Georeference photo

(Michele) #1

Good evening, I am a new user of the forum, I work in Italy and unfortunately English is not my mother tongue and I apologize for the inevitable inaccuracies due to the use of a translator.
I deal with topographic surveys including drone photogrammetry.
I use a UAV with a Pixhawk 2.1 flight controller (black cube), GNSS Here + with RTK corrections (both reference and via NTRIP), sony Alpha 5100 camera; all managed by Mission Planner.
I’m having problems with photo geotags.
The Mission Planner procedure returns Latitude and Longitude (ellipsoid coordinates WGS84) while the elevation is orthometric (above sea level); it is not clear to me whether it is calculated using an EGM96 type geoid model or in another way.
This means that at the end of the procedure I have a non-homogeneous set of coordinates and I cannot correctly apply the conversion parameters from ellipsoidal coordinates to rectilinear coordinates.
In Italy the gratings provided by the Military Geographical Institute are used, which contain a very detailed geoid model defined in 2005.
I ask the programmers of the Mission Planner software: is there a way to extract the ellipsoidal height instead of the orthometric height?

#2

@Michael_Oborne

(Michael Oborne) #3

The altitude returned is that of the autopilot/GPS you are using. Typically in egm96 using the 15 degree grid. Are you logging raw data to the dataflash log?

(Michele) #4

I suppose you’re referring to the * .log file I downloaded to georeference the photos.
It contains ellipsoidal latitude and longitude but the elevation is orthometric I am sure of it. I can’t tell you anything else because I didn’t do the drone building and the Mission Planner configuration. If you want I can send you the log file to verify.

(Michael Oborne) #5

I would expect the current altitude listed to be the egm96 value. To get the ellipsoid value would either need to ppk with the raw data/use ground control/ or minus the egm96 geoid to get the ellipsoid values. Currently there is no direct way to get the GPS reported ellipsoid value. It is possible but would require custom firmware at the moment. The reason for the egm96 alt is all srtm terrains data is in this format already. Also I cannot guarantee the geoid accuracy output by the ublox GPS.

1 Like
(Michele) #6

Thank you for your reply.
Confirm what I thought.
I am aware that using EGM96 is indispensable for having the orthometric dimensions compatible with existing maps (Google or other). However, my need is different as I use photogrammetry to create topographic surveys and I do not calculate the altitudes using EGM96. I therefore need to use the ellipsoidal height and then calculate the altitude above sea level using other geoid models. Certainly the ellipsoidal height is a datum that the flight control unit has (provided by the GNSS receiver), and which it used to calculate the EGM96 orthometric dimension. My request is justified by the fact that I am checking the accuracy of the RTK data to be able to eliminate or at least limit the use of ground control points.
I was told that DJI already with Phantom RTK returns ellipsoid WGS84 data (latitude, longitude, ellipsoid height).

(Michael Oborne) #7

it is possible to get the ellipsoid height by logging raw_data or changing the geoid model in the gps to always uses another geoid. this however breaks things like terrain following. but that depends on your needs.

(Michele) #8

I’m sorry, but I didn’t understand your answer, you could be more detailed; unfortunately I have little experience with Mission Planner.
How should I proceed and what should I do to register raw_data.
I’m not interested in changing the geoid model. It is sufficient to unload the ellipsoidal height.

(Michele) #9

I looked through the various Mission Planner menus and I think I found where to set the parameter you indicated: precisely on the “Configtuning - Full parameter list” menu. Scrolling through the list I found the parameter records raw data and I set the value 1.
Questions:

  1. will I find the ellipsoidal height recorded on the log file?
  2. Starting the “Geo refi images” procedure on the photo exif and on the csv file is the EGM96 dimension or the ellipsoidal height inserted?
    I ask you for more info as I would be interested in herelink:
  3. Is it possible to use Mission Planner with herelink?
  4. Will there be a version of Mission Planner for Android in a short time?