Remote sensing is the acquisition of information about an object without physical contact
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].
Source of radiation:
- Passive remote sensing: record emitted or reflected radiation, typically sunlight;
- 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);
- Sonar: (passive) vessel, animal; (active) underwater objects;
- Ultrasound sensor: (active) sea level, tide;
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:
- 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.
- Oblique image: for (human) validation.
- 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;
Resolution of a sensor: smallest difference between indications of a sensor
that can be meaningfully distinguished [@ISO 19101-2].
- Radiometric: bit, 8 ~ 14; [256 ~ 16384 levels]
- Spectral: μm per band, 0.10 ~ 2.1; [visible spectrum 0.39 ~ 0.70 μm]
- Spatial: meter per pixel, 1 ~ 1000;
- Temporal: for time-series studies, cloud-averaged image for deforestation and mapping.
Sensor model: description of the radiometric and geometric characteristics of a sensor
Calibration: quantitatively define a system's response to known, controlled signal inputs
Validation: assess the quality of the data products derived from system outputs, by different means
- 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†:
- Level 0: Raw data
- 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;
- Level 2: Derived geophysical variables, at full instrument resolution.
- 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
Earth Observation Data Products
See the main article about Earth Observation Data Products.
- 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];
- 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