18-Geol-B2 Terrain Analysis · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2017 — 04-Geol-B2 Terrain Analysis. Three-hour, open-book exam; approved Casio/Sharp calculator permitted. The paper prints SIX questions; per the instructions only the first five as they appear in the answer book are marked (100 points total), but all six are answered below so the set stands as a complete study resource.
Reference texts: Lillesand, Kiefer & Chipman, Remote Sensing and Image Interpretation (7th ed.) — primary reference for spectral/spatial/radiometric resolution, radar imaging geometry, Landsat sensor comparisons, and image-interpretation elements (Q1, Q2, Q3, Q5); Sabins, Remote Sensing: Principles and Interpretation (3rd ed.) — radar depression-angle geometry, albedo, atmospheric correction, Landsat 8 TIRS (Q1, Q2, Q5); Mollard, J.D. & Janes, J.R., Airphoto Interpretation and the Canadian Landscape (Energy, Mines and Resources Canada, 1984) — the exam's own required reference for the stereopair interpretation questions (Q4, Q6); Van Zuidam, Terrain Analysis and Classification Using Aerial Photographs — slope-form and karst terrain-classification context (Q1, Q6).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
a) Synthetic Aperture Radar (SAR) [5 marks]. SAR is an active, side-looking microwave imaging system that synthesizes a very long "virtual" antenna by exploiting the platform's forward motion: as the sensor flies past a ground target, it transmits a sequence of pulses and records the amplitude and phase (Doppler history) of each return. Coherently combining these returns computationally is mathematically equivalent to having used a physical antenna as long as the whole flight path over which the target stayed illuminated, so SAR's azimuth (along-track) resolution is set by the true antenna length alone and does not degrade with range or altitude — the opposite of the real-aperture case in Q2b, where resolution worsens linearly with range. Because it is active and operates in the microwave window, SAR images day or night and through cloud, smoke, and most precipitation, which real-aperture radar shares but optical sensors cannot. SAR underlies RADARSAT sea-ice and ship monitoring, and interferometric SAR (InSAR) for millimetre-scale ground-deformation and DEM generation.
b) Atmospheric windows [5 marks]. Atmospheric windows are the wavelength bands in which the atmosphere's constituent gases (chiefly water vapour, CO₂, and O₃) absorb and scatter only a small fraction of incident electromagnetic energy, so radiation in those bands reaches (or leaves) the Earth's surface with little attenuation. Sensors are deliberately designed to operate inside windows — the visible/near-infrared window (≈0.4–1.3 μm), two shortwave-infrared windows (≈1.5–1.8 and 2.0–2.4 μm), the thermal-infrared window (≈8–14 μm), and the microwave region (> ≈1 cm), which is almost entirely transparent regardless of cloud or weather. Between the windows — e.g. ≈2.5–3.5 μm and ≈5–8 μm, dominated by CO₂/H₂O absorption — the atmosphere is effectively opaque and no useful surface signal can be recorded from those wavelengths.
c) Spatial resolution [2.5 marks]. Spatial resolution is the smallest ground separation between two objects that a sensor can record as two distinct features, rather than blending them into a single averaged pixel. It is governed by the sensor's instantaneous field of view (IFOV) and the platform's altitude (ground IFOV ≈ altitude × IFOV in radians for an optical scanner) or, for a real-aperture radar, by the antenna's beamwidth and range (Q2b). Two targets are resolved only when their angular separation, as seen by the sensor, exceeds this limit.
d) Atmospheric correction [2.5 marks]. Atmospheric correction is the processing step that removes or compensates for the atmosphere's scattering and absorption effects (haze, Rayleigh and Mie scattering, gas absorption) from the raw at-sensor radiance recorded by a sensor, so that the corrected image approximates the true ground-leaving spectral reflectance. Common methods are dark-object subtraction (assumes a known-dark target, e.g. deep water or shadow, should read near-zero reflectance), radiative-transfer modelling (e.g. 6S, MODTRAN, using known atmospheric conditions), and empirical line calibration against ground targets of known reflectance measured at the time of overflight.
e) Slope [2.5 marks]. Slope is the rate of change of ground elevation with horizontal distance, expressed as the ratio rise/run, as a percentage (rise/run × 100), or as an angle θ = arctan(rise/run) from the horizontal. On stereo airphotos, slope is read from vertical exaggeration in the stereo model or inferred indirectly from the spacing/curvature of tonal and shadow patterns that mimic contour lines.
f) Albedo [2.5 marks]. Albedo is the fraction of incident shortwave (solar) radiation that a surface reflects rather than absorbs, ranging from 0 (perfectly absorbing) to 1 (perfectly reflecting). It is a first-order control on a surface's energy balance and hence its temperature: fresh snow has a high albedo (≈0.8–0.9), dense vegetation and dark wet soil are low (≈0.1–0.2), and open water is very low (≈0.05–0.1) except at low sun angles. Albedo differences help distinguish surface types in visible-band imagery and set the baseline against which thermal-infrared anomalies (Q5b) are interpreted.
