Author Topic: Questions regarding georeferencing, scaling and DEM  (Read 737 times)


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Questions regarding georeferencing, scaling and DEM
« on: October 03, 2023, 03:38:18 PM »
Hello, everyone!

I'm generating underwater (reefscapes) orthomosaics, and reading the user's manual I'm not being able to solve some issues regarding the topics mentioned in the title of this post.

1. Georeferencing
First of all, there's the issue of not being able to acquire georeferenced data from the photo's metadata, as they are obviously not avaiable underwater. I did, however, get one handheld GPS position from the boat, but I don't know how (or if it's possible, even) to use this single GPS reference of the project. If so, how can I input this information on the mosaic?

2. Scaling
Another issue I'm having is regarding GCPs. Along the reefscape, I  evenly distribute 5-7 metal bars measuring exactly 0.5m to use as scale bars. However, if I add one scale bar in a single metal bar and update the model, apparently the scaling works perfectly, which I verify using the ruler tool to measure the other bars, but I reckon that ideally we should use at least 3 scale bars. The problem is that, if I add markers and scale bars for every metal bar on the scene, then I get huge amounts of error. What could be causing this?

3. DEM
One important thing for measuring reefscapes is rugosity and structural complexity, and for these metrics I need to generate proper and accurate DEMs. I understand that I need to align the X and Y axis of the model on the grid, but I'm having trouble understanding how the Z axis behaves. I get that if I position the model (and it's region, of course) 'under' the grid, the points will be referenced as negative Z values, which is what I need, as I'm working underwater, so the depth should be expressed as negative Z. But how can I manually (and accurately) input the Z value (in other words, depth) of a GCP that I know the measured depth using a depth meter if the coordinates of a marker are related to the position of the model to the grid?

Thank you all in advance!