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Author Topic: Help with source values  (Read 2734 times)

Emma

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Help with source values
« on: June 03, 2019, 02:37:04 AM »
Hi everyone,

I'm quite new to photogrammetry and I'm trying to build DEMs of gullies to track erosion over time.

I’m using 12-bit coded targets set exactly 1m apart as scale bars. I want to be able to check the markers in the reference plane so I can optimise them to reduce the error. I also want to do this with the cameras, but at the moment, I can’t check any of them (only the scale bars). The RMS reprojection error for the chunk is 0.161 and for the markers the error ranges from 0.22 to 0.81 (pix).
I know that I can’t check them until I have source values in the reference plane, but I have no idea how to retrieve these values as I don’t have co-ordinates from my camera (the resolution of a GPS enabled camera isn’t high enough for this project, hence the use of local coordinates, targets and scale bars) and I don’t have real life measurements. Is there any way to be able to optimise my marker locations/retrieve source value data?

Any help is much appreciated!

Thanks
Emma

Eichhorn18

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Re: Help with source values
« Reply #1 on: June 03, 2019, 08:51:04 AM »
I want to be able to check the markers in the reference plane so I can optimise them to reduce the error.

What exactly do you mean by this? What do you think can be optimized, and how would you optimize it?

Emma

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Re: Help with source values
« Reply #2 on: June 04, 2019, 04:01:44 AM »
I want to be able to optimise, and therefore reduce the error of marker placement for a more accurate model - does that make sense?

Eichhorn18

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Re: Help with source values
« Reply #3 on: June 04, 2019, 10:49:46 AM »
I want to be able to optimise, and therefore reduce the error of marker placement for a more accurate model - does that make sense?

It makes sense what you're saying in so far as I understand you want to measure the accuracy of your model compared to the real world. I don't think marker placement will increase your accuracy in a meaningful way compared to the quality of your images, number of images, orientation of the images relative to your object, lighting conditions (presence of shadows between images).

Your markers should be placed around the extent of your model such that you have markers that encompase the object of interest in all three dimensions if possible. You said in your other post that you're photographing erosion gullies. I presume that these are somewhat inclined on a hillside? This incline may be sufficient elevation change that you don't need markers above the gully. How exactly do you know that your markers are 1 m apart exactly?

I have not worked with coded markers but I would hope that when you identify the markers automatically in metashape that you could then import and assign the coordinates to some of them, then refresh your model, and measure the coordinates of the others. This will give you some measure of accuracy.


Emma

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Re: Help with source values
« Reply #4 on: June 05, 2019, 04:26:52 AM »
Thanks for you reply! I found it very helpful!

You are correct, that was one of the difficulties of image-capture as I want to get a really high quality image set but I can't walk inside my study area (~10m x 2m) as that would disrupt the topography and lead to differences caused by trampling, rather than erosion. It would make image capture a lot easier if I didn't need markers above the gully too. I have calibrated scale bars from Cultural Heritage Imaging with coded targets on them. They're calibrated to 1/10th of a millimetre so they're pretty accurate.

I've just done a test run using some steps on my university campus, and I assigned co-ords of 0,0,0 (x,y,z) to the bottom left target - I'm hoping that, with that co-ordinate, and the accurate scale bars, my DEM model will produce some accurate measurements.

Eichhorn18

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Re: Help with source values
« Reply #5 on: June 05, 2019, 11:13:47 AM »
I've just done a test run using some steps on my university campus, and I assigned co-ords of 0,0,0 (x,y,z) to the bottom left target - I'm hoping that, with that co-ordinate, and the accurate scale bars, my DEM model will produce some accurate measurements.

It's likely that the raw reconstruction of your scene is pretty close to the correct scale if you're using a camera that retains the focal length, aperature, etc. in the EXIF data. The scale bars will obviously increase the accuracy, however your scale is likely correct un-adjusted, however--what is probably incorrect is the orientation of your model in real space. Unfortuantely setting one marker in your scene as 0,0,0 wont be enough to get the origitation (roll, pitch, yaw). Ideally you would have at least 3 points encompasing your space in 3D to get the orientation correct. I'm sure there are more scientificly rigerous statements on how many points you should have. If you are hoping to do any numerical calculations on volume or elevation models to get flowpaths on your gully, you'll need to orient your model correctly to gravity (I assume your application is scientific in nature).

Emma

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Re: Help with source values
« Reply #6 on: June 06, 2019, 03:35:52 AM »
Yes, that appeared to be the case yesterday, so I inputted values to two other markers and manually orientated my model when creating the DEM so that I had an exactly top-down view - this definitely appeared to produce much more accurate measurements. Thank you so much for your advice - it's been rally helpful in clearing things up for me!