Remote sensing is the acquisition of information about an object without physical contact [@ISO 19101-2]. This term is mostly in contrast to on-site observation.

Radiance: at a point on a surface and in a given direction, the radiant intensity of the surface divided by the projection area [@ISO 31-6].

Band: range of wavelengths of electromagnetic radiation that produce a single response by a sensing device [@ISO 19101-2].

Scene: spectral radiances of a view of the natural world as measured from a specified vantage point in space at a specified time [@ISO 22028-1].

Categories

Source of radiation:

  1. Passive remote sensing: record emitted or reflected radiation, typically sunlight;
  2. Active remote sensing: emit signal from aircraft or satellites and measure reflected radiation;

Type of radiation:

  • Visible light: CCD (Charge-coupled device, 感光耦合组件);
  • Infrared;
  • Sonar: (passive) vessel, animal; (active) underwater objects;
  • Ultrasound sensor: (active) sea level, tide;

Photogrammetry

Photogrammetry (摄影测量法) is a traditional and less sophisticated method of remote sensing, compared with satellite imagery.

Private vendors: Pictometry (Southwest US, ~95 aircrafts);

Drones have been used for photogrammetry in recent years.

Types and uses:

  1. Orthoimage: displacement of image points due to sensor orientation and terrain relief has been removed by orthogonal projection to a reference surface; for measurement and map making.
  2. Oblique image: for (human) validation.
  3. Stereophotogrammetry: topographic maps (地形图), rapid 3D mapping (Saab Group)

Earth Observation Satellite

Earth observation (EO) is the acquisition of satellite imagery of Earth.

Instrumentation technology and applications:

  • Radar (radio detection and ranging, 雷达):
    • aerial traffic control (speed limit enforcement);
    • large scale meteorological data (precipitation, wind velocity);
    • radar altimeter: seafloor, ocean wave height and wavelength;
  • LiDAR (light detection and ranging, 光学雷达):
    • weapon ranging (测距), laser-homing projectiles (激光自动寻的导弹);
    • vegetation, chemical concentration in the atmosphere;
  • InSAR (interferometric synthetic aperture radar, 干涉合成孔径雷达):
    • precise, large-scale digital elevation model, land cover and land use;

Data processing

Resolution of a sensor: smallest difference between indications of a sensor that can be meaningfully distinguished [@ISO 19101-2].

Imagery resolutions:

  1. Radiometric: bit, 8 ~ 14; [256 ~ 16384 levels]
  2. Spectral: μm per band, 0.10 ~ 2.1; [visible spectrum 0.39 ~ 0.70 μm]
  3. Spatial: meter per pixel, 1 ~ 1000;
  4. Temporal: for time-series studies, cloud-averaged image for deforestation and mapping.

Sensor model: description of the radiometric and geometric characteristics of a sensor [@ISO 19101-2].

Calibration: quantitatively define a system's response to known, controlled signal inputs [@CEOS WGCV].

Validation: assess the quality of the data products derived from system outputs, by different means [@CEOS WGCV].

Corrections:

  • Georeference: matching points on image to established benchmark;
  • Radiometric correction: convert monochromatic scale to radiance values;
  • Topographic correction: recover reflectivity in horizontal conditions from terrain-affected radiance values;
  • Atmospheric correction: transform gray-scale value to eliminate atmospheric haze;

Data processing levels†:

  1. Level 0: Raw data
  2. Level 1: Reconstructed data
    • (1a): Unprocessed data at full instrument resolution, time-referenced and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters;
    • (1b): Level 1a data processed to sensor units;
  3. Level 2: Derived geophysical variables, at full instrument resolution.
  4. Level 3: Variables mapped on uniform space-time grid scales.

Level 1 data is the most fundamental record with significant scientific utility. Level 2 data is the first directly usable data for most scientific applications, variables including ocean wave height, soil moisture, ice concentration, etc. Level 3 data is smaller and have regular spatial and temporal organization.

† As defined by NASA and EUMETSAT.

Earth Observation Data Products

See the main article about Earth Observation Data Products.

Elevation Models:

  • Digital elevation model (DEM): a coverage from 2-dimensional coordinates to elevation values.
  • Digital surface model (DSM);
  • Orthorectified radar intensity images (ORI);

A list of DEMs, compiled by Peter L. Guth of US Naval Academy.

Publicly available DEMs:

  • NASA JPL Shuttle Radar Topography Mission (SRTM):
    • SRTMGL1 (version 3, 2013): global land, 1 arc-second (~30 meter) [@JPL2013];
  • David Sandwell of UCSD:
    • SRTM30_PLUS (v11, 2014-11-29): 30 arc-second [@Becker2009];
    • SRTM15_PLUS (v1, 2014-11-29): nominal/partial 15 arc-second (~500 m) [@Sandwell2014];
  • General Bathymetric Chart of the Oceans (GEBCO):
    • GEBCO_2014 (version 20150318): 30 arc-second [@Weatherall2015];
  • Christian Hirt of Curtin University and Moritz Rexer of TU Munich:
    • Earth2014 (2015): 1 arc-minute topography, bedrock, ice-sheet, and shape [@Hirt2015];
  • NOAA:
    • ETOPO1 Global Relief Model (2009-08): 1 arc-minute [@Amante2009];
  • ASTER Global Digital Elevation Model (GDEM);
  • Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010);

🏷 Category=Geographic Information System