recorded by the sensor It can be useful to view objects from multiple angles for stereoscopic analyses Some features reflect light differently in different directions Isotropic vs. anisotropic reflectance Example The Multi-angle Imaging SpectroRadiometer (MISR) sensor has 9 view angles

Resolution Caveats Some remote sensing initiatives have multiple satellites/sensors, with later versions featuring improvements (i.e., better resolutions) E.g., Landsat MSS, TM, ETM+ Some satellites have more than one sensor on them, so resolutions are associated with the sensors and not satellites E.g., Earth Observing System (EOS) – 2 satellites that each contain MODIS, ASTER, etc. sensors

Landsat Thematic Mapper (TM) Resolution Example Landsat Thematic Mapper (TM) Spatial Resolution = 30 meters Greenville, NC

Landsat TM Spectral Resolution Wavelength (in micrometers) Description 1 2 3 4 5 6 7 0.45 - 0.52 0.52 - 0.60 0.63 - 0.69 0.76 - 0.90 1.55 - 1.75 10.4 - 12.5 2.08 - 2.35 Blue-green Green Red Near-IR Mid-IR Thermal Mid-IR

Spectral Signatures Complete Signature Landsat TM Signature Note the overall loss of detail This is the case for hyperspectral imagery * TM bands 5, 6, & 7 aren’t on this graph

Spectral Regions – Landsat TM BAND 1 (Visible Blue) PANCHROMATIC BLUE GREEN RED NEAR IR SHORT WAVE IR MID- WAVE IR LONGWAVE IR 0.4 0.5 0.6 0.7 1.1 3.0 5.0 14.0 0.45 - 0.52 mm Blue Visible - Used for bathymetry - water penetration to about 50m in clear water. Necessary for a true color image Illuminates Materials in Shadows Water Penetration for Bathymetry Soil / Vegetation Differentiation Deciduous / Coniferous Differentiation

Band 1 Band 1 is used with bands 2 and 3 to construct true color composites to assess a variety of features.

Spectral Regions – Landsat TM BAND 2 (Visible Green) 0.52 - 0.60 mm Water Penetration for Bathymetry Clear and Turbid Water Contrast Discrimination of Oil on Water Green Reflectance Peak of Healthy Vegetation BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Green Visible - Peak chlorophyll reflectance band, used for vegetation analysis. Also bathymetry - water penetration to about 40m.

Band 2 Band 2 is used with bands 1 and 3 to construct a true composite and bands 3, 4, and 7 to construct near and shortwave infrared composites to assess a variety of features.

Spectral Regions – Landsat TM BAND 3 (Visible Red) 0.63 - 0.69 mm Vegetation Differentiation Chlorophyll Absorption Limited Water Penetration for Bathymetry BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Red Visible - Provides added value information to vegetation analysis.

Band 3 Band 3 is used with bands 1 and 2 to construct true color composites and bands 4 and 2 to construct near infrared composites to assess a variety of features.

Spectral Regions – Landsat TM BAND 4 (Near Infrared) Vegetation Analysis Shoreline Mapping Landcover Discrimination 0.76 - 0.90mm BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Near Infrared - This is the primary band used to assess vegetation. Analysts can also assess camouflage detection and land water delineation using band 4.

Band 4 Band 4 is used with bands 3, 5, 7 and 2 to construct near and shortwave infrared composites to assess a variety of features.

Spectral Regions – Landsat TM BAND 5 (Short-wave Infrared) 1.55 – 1.75mm Fire Mapping Discrimination of Oil on Water Moisture Content of Soil and Vegetation Snow / Cloud Differentiation Vegetation Analysis BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Short-wave Infrared - Camouflage detection, change detection, vegetation analysis.

Band 5 Band 5 is used extensively with bands 3, 4, 6, and 7 to construct shortwave and longwave infrared composites to assess a variety of features.

Spectral Regions – Landsat TM BAND 6 (Long-wave Infrared) 10.4 – 12.5 mm Thermal Analysis Vegetation Density Urban Heat Islands BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Long-wave Infrared - Thermal analysis.

