Glossary.
Plain-language definitions for 46+ key terms in space-domain geographic information systems. Use this page as a reference while working through the course.
- WGS84
- World Geodetic System 1984. The geodetic datum used by GPS and most satellite-derived coordinates. EPSG:4326 is its geographic (lat/lon) form.
- EPSG
- European Petroleum Survey Group code — a registry of coordinate reference system identifiers. EPSG:4326 = WGS84 lat/lon. EPSG:3857 = Web Mercator.
- Ellipsoid
- A geometric approximation of Earth's shape: bulged at the equator, flattened at the poles. WGS84 ellipsoid: equatorial radius 6378137 m, flattening 1/298.257223563.
- Geoid
- The equipotential surface of Earth's gravity field that best matches mean sea level. Differs from the ellipsoid by up to plus or minus 100 m globally.
- EGM2008
- Earth Gravitational Model 2008. The standard global geoid model published by NGA, accurate to about 15 cm.
- Mercator (Web Mercator)
- EPSG:3857. The conformal projection used by Google Maps, Mapbox, and most slippy web maps. Distorts area near the poles.
- UTM
- Universal Transverse Mercator. Earth divided into 60 6-degree-wide zones. Conformal and nearly equidistant within a zone. Cape Canaveral is UTM Zone 17N (EPSG:32617).
- Equirectangular
- Plate carree projection where latitude and longitude map directly to y and x. Cheap, common for global imagery. Distorts near the poles.
- MGRS
- Military Grid Reference System. NATO-standard coordinate notation using a global grid with variable precision (km to m).
- Vector data
- GIS data representing the world as discrete geometric objects (points, lines, polygons) with attributes. GeoJSON is the most common format.
- Raster data
- GIS data as a grid of cells, each holding a value. Satellite imagery is raster. GeoTIFF is the standard format.
- GeoJSON
- An open standard format for encoding geographic features in JSON. Used universally on the web.
- GeoTIFF
- A TIFF image with embedded georeferencing metadata. The standard raster format in GIS.
- COG
- Cloud-Optimized GeoTIFF. A regular GeoTIFF organized so HTTP-range-request access fetches just the needed bytes without downloading the whole file.
- Zarr
- A format for chunked, compressed, multi-dimensional arrays. Standard for cloud-native gridded data (climate reanalysis, time series).
- STAC
- SpatioTemporal Asset Catalog. A spec for cataloging geospatial assets with searchable APIs. Major STAC catalogs: Microsoft Planetary Computer, AWS Earth Search.
- PostGIS
- The spatial extension to PostgreSQL. Adds geometry and geography types, hundreds of ST_ functions, GIST spatial indexes.
- GIST index
- A PostgreSQL index type using an R-tree for spatial queries. Essential for performance on PostGIS geometry columns.
- Spatial join
- An operation that attaches attributes from one layer to another based on a spatial relationship (within, intersects, etc.) rather than a key match.
- TLE
- Two-Line Element set. NORAD's plain-text format encoding a satellite's Keplerian orbital elements plus drag and perturbation terms in 70 characters per line. Distributed by Space-Track.org and CelesTrak.
- SGP4
- Simplified General Perturbations 4. The standard orbital propagation algorithm for use with TLEs. Accurate to about 1 km for a fresh TLE.
- Keplerian elements
- The six orbital parameters that uniquely describe an orbit shape and orientation: semi-major axis, eccentricity, inclination, RAAN, argument of periapsis, true anomaly.
- Ground track
- The path on Earth's surface traced by the sub-satellite point of a satellite over time.
- Sub-satellite point
- The point on Earth's surface directly below a satellite.
- LEO
- Low Earth Orbit, under 2000 km. ISS, Starlink, Hubble, Landsat, Sentinel-2 all operate here.
- MEO
- Medium Earth Orbit, 2000 to 35786 km. GPS, GLONASS, Galileo operate here.
- GEO
- Geostationary Orbit, 35786 km altitude over the equator. GOES, Himawari, most communications satellites.
- Sun-synchronous orbit
- A nearly polar orbit at about 98-degree inclination where the satellite passes the equator at the same local solar time each orbit. Used for Earth observation.
- ABI
- Advanced Baseline Imager. The primary instrument on GOES-R series satellites. 16 spectral bands from visible to longwave IR.
- GOES-R
- NOAA's geostationary weather satellite series. GOES-18 (West, 137.2W) and GOES-19 (East, 75.2W) are the operational satellites.
- Himawari-9
- JMA's geostationary weather satellite at 140.7E. Covers East Asia and the western Pacific.
- Band 7
- GOES-R ABI Band 7 at 3.9 micrometers — the mid-wave infrared band used for thermal hotspot detection, including rocket plumes and wildfires.
- Brightness temperature
- Temperature of a perfect black body that would emit the observed radiance. Computed via the inverse Planck function. Standard unit for thermal IR analysis.
- NDVI
- Normalized Difference Vegetation Index, computed as (NIR - Red) / (NIR + Red). High for healthy vegetation, low for bare soil.
- SAR
- Synthetic Aperture Radar. Active microwave imaging that sees through clouds and works day or night. Sentinel-1 is the workhorse civilian C-band SAR.
- InSAR
- Interferometric SAR. Phase-difference analysis between two SAR acquisitions, capable of measuring ground deformation to the millimeter.
- Parallax in remote sensing
- Apparent shift of a high-altitude feature (e.g. rocket plume at 50 km) as seen from a satellite, compared to its true ground position. Must be corrected for accurate geolocation.
- CesiumJS
- The open-source 3D globe library. Industry standard for serious web-based 3D GIS.
- MapLibre GL JS
- The community fork of Mapbox GL JS. WebGL-based vector tile renderer for web maps.
- Vector tiles
- Pre-indexed pyramid of small geographic data tiles served as Protocol Buffers. Smaller, smoother, and more flexible than raster tiles.
- PMTiles
- A single-file alternative to MBTiles that can be served directly from S3 via HTTP range requests — no tile server needed.
- ITAR
- International Traffic in Arms Regulations. US law controlling export of defense-related articles and services, including some satellite imagery.
- NOTAM
- Notice to Air Missions (FAA). Pre-launch safety advisory defining airspace exclusion for launches.
- FIRMS
- Fire Information for Resource Management System. NASA's near-real-time wildfire hotspot data, used to filter out fire false-positives in plume detection.
- AIS
- Automatic Identification System. Maritime vessel tracking transmitted on VHF and aggregated globally.
- ADS-B
- Automatic Dependent Surveillance-Broadcast. Aircraft transponder data broadcasting position and identity.