Gap in Elevation Data

We recently acquired a Reach RS + antenna to measure GCPs for drone mapping projects. We are testing this solution with a national NTRIP provider (Orpheon / France) to achieve centimeter accuracy.

This week, to verify the good functioning, I measured known points that we had collected 6 months earlier for an old modeling project (using a Leica antenna connected to the same NTRIP network).
The precision obtained in XY is centimeter, there is a difference in average measurements less than one centimeter.
However, the elevation data show a difference of about 47m.

GCP1 LEICA 7.75957778 48.58578098 141.277
GCP2 LEICA 7.76043817 48.58616646 140.823

GCP1 REACH 7.75957757418 48.5857807328 188.977142009
GCP2 REACH 7.76027878753 48.5860843939 188.598433953

Can someone explain to me how this gap is possible? Knowing that the elevation data from the previous survey are correct.

Thanks in advance for your answers,

Julien

These are “Above sea level”

These are GNSS ellipsoidal

If you wanna convert yourself: http://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html

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Hi Julien,

Like many drone users you are confused between ellipsoid and geoid.
You have substract geoid from IGN69 model. Your height is ellipsoidal whereas Leica applied a geoid.
In this part of France, you have to substract 47.7186. So elevation become GCP1 = 141,258542009, GCP2 = 140,879833953

Sylvain

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That’s what I thought! But having no good knowledge in the subject, I needed a precise answer. Thanks also for the conversion tool!

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Thank you Sylvain, can you tell me how I can find the value to be subtracted according to the study area?

You simply have to download the grid from IGN and then using GIS to find it ! May be there is a better way but I’m a GIS guy …

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