18-Geom-B1 Digital Terrain Modelling · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2014 — 3 hours, closed book (any non-communicating calculator permitted). TWELVE numbered questions constitute a complete paper; each is of varying value and the schedule totals 100 marks. Most answers are required in essay format, so clarity and organization are graded. All twelve questions are solved below for completeness.
Reference texts: Li, Zhu & Gold, Digital Terrain Modeling — Principles and Methodology (CRC Press, 2005); Maune (ed.), Digital Elevation Model Technologies and Applications: The DEM Users Manual (2nd ed., ASPRS, 2007); Wilson & Gallant, Terrain Analysis — Principles and Applications (Wiley, 2000); Wolf, Dewitt & Wilkinson, Elements of Photogrammetry with Applications in GIS (4th ed., McGraw-Hill); Natural Resources Canada High-Resolution DEM (HRDEM) and CDEM product specifications. Canadian datums throughout (NAD83(CSRS), CGVD2013).
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.
Given. A digital satellite image (with a rigorous sensor model or rational polynomial coefficients) and a DSM of the scene.
Find. How the DSM is used to rectify (orthorectify) the imagery.
Method. High-resolution satellite imagery carries relief displacement just like an aerial photo, and for a true orthoimage the displacement of all raised objects—not only the terrain but buildings and canopy—must be removed. The DSM (which, unlike a bare-earth DEM, records the tops of those objects) supplies the correct height for every ground/object point. Rectification proceeds by the same differential principle: for each output ortho pixel at map position $(X,Y)$, the DSM gives the surface height $Z$; the satellite rigorous sensor model or its rational function model (RPC) then maps $(X,Y,Z)$ into the raw image line/sample, whose radiometric value is resampled into the ortho. Using the DSM (rather than a DEM) means building roofs and elevated features are placed in their correct planimetric position and their lean is removed, producing a true orthophoto. It also lets occluded areas behind tall objects be identified (and, in multi-image processing, filled). A bare-earth DEM would correct only the terrain and leave buildings displaced, so the DSM is essential for accurate rectification of high-resolution urban satellite scenes.