Band 6 Landsat has the ability to sense EM radiation in the LWIR region using band 6. Unlike other TM bands, band 5 has a spatial resolution of 60 meters on Landsat 7(etm+) (120 meters on 4 & 5… TM) It can be useful for geologic mapping, vegetation classification, vegetation stress detection, soil moisture content, fire management, thermal pollution, and ocean current studies. Band 6 is used with bands 5 and 7 to construct infrared composites to assess a variety of features. There is also a landsat TM panchromatic band with 15 m resolution ** The spatial resolution of Landsat TM band is ≠ that of the other bands

Spectral Regions – Landsat TM BAND 7 (Mid-wave Infrared) 2.08 – 2.35 mm Solar Reflectance From Metal Roofs Smoke Penetration Daytime Reflectance Mixed With Emitted EM Radiation Nighttime Emitted EM Radiation BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR 14.0 5.0 3.0 1.1 0.7 0.6 0.5 0.4 PANCHROMATIC Mid-wave Infrared.

Band 7 Band 7 is used extensively with bands 2, 4, 5, and 6 to construct shortwave and longwave infrared composites to assess a variety of features.

Landsat Bands Band 1 Band 2 Band 3 BLUE GREEN RED NEAR IR SHORT WAVE IR MID- LONGWAVE IR Review the differences in features between the bands.

Resolution Example AVHRR English Channel Spatial Resolution: 1 km (swath width = 2700 km) Temporal Resolution: daily Spectral Resolution: 5 bands (1 visible, 1 NIR, 1 mid-IR, 2 thermal) Radiometric Resolution: 8-bit (cells 0-255 for each band) View Angle Resolution: 1 view angle English Channel

Resolution Example SPOT 5 Palm Springs, CA Spatial Resolution: 20m, 10m, 2.5m (~ 60 km footprint) Temporal Resolution: 1 day Spectral Resolution: 5 bands (1 red, 1 green, 1 NIR, 1 mid-IR, 1 pan) Radiometric Resolution: 8-bit (cells 0-255 for each band) View Angle Resolution: unlimited (movable sensor) 20 = mid-IR 10 = Red, Green, NIR 2.5 = Pan Palm Springs, CA

Resolution Example IKONOS Sydney Olympic Park Spatial Resolution: 4m, 1m (swath width ~ 11-13 km Temporal Resolution: 3-5 days (144 days for nadir) Spectral Resolution: 5 bands (1 red, 1 green, 1 blue, 1 NIR, 1 pan) Radiometric Resolution: 11-bit View Angle Resolution: unlimited (movable sensor) Sydney Olympic Park

Image Display Graphics display devices use three color guns Red, Green, Blue All colors can be formed from various combinations of these 3 colors (which is why they’re used in CRT computer/TV screens) The brightness values (BV) to be displayed will often have an 8-bit range 0 to 255 In remote sensing, we assign one band to each color gun to give color to the image

Spectral Bands vs. Image Displayed For digital data the spectral bands of the imagery can be displayed in any way For example, red reflectance can be assigned to green light (on screen) or vice versa Features we see on screen are usually ≠ what we would see with our eyes in terms of color This is important because we can’t see in infrared etc., but we can still display this data by assigning it to a color that we can see

Image Display For a single band, the resultant color will be grayscale All three colors display the same value, so the colors are shades of gray Band 1 Band 1 Band 1 Red color gun Green color gun Blue color gun

Landsat Image Single Bands

Image Display For a multi-band image, the resultant color will depend on which bands are assigned to which color guns Red (3) Green (2) Blue (1) True Color Composite (321) Red color gun Green color gun Blue color gun Near Infrared (4) Red (3) Green (2) False Color Composite (432) Red color gun Green color gun Blue color gun

Color composite image Color Composite Image Band A Band B Band C Blue color gun Green color gun Red color gun

Landsat TM Composites 432 743 321 The order of the numbers indicates the color gun attributed to the bands red, green, and blue color guns. The first number gets displayed using the red color gun and so on. For example, in a 432 composite band 4 (NIR) reflectance gets displayed as red, band 3 (red) reflectance gets displayed as green, and band 2 (green) reflectance gets displayed as blue.