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Author Topic: French Geoid models  (Read 4762 times)

Yoann Courtois

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French Geoid models
« on: May 17, 2017, 05:12:21 PM »
Hi !

I usually work with UAV pictures which are georeferenced in WGS 84 global coordinates system with a metric accuracy.

But, as I want to accurately georeference my models in french RGF / CC46 projected coordinates system, I used GCPs known in this system.
As well, I don't want my vertical coordinate to be in ellipsoid height, so I've downloaded the NGF-IGN69 geoid model on PhotoScan website as it is the used vertical datum in France. I've created my own system in PhotoScan by merging RGF / CC46 system and NGF-IGN69 vertical datum. Looks good !

But, when I convert pictures coordinates from WGS84 system to RGF / CC46 - IGN69 system, I have a big problem with Z coordinate. Here is an example with a flat survey 30m above a flat area:
- Average altitude of pictures according to EXIF data (WGS84) = 88m
- Average altitude of pictures after convertion (RGF / CC46 - IGN 69) = 38.50m
- Average altitude I should get = Average altitude of the floor (RGF / CC46 - IGN 69) + flight height = 175 + 30 = 205m

May somebody can explain me what's wrong in this ?

Regards
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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Alexey Pasumansky

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Re: French Geoid models
« Reply #1 on: May 17, 2017, 05:17:17 PM »
Hello Yoann,

Can you post the PRJ file that you are using and also specify how the elevation data is captured in WGS84 system?
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: French Geoid models
« Reply #2 on: May 17, 2017, 05:53:59 PM »
Hi Alexey,

I just have sent you a mail (support@agisoft.com) with a link to download my project (.psz)

WGS84 pictures coordinates are coming from the UAV's GNSS.
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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Alexey Pasumansky

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Re: French Geoid models
« Reply #3 on: May 21, 2017, 06:36:15 PM »
Hello Yoann,

Thank you for providing the project file.

I see that the camera altitude is written by DJI to the image EXIF: GPSAltitude = AbsoluteAltitude. Are you sure that these values are related to the height above ellipsoid? From what I have seen for DJI datasets this "AbsoluteAltitude" value usually is different from the actual height above ellipsoid value. Is't this value estimated by the barometer?
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: French Geoid models
« Reply #4 on: May 21, 2017, 06:57:42 PM »
Hello Alexey !

Thanks to have spend time on my project !

I've never noticed that "Absolute Altitude" before ! Indeed I've seen several times there is problem with altitudes but I've never taken care about as I use GCPs, I always uncheck pictures positions after alignment.
But since I download our real french geoid, it showed me there is a real problem somewhere else !

Gonna spend time on look at that point in the following days. Let's see if we can configure our UAV differently !

Regards
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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Alexey Pasumansky

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Re: French Geoid models
« Reply #5 on: May 21, 2017, 07:06:05 PM »
Hello Yoann,

I think you can modify the altitude values for all the cameras as RelativeAltitude + Take-off-altitude, providing that you can measure the altitude of the take off point above the WGS84 ellipsoid (or above the geoid).
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: French Geoid models
« Reply #6 on: May 21, 2017, 07:14:37 PM »
Hello Alexey !

Indeed, I guess there is this possibility !
But it would be crazy if the WGS84 altitude is not available as we already get X and Y coordinates with a quite good accuracy for such device. Even GNSS planimetric accuracy is better than altimetric accuracy, this last one would be much better than a barometer measurement !
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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