C+S December 2020 Vol. 6 Issue 12 (web)

After the final triangulations were finished, Lafica then moved all the photos to his photogrammetric workstation to produce a DTM and view the imagery in stereo and create the precise 3D topographic map. He compiled terrain break lines across the entire cliff face––from the rocky base of the bluff to the top––that were accurate to 0.01 ft and symbolized planimetric features such as dense trees and brush, struc- tures, fencing and scattered rocks and boulders. He also delineated the MHW contour line along the shore as well as the water surface eleva- tion line to precisely illustrate the position of the tide line at the time of data acquisition in relation to the MHW line––a critical map element for Reichardt’s customer. “With conventional surveying, it’s difficult to scale the face of a cliff to get equal elevation for the survey to the MHW line,” says Reichardt. “And it’s challenging to acquire the detail of a rocky bluff. But Robert nailed it. The map had a perfect MHW line, the water surface elevation line, the exact- ness of the break lines and all the rocks and structures. It’s an incredible amount of detail that I think only a photogrammetrist can provide.” Lafica also created orthophotos and then used Inpho’s OrthoVista module, the photogrammetry tool for creating seamless orthomosaics, to stitch each orthorectified image together. In about two hours, the software mosaicked the orthophotos into one seamless, color-balanced and geometrically correct orthomosaic of the entire AOI at a 2D ac- curacy of 1.2 in. “OrthoVista is exceptionally good at mosaicking,” says Lafica. “To date, I haven’t found another product that can do it better.” About one week after the field survey, Lafica delivered the 3D topo- graphic map, the orthomosaic and a vector-based DTM to Reichardt, answering any question marks around the uncertainty of processing and mapping oblique UAS images in 3D. “This was a significant achievement,” says Lafica. “Without the Inpho technology, I wouldn’t have been able to deliver on this project. We proved that we can map in stereo from nadir drone imagery about three years ago, but this is beyond what I thought I could achieve in stereo mapping. Most importantly, this success gives us the opportunity to offer a unique service and build new business.” From Reichardt’s perspective, that new business is already waiting. “My clients are ecstatic with the mapping Robert provided,” says Reichardt. “They said they’ve never seen anything that good before. Traditionally, they’ve used conventional methods to acquire bluff surveys and capture the MHW line, which is ultra important for their coastal work, and it’s been costly and not as precise. Seeing this ortho- mosaic and topo map has transformed their traditional ways. They’re going to ask for more maps like these in the near future.” Given that, if Reichardt approaches Lafica about acquiring a cliff-face survey again, his response may still be frank, but he’ll likely answer with a much more assured, “I can do that.”

“It took me 2.5 days to complete this cliff survey 13 years ago with my conventional methods,” says Reichardt. “Robert and I did it in one eve- ning. That’s a huge time savings, but even more amazing is the end result.” Full frontal mapping in 3D To begin the mapping process, Lafica first needed to tie photos together to orientate the project. Because he captured both nadir and oblique images, he processed the two image sets separately, beginning with the nadir imagery. Lafica loaded the 127 nadir photos, the UAS GNSS data and the GCP positions into Trimble’s Inpho UASMaster photogrammetric soft- ware to automatically triangulate the images. Processing the images in batches, it automatically pinpointed 6,368 common features or tie points (TPs) with multiple connections across the images to match im- ages to each other. After the initial triangulation, Lafica then zoomed into each photo to locate the control point targets and attached precise coordinates to each target in the image. With that additional georefer- encing, UASMaster then performed the final triangulation, connecting all the imagery to the ground control and setting the images in the correct positional accuracy for orthorectifying and mapping. “The software is incredibly good at automatic aerial triangulation and it saves us inordinate time,” says Lafica. “Without it, we’d have to find common image points and tie them together manually, which could take days. UASMaster triangulated the photos in one hour. And the matching capabilities are particularly significant along the beach which is typically homogenous and low-textured.” The true test of the software’s triangulation capabilities came next, when Lafica loaded the 80 oblique images into UASMaster for aerial triangulation. To his surprise, the software automatically deciphered the unique characteristics of off-nadir imaging and processed them just as well as traditional UAS photos––all in 45 minutes. “One of the reasons we use UASMaster is because of its photogram- metric power,” says Lafica. “But to accurately tie the oblique images together, the software needed to use different routines and algorithms to figure out puzzles like the camera lens distortion values. And it did. That was a level of smart I didn’t expect.” Lafica used Trimble’s Inpho OrthoVista module to mosaic the UAS orthorectified images into a seamless orthomosaic at a 2D accuracy of 1.2 inches.

MARY JO WAGNER is a Freelance Writer, Editor, and Media Consultant based in Vancouver, BC. She can be reached at mj_wagner@shaw.ca.


december 2020


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