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Calculating "Camera to facade distance"

I have taken a series of overlapping oblique images of a building facade with an iPhone camera (with geotags) and generated a point cloud and orthomosaic in Pix4D. When I import the point cloud and orthomosaic into eCognition, it shows that the point at the ground adjacent part of the facade has a lower Z value and the point of the upper part has a higher Z value but I need “camera to facade distance” as Z value to help extract cracks on the facade using OBIA in eCognition.

Later I generated point cloud and mosaic by deleting geolocation information and selected “Arbitrary”
(output) coordinates in Pix4D. Still, the Z value is the height above the ellipsoid (according to WGS 1984), not the distance from camera position to facade. What can I do?!

DSM|541x500

Hi @m.s.hossain-2,

Thanks a lot for your feedback. Your feature request has been forwarded to the product team.

Best,

Hi Nikoleta,

Thank you for your reply and the action taken. I am eagerly waiting for the ultimate response from the product team.

Best regards,
Muhammad Selim Hossain

Hi Muhammad,

Pix4Dmapper will not export “camera to facade distance” as Z value by default.

One thing you could do is delete the image geolocation and add an orientation constraint:

Hi Christina,

Thank you for clarifying your position with regard to the problem and also for the idea to find the solution.

I am going to experiment with your idea.

Best regards,
Selim

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Hi Christina,

I am going to experiment with your idea.

By changing orientation while taking orientation constraints, I got the “Camera to facade distance” which varies object to object (object depths) on facade. But it shows in negative numbers.

Is there anything to do to get the distance in positive numbers?

I look forward to hearing from you.

Best regards,
Selim

Hello Selim,

One thing you could try is to add some arbitrary GCPs that will give an arbitrary coordinate system to your facade. For example, you could add a GCP at the lower left part of your facade with coordinates 0,0,0.

Similarly, you should add more GCPs so that the numbers come out positive.
I believe the numbers came out negative because you removed the image geolocation and this lead to an arbitrary coordinate system.

I hope it helps.
Christina

Hi Christina,

Thanks for the nice idea!

Yes, you are right. I have removed the image geolocation and taken orientation and scale constraints.

I understand that I can add the coordinates 0,0,0 at the lower left part of my facade as a GCP.

If I add another GCP at the lower right corner, the coordinates should not be 0,0,0 I guess. So, how to find out the coordinates of that point?

One thing I can do: after inserting 0,0,0 coordinates at the lower left corner, I can check on the facade image what coordinates at the lower right corner come up.
Then, I can add that as its GCP. Does that make sense?

I look forward to hearing from you.

Best regards,
Selim

Hi Selim,

I was thinking something like this:
gcps_arbitrary

GCP1 (0,0,0)
GCP2 (X,0,0)
Where X is the real distance between point 1 and 2 in one direction
GCP3 (X,Y,Z): a point in the direction of the camera and the facade in order to define the Z direction

I hope it makes sense :slight_smile:
Given that this is an application that is not officially supported by Pix4Dmapper, unfortunately, I cannot give further assistance.

Currently, the software does not export “camera to facade distance” as Z value by default.
But you can give it a try with arbitrary GCPs. I’ve seen similar projects in the past from other customers.

Christina

Hi Christina,

Really interesting! Thanks a lot. I am going to experiment it today. I will let you know the results.

Best regards,
Selim

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