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Background: Aerial photos / LiDAR.

Analysis of stereoscopic (3D) aerial photos and LiDAR provides fast, affordable access to a wide range of useful information. It can be used to identify and characterize the best route for linear infrastructure (roads, railway lines, power lines, pipelines), track site changes over a number of years and identify potential borrow sources (quarries and sandpits) or any other observable feature on the aerial photos. Stereoscopic analysis has much broader potential than 2D visualizations like Google Earth (ref.: Poly-Géo White Paper), but it also has limitations, in particular stemming from photo quality and resolution/scale, as well as the types of terrain to be analysed. Densely wooded areas, for instance, can make interpretation of surface materials very difficult, even hazardous.

Example of a Digital Terrain Model (DTM) generated from LiDAR surveys.

Fortunately, this limitation can be overcome by airborne laser scanning, or LiDAR (light detection and ranging), because computer processing can classify the raw data in such a way as to remove shrub and tree cover or other unwanted elements. The resulting digital terrain models (DTMs) and shaded relief maps (hillshading) provide a realistic and detailed (digital) view of the ground surface without trees or shrubs. Landforms that are otherwise invisible are revealed, providing a great deal of information that is key to an informed analysis. Notwithstanding the exceptional results obtained, DTMs are not a panacea, however, and an accurate interpretation ultimately depends on the keen eye of geomorphologists and various earth scientists. When the data are available, combined analysis of 3D aerial photos and DTMs is by far the best option. DTMs should be considered an essential complement to traditional photo-interpretation, but not an end in themselves.

The observation of landforms and a range of other visual clues throws open the door to understanding and knowledge for the geomorphologist. The examples below demonstrate the strong complementarity of the two methods of analysis.
Mots clés : Aerial photos / LIDAR

Case #1
3D analysis of the aerial photos of this site made it possible to determine, with a good degree of confidence, the nature of the materials present, the excavation limits of the granular borrow areas, the potential volume of exploitable material and a host of other elements relevant to the analysis and understanding of the site. Without the support of the LiDAR data, however, it would have been impossible to identify the ancient shorelines of a proglacial lake that had developed on the surface of this large sand and gravel deposit or to measure the depth of the gullies carved into the surface of the deposit, which is generally a good indicator of a stratigraphic contact with a more erosion-resistant material.

Move the cursor from right to left to reveal the Digital Terrain Model (DTM)

Case #2
The esker segment, the kettles (closed depressions resulting from the melting of ice blocks following the deposition of sand and gravel around their edges during deglaciation), portions of dunes and the small rocky hill were clearly visible in the stereoscopic aerial photos. DMT analysis, however, reveals a host of details that were not visible in the photos. For example, a series of particularly well-developed shorelines (raised beaches) can be seen on the east and north sides of the esker, indicating that wave energy was significantly greater on these sides, meaning that they have the greatest potential for coarse-textured granular material. The parabolic dunes in the upper left side of the hillshade model were partially visible on the aerial photos, but the ridge could only be traced in the cleared areas. The small dune just northeast of the rocky hill was undetectable on the photos.

Move the cursor from right to left to reveal the Digital Terrain Model (DTM)

Case #3
The very dense forest in this sector greatly complicates the observation of the elements and clues needed to properly interpret the surficial materials, even for an expert. While the main landforms (riverbanks, river terraces, esker segments, kettles, ancient river channels, main gullies, rocky hills, etc.) are easily discernible, detailed interpretation is almost impossible over much of the area. The hillshade map, generated from LiDAR data with vegetation removed, reveals a truer picture. It clearly shows all the ancient river channels, the esker segments, the network of gullies on the east side of the hill, the depth of the gullies and a host of other details that facilitate analysis and greatly improve the accuracy of the interpretation. Examination of the DTM also revealed the presence of a relatively thick till deposit on the south side of the hill, which was not identifiable from the air photos alone..

Move the cursor from right to left to reveal the Digital Terrain Model (DTM)

For over 30 years, major players in the transportation, hydroelectricity, natural resources and mining sectors turned to Poly-Géo for guidance in the planning, the identification, exploration and exploitation of sources of construction materials.

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