Remote Sensing and Geomatics
Remote Sensing and Geomatics covers electromagnetic radiation, sensor resolutions, platforms, GIS data models, GPS and photogrammetry.
Core rules
- Remote sensing uses reflected or emitted EMR; atmospheric windows such as visible, near-IR and thermal IR allow observation.
- Four resolutions: spatial (pixel size), spectral (number and width of bands), radiometric (bit depth) and temporal (revisit period).
- Passive sensors use sunlight; active sensors such as radar supply their own energy and can image through cloud.
- GIS: vector data use points, lines and polygons; raster data use cells; topology preserves adjacency and connectivity.
- GPS positioning uses trilateration; four satellites are needed to solve x, y, z and receiver clock bias.
- Photogrammetric scale = f/H, where f is focal length and H is flying height above ground.
Worked example
An aerial camera has f = 152 mm and flies 3000 m above ground. Find the photo scale and ground sample distance for a 20 µm pixel. Scale = f/H = 0.152/3000 = 1/19,737, written 1:19,737. GSD = pixel size × scale denominator = 20 µm × 19,737 = 0.395 m. Thus one pixel covers about 39.5 cm on the ground.
Traps
Using height above sea level instead of above ground; inverting the scale; confusing spectral resolution with spatial resolution; thinking three GPS satellites are enough for a 3D fix; treating raster cells as topologically connected lines.