Remote sensing and modeling in forestry Lecture 4 Resolutions and sensors

Remote sensing and modeling in forestry Lecture 4 Resolutions and sensors Dario Papale Contributions: Vern Vanderbilt, TA- Quinn Hart, CCRS

Detector DN Filter 16 t0 to t1 25 t1 to t2 t2 to t3 10 EM energy EM energy that reach the detector generate an electric signal. This signal is sampled in a time interval (dt) and recorded as digital number (DN). Only a subset of wavelength pass through the filter and reach the detector. They are out “spectral band” How a TLR system works

Incident radiation – DN conversion DN Ll [Wm-2sr-1mm-1] noise

CCRS Characteristics of the TLR system • mirror • detectors • IFOV (Instantaneous field of view) • Ground resolution • FOV (Field of view) • swath (image width)

Different sensors

Platform characteristics Sensor characteristics • Technical characteristics • Orbit • Technical characteristics • Spectral characteristics • Geometric characteristics • View characteristics Image characteristics To chose the sensor an number of aspects and characteristics must be considered Resolutions: Spectral, Radiometric, Geometric, Temporal

Geostationary Rotates at the same Earth speed. Look at the same area on the ground continuously Platform characteristic: the orbit Sun synchronous quasi polar Come back over the same point after a time T, function of speed and hight, in the same solar time conditions

Inclination defines also the extension of the ground area explored: with low angles larger areas close to North and South poles are missed. Platform characteristic: the orbit Orbit Ground track i= inclination angle

The resolutions • Spectral: indicates the number and characteristics of the spectral bands where data are acquired. • Radiometric: sensitivity of the detector to receive EM energy and codificate it in DN. Sensors with higher sensitivity are able to recognize and register smaller differences in the incoming radiation quantity. In pratice it is the number of DN levels used to sample and registed the original signal. • Geometric: size of the elementar area on the ground from which the EM energy in measured (pixel). • Temporal: period of time between two subsequent acquisition of the EM signal from the same area.

Spectral resolution • Number, width and position of the acquisition spectral bands. • The band is a region of the spectrum where the detector is sensible. • Multispettral sensors have few, wide bands • Hyperspectral sensors have many narrow bands

Remote sensing of vegetation Wavelength (nm)

Spectral resolution Multi-spectral hyper-spectral

Radiometric resolution 2-bit = 4 radiance levels 8-bit = 256 radiance levels

Radiometric resolution 3 bit (8 levels) 2 bit (4 levels) 1 bit (2 levels) 8 bit (256 levels)

Spatial resolution In general, the spatial resolution should be less than half of the size of the smallest object to investigate, to be sure that at least one pixel will be fully on the object.

SPOT5 => 10 m Quick Bird => 2,8 m Landsat 7 ETM+ => 30 m

Reflectance is also function of the Sun-Surface-Sensor systme (BRDF) Observing an area or an object by multiple point of view gives more info Angular resolution There are today available sensors from satellite and airborne platforms that can see the surface from different angles: a new resolution is introduced: angular resolution

Sensor’s resolutions and description 30km x 30km Spatial Size of the pixel (IFOV) Size of the image Spectral Number of bands Wavelengths of central bands Wide of the band (FWHM) Temporal Hour of the acquisition Time between two acquisitions of the same area Radiometric Radiance registration - Bits/Pixel Noise 30m x 30m

Resolutions today available Spatial: 0.6 m - 4m 10 - 30m 100 - 500m 1k - 8km Temporal: 0.5 hr Daily Weekly Bimonthly Spectral: Pancromatic (1) Multispetral (2-6) Hyperspectral (10-100)

Relations between resolutions Swath size Pixel size Pixel size Bands width Time between acq. Swath size

Remote sensing and modeling in forestry Lecture 4 Resolutions and sensors