Slope Analysis
As we have discussed before Digital Elevation Models help us understand subtle changes in depth (or height) of the subject. We also think they are art in their own right.
Tucked away in the 1.4 release of Agisoft Photoscan Pro is a preference that adds slope analysis. The colours applied to the model are transformed from conveying height to showing the angle of the slope of the 3D mesh.
Here’s an example. First, we have the 2D orthophoto as derived from the model:
Ortho photo view of the lower cargo deck of the SS Thistlegorm. Norton H16 motorcycles can be seen in loaded in the rear of Fordson WOT3 trucks. Aircraft accumulator trollies can be seen to the left and at top in the load bed of a Leyland Retriever lorry. Image © Simon Brown.
Then we have the DEM set to altitude mode, in this case showing depth of water over the lower deck of the SS Thistlegorm:
Digital Elevation Model showing an altitude (or in our case, depth) view of the lower cargo deck of the SS Thistlegorm. Norton H16 motorcycles can be seen in loaded in the rear of Fordson WOT3 trucks. Aircraft accumulator trollies can be seen to the left and at top in the load bed of a Leyland Retriever lorry. Image © Simon Brown.
And now the same DEM but with the visualisation set to slope analysis:
Digital Elevation Model showing slope analysis view of the lower cargo deck of the SS Thistlegorm. Norton H16 motorcycles can be seen in loaded in the rear of Fordson WOT3 trucks. Aircraft accumulator trollies can be seen to the left and at top in the load bed of a Leyland Retriever lorry. Image © Simon Brown.
The P47-D Crash Site
In the example above, edges of the individual lorries as loaded onto the Thistlegorm stand out in sharp red-lined relief. When applied to the P47-D crash site the visual information conveyed really makes the scattered components stand out clearly:
The P47-D crash site in Weymouth Bay with slope analysis applied. Image © Simon Brown.
Preference not Processing
The good news is viewing slope analysis is a preference. Once the DEM is calculated viewing the slope analysis is quick and needs no further processing.
Change the DEM mode from Altitude to Slope – simple as that.
So we have another means to present data and if (and its a big ‘if’) we are ever granted a permit to investigate the Republic Thunderbolt P47-D crash site further this tool will be invaluable.