A flood modeler needs to determine which areas would be inundated if a river rises 3 meters. Why is a LiDAR-derived DTM preferred over an SRTM DEM for this analysis?
ASRTM has global coverage while LiDAR is only available locally
BLiDAR DTM has centimeter-level vertical accuracy and represents bare earth, while SRTM (~30m resolution, ~10m vertical accuracy) is a DSM that includes canopy and buildings, overestimating surface elevation in vegetated and urban areas
CSRTM data is classified and unavailable for civilian flood modeling
DLiDAR produces colored elevation maps while SRTM produces grayscale
Flood models require accurate bare-earth elevation to determine flow paths and inundation extent. SRTM's C-band radar reflects off vegetation canopy and building roofs, producing a DSM that overestimates ground elevation -- potentially hiding low-lying areas from the flood model. SRTM's ~10m vertical uncertainty also exceeds the 3m flood scenario. LiDAR penetrates vegetation (via last returns) to map bare earth with ~10 cm accuracy, revealing the true terrain that water would follow.
Question 2 True / False
The terms DEM, DTM, and DSM all refer to the same type of elevation data.
TTrue
FFalse
Answer: False
DEM is a general term for any gridded elevation representation. DTM specifically represents the bare-earth terrain surface (no vegetation or buildings). DSM represents the elevation of the highest surface including tree canopy, buildings, and other structures. The distinction matters enormously: a DTM is needed for flood modeling (water flows on the ground), while a DSM is needed for viewshed analysis (visibility is blocked by trees and buildings) or urban planning.
Question 3 Short Answer
Explain how InSAR generates a DEM from two SAR images acquired from slightly different orbital positions.
Think about your answer, then reveal below.
Model answer: Two SAR antennas (or one antenna in two orbital passes) view the same terrain from slightly different positions, creating a baseline. The phase difference between the two radar returns for each pixel is related to the path length difference, which depends on the viewing geometry and the terrain height. By knowing the precise baseline geometry and unwrapping the phase (which repeats every 2-pi radians), the height of each pixel can be calculated trigonometrically. This is how SRTM generated near-global elevation data in 11 days using a fixed 60-meter baseline on the Space Shuttle.
InSAR elevation measurement is essentially parallax (like stereo vision) measured through phase differences rather than pixel offsets, achieving high precision from the sub-wavelength sensitivity of phase measurements.