Depth scaling introduces huge error to scale bars

[Carol_Coral]

Dear Metashapers,

I hope you can help me figure out what is happening with my scaling.

As I mentioned on previous post, I do have underwater squared structures loaded with corals that I monitor using Structure from Motion photogrammety. Photos are taken from a distance of about 2m between the structure bottom and the camera. We have placed four "ruler bars" on the platform every time the pictures are taken. The main purpose of these bars is to be used as reference for scaling.

I've been creating DEM and Orthomosaic models for these structures using Metashape. However, I'm facing a problem with the scaling. I do add the markers (GCP) according to the location of the "ruler bars" to later use them to set my scale bars. In addition, I add another marker GCP at the center point of each of the four "ruler bars" and I use those markers for depth scaling.

Once I've created the four scale bars, I set the size in m of each scale bar and use the "update transform" to scale the model. After this step I do change the depth of my depth markers by changing the value under the Z column. I set the Z value to -2m. Then, after selecting those four depth markers, I use the "update transform" bottom. The resulting model has the scale bars messed up with an error of 1 to 2m, which is unnaceptable.

I'm not sure how much error does this introduces to the DEMs. I'm using these models to monitor growth of corals, so I use the orthomosaics to determine area and DEM to determine surface area.

I've tried to "update transform" the scale bars and depth markers simultaneously but it also results in huge errors associated to the scale bars. 

Can someone please enlight me with possible solutions of any idea of why is this happening.

Are X, Y and Z scales tight to each other?

Thanks in advance for your help!


   

[Alexey Pasumansky]

Hello Carol_Coral,

What values do you input to X and Y columns for the created markers? If you have created markers just by using Add Marker option in the Model view before scaling the project, then the default Source coordinates for such markers would correspond to the arbitrary internal coordinate system and you should fix them before using together with the scale bars.

You can just input some rough values, according to the markers distribution and set low accuracy for markers XY values and higher for Z, before using Update Transform.

[wgreenePIMS]

Hi there,

I also use Metashape for this purpose frequently! Like Alexey said, the best way to get around this is to set very low accuracy for the X and Y values and high accuracies for the Z coordinates. Your scale bar accuracies should be pretty high based on your measurements (probably an accuracy of .5mm is reasonable), and for the depth GCPs, your Z accuracy should probably be around 0.1 meters or thereabouts if you're using a dive watch to collect depths. The XY coordinates will be arbitrary (unless you're setting up some kind of specific local coordinate system) so you should set the accuracies of those to a very high value like 100m so that Metashape won't try to take those into account with the same "weight" as the scale and depth. To do this you'll want to type in something like 100/0.1 in the accuracy column for the depth GCPs. The / tells the program that you're setting the XY and Z accuracies separately. Then if you update the transform, you should get good depth and proper scaling at the same time!

[CastroCardoso]

Hello, Carol.

I work with orthomosaic for reef system environment as well and feel we could exchange a lot.
I also feel, by the way you write and your username, that you're Brasilian, which I also am. If that's true, we have even more to exchange considering the idiossincrasies of our reefs.
Hope we can get in touch.

Regards,