French Geoid models

[Yoann Courtois]

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

[Alexey Pasumansky]

Hello Yoann,

Can you post the PRJ file that you are using and also specify how the elevation data is captured in WGS84 system?

[Yoann Courtois]

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.

[Alexey Pasumansky]

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?

[Yoann Courtois]

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

[Alexey Pasumansky]

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).

[Yoann Courtois]

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 !