Lecture 4 The Golden Age of Geospatial Data

Sensor Types

• two basic types, both detect electromagnetic radiation

  • passive sensors absorb radiation coming from earth, no emission

    • multispectral and hyperspectral imaging

  • active sensors emit electromagnetic radiation

    • RADAR and LiDAR imaging

Passive Sensors

• ‘optical’ passive satellites capture spectral bands on the visible spectrum, with some capabilities at wavelengths just shorter and longer than visible

  • typical satellite image (e.g. google maps) is a combo of red, green and blue bands

• many optical satellites in orbit

• can also use band ratios to figure out things 

  • vegetation (NDVI), snow (NDSI), water (NDWI), sediment, urban areas, etc.

Spectral Band

• dataset consisting of one wavelength on the EMS

Optical Satellites

• Sentinel deployed by European Space Agency

• Landsat deployed by NASA

NDVI

• Normalised Difference Vegetation Index (NDVI)

  • tells you about the density of plants

  • =  (NIR - RED) / (NIR + RED)

  • NIR = near infrared band, RED = red band

  • trees absorb visible red light spectrums, increased NDVI means denser vegetation

Active Sensors

• RADAR

• LiDAR

RADAR

• can see through clouds (X-Band, C-Band), also trees depending on wavelength (L-Band)

• used for biomass, water, interferometry

Radar Interferometry

• radar interferometry combines multiple images to map topography and surface shifts, useful for volcanoes

  • can measure centimetre scale changes from orbit

LiDAR

• Light Detection and Ranging

• shoots a laser that bounces back, calculate distance based on speed of light

• can shoot many, many points to get a dense ‘point cloud’ 

• used for biomass and topography

• can see through trees

• Digital Surface Model includes trees, buildings, etc. but Digital Terrain Model only shows earth surface

Research based on global satellite data

• Global GHG emissions from biomass estimates, Harris et al 2021

• Tidal wetlands mapping, 

• Changes in live terrestrial biomass

• Mass balance of Greenland Ice Sheet