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Author Topic: Distorted Ortho  (Read 4478 times)

kikbak

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Distorted Ortho
« on: December 26, 2014, 07:37:47 PM »
Hi All,

I'm not having much luck in creating a nice ortho.  As seen the below picture there is a lot of distortion around the edge of the roof and a double staircase effect.  What is a typical cause of this type of error?  Overlap? Altitude of Photos?  Camera position data?

Thanks so much!
mb

stihl

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Re: Distorted Ortho
« Reply #1 on: December 27, 2014, 07:41:31 AM »
Improper geometry of the roofline and the staircase is due to noise and/or inadequate height data (point data) of those locations in your dense point cloud.

Increasing the overlap and/or lowering the GSD of the images (ie flying at a lower altitude) will increase the amount of points that can be generated during alignment process and dense cloud generation. Thus creating a more complete pointcloud which will improve the geometry. Unless extensive use of oblique imagery is used, camera position data isn't that important. I assume you used nadir only images.

kikbak

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Re: Distorted Ortho
« Reply #2 on: December 29, 2014, 02:24:42 AM »
Stihl,  thank you for the feedback!  I will keep experimenting with the altitude and overlap.  I noticed that the altitude data recorded by my Canon SX260 HS in the EXIF was vary inaccurate for some reason.  For instance a programed autopilot mission was flown at 30m but the data in the EXIF showed altitudes ranging from 15m - 100m.  I manually edited the values in the EXIF (setting all to 32m ASL) and reran the process.  This helped clean up much of the error but there is still some minor distortion in the same areas but not nearly as bad. 

Thanks again for your help with this issue!

stihl

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Re: Distorted Ortho
« Reply #3 on: December 29, 2014, 04:36:13 AM »
That's a good solution to an odd problem. Not sure why it would range so much, seems broken.

What you can do is make a mesh of only the ground points by using the automatic ground filtering option to make the ground points as one selection. Then make a mesh of only those points. This will increase object/building lean but it should also eleminate artifacts.

hareem

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Re: Distorted Ortho
« Reply #4 on: January 05, 2015, 09:55:32 AM »
Thanks stihl,

it worked  :)

Is there any advantage in using a gps unit with the d800 for creating a 3D model of a building? will it make the photo align faster?

thanks,
hareem
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stihl

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Re: Distorted Ortho
« Reply #5 on: January 05, 2015, 05:02:51 PM »
Hareem,

Yes the advantage of using image positions (GPS) is that it will decrease the processing time for alignment if you align the photos with Pair preselection set to  'Reference' (v1.10)

In theory your 3D model will also then be geo referenced in an absolute sence but without the use of ground control points the mean absolute deviation of the model will be several meters but it's relative accuracy (the model scales) might also be better than aligning without image positions.

mwillis

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Re: Distorted Ortho
« Reply #6 on: January 06, 2015, 12:52:09 AM »
The camera uses the GPS for its altitude values which are known to vary wildly. The autopilot uses a barometer for its Z vales.  The barometer can be relatively acurate to about 30 cm. You can purchase a data logger that uses your GPS values for X and Y but uses its own barometer for Z values.

Hope that helps. -Mark