Photogrammetry and Remote Sensing
This domain tests the geometry of aerial and satellite imagery, stereo coverage, orthorectification, and sensor characteristics. Expect scale and flying-height computations, relief displacement, ground sample distance, and classification accuracy measures.
Image geometry
- Photographic scale at the negative equals focal length divided by flying height above ground, in the same units: scale = f / H.
- Relief displacement is radial from the principal point (nadir for a vertical photo): d = r times h / H, where r is the radial distance from the center, h the object height, and H the flying height. Tall objects lean away from the center.
- Stereo coverage uses about 60 percent endlap and 20 to 30 percent sidelap so every point appears on at least two frames; parallax produces the vertical exaggeration of the stereo model.
- Orthophotos are differentially rectified with a DEM, removing tilt and relief displacement, but their horizontal accuracy is limited by the DEM and control used.
Digital sensors
- Ground sample distance for a digital frame camera: GSD = pixel size times H divided by f.
- LiDAR delivers direct three-dimensional points with intensity and class codes; vertical accuracy is commonly specified as RMSE in the z coordinate.
- Multispectral indices: NDVI = (NIR - Red) / (NIR + Red), ranging from -1 to +1. Healthy vegetation is high; water and snow are low or negative.
- Classification accuracy comes from a confusion matrix: overall accuracy, kappa, and the distinction between producer accuracy (related to omission error) and user accuracy (related to commission error).
Worked example
A digital camera has 12-micrometer pixels, a 100 mm focal length, and flies 1,000 m above ground. GSD = 0.000012 m times 1,000 m divided by 0.100 m = 0.12 m per pixel. Reasoning check: longer focal length or lower altitude shrinks the GSD and improves detail. For scale comparison, a film camera with a 152.4 mm lens at 3,000 m above ground gives a scale of 0.1524/3,000 = 1:19,685. Relief displacement for a 30 m building at 60 mm radius on that same photography: d = 60 mm times 30/3,000 = 0.6 mm on the negative.
Traps that cost marks
Mixing units in scale or GSD computations, especially millimeters with meters; using height above datum instead of above ground; applying relief displacement measured from a photo corner rather than the principal point; assuming an orthophoto is perfectly accurate regardless of DEM quality; reading user and producer accuracy as the same quantity; reporting overall accuracy alone while ignoring class-level errors.