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Author Topic: Measuring photogrammetry resolution  (Read 4753 times)

outsider

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Measuring photogrammetry resolution
« on: June 06, 2018, 10:57:02 PM »
How does one go about measuring the resolution that has been captured with a bunch of photos and the photogrammetric process? By resolution I mean the detail frequency (?is that even a term?) that the point cloud and/or mesh is able to reconstruct.

And I'm not talking about what the theoretical cm/pixel resolution that a certain sensor should be able to capture. I'm talking more about how to measure the highest frequency of detail a certain set of photos can capture, which will of course be influenced by the sensor, lens, distance to subject, f/stop...( and likely other factors).
I know I can set the reconstruction settings to High, or Ultra, or Max, but once the model is complete, what tools are available to tell me how much detail was captured in the point cloud or the mesh, if I compare with a real-life object.

Are there any proven workflows?
Any tips?
Any tools you've seen/used?

SAV

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Re: Measuring photogrammetry resolution
« Reply #1 on: June 07, 2018, 06:54:24 AM »
Hi outsider,

I guess the term you are looking for is ground sampling distance (GSD).
GSD is the distance between pixels measured on the ground/object (e.g., 7 mm/pixel) and has an inverse relationship to spatial resolution (e.g., 10 pixel/mm).

The GSD will dictate the smallest features that you'll be able to identify in your imagery. For example, if the feature is only 2cm wide and your GSD is 2cm/pixel, then this feature will only be visible in 1 pixel, hence probably quite difficult to identify.

The GSD depends on the distance to the object, physical size of the camera sensor, sensor resolution and focal length (see formula below).

Code: [Select]
GSD = (DISTANCE x SENSORwidth) / (IMAGEwidth x FOCALLENGTH)
All the best.

Regards,
SAV

outsider

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Re: Measuring photogrammetry resolution
« Reply #2 on: June 07, 2018, 03:16:36 PM »
Thanks for the reply SAV,
Both GSD and Spacial Resolution are purely theoretical numbers. And while they are useful to a degree, it's like judging a photograph purely based on the megapixels the sensors. We all know there's more to how sharp an image is, then just the sensor.

To measure the ability of a camera/lens combination to capture data, charts like these are used to determine the resolution of the lens/sensor combination. Change the lens, and the captured chart will show different results (more or less resolution depending if the lens is able to resolve more or less detail)


Camera resolution test chart from ISO 12233:2000

Similarly in photogrammetry, the lens plays a HUGE part in the resolution the camera can capture, which is why I called GSD and Spacial Resolution purely theoretical. Depending on what lens is used (and how the entire system is used) you can get closer or further to that theoretical number.

Would I be wrong to assume that there's no standardized method to measure Actual Spacial Resolution (not theoretical) with some sort of equipment similar to what the chart above does for photography?
 

SAV

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Re: Measuring photogrammetry resolution
« Reply #3 on: July 16, 2018, 12:09:06 PM »
Hi outsider,

GSD is the best we have ;-)
It basically represents the best possible 'resolution' you can achieve, which does not say much about the quality of the images or resulting model.
For example, even though the GSD is let's say 5mm/pixel, it does not mean that you can properly resolve or identify features that are even 2cm big in your imagery or model. The ability to identify and model such features depends on the image quality, which (as you pointed out) is controlled by many factors such as camera settings, quality of lens, dynamic range of sensor, image blur, .....

Regards,
SAV

mcstieg

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Re: Measuring photogrammetry resolution
« Reply #4 on: August 27, 2020, 11:05:45 PM »
Metashape calculates the Pixel size (GSD?) for the build orthomosaic step. Does anybody know how the distance in this formula is calculated?
Is it the average distance between cam hight and hight of model points? Or something else?

Thank you!