A Ground Reference Image is an image that is recorded on the ground at the take off location. It doesn't matter where the camera is pointing at the time. The image will not be included in the processed outputs.
As of the December 2015 firmware releases DJI changed their JPG metadata format to use the GPS derived altitude instead of the relative barometric altimeter reading they had used previously. This is good news for general purpose geotagging since the above sea level number is now reflected. This is very bad news for users that are trying to use photogrammetry to accurately reconstruct a scene because the GPS derived altitude is notoriously noisy and unreliable.
Maps Made Easy adjusts uses a Ground Reference Image to fix the image tags back to a reliable altitude using the following method:
- Detect a Ground Reference Image (-3m to +3m) to determine the takeoff location's above sea level (ASL) elevation using either SRTM data or Google Elevation Service.
- Get the above ground level (AGL) barometric altitude value that is embedded in each image and add that to the determined starting altitude.
- Use the newly created and much more accurate above sea level (ASL) value instead of the inaccurate value the EXIF tag.
Map Pilot automatically records an image at the takeoff point after the start of every mapping flight.
Include this image in your upload to Maps Made Easy for proper elevation offset calculation and elevation readings.
NOTE: Due to how Maps Made Easy uses ground reference images to correct for the use of the barometric altimeter in its elevation processing, it is highly recommended to take off from an area of undisturbed ground. Taking off from man-made structures or in areas of heavily modified earth will not correspond well to the reference values we get from the NASA SRTM elevation data and will result in elevation offsets.
Important for Multiple Takeoff Locations: If you are taking off from multiple takeoff locations, DO NOT upload the ground reference images with your data. We only use one ground reference image for the altitude corrections and if your reference images were taken from multiple elevations there is no way to control which on gets used. Uploading ground reference images from multiple elevations will likely do more harm than good.
Do you include this image in the selection with all the main images or separately when it prompts on the EXIF file request? Wasn't clear when and where I should include the file from the ground, included it as part of the full set.
The ground reference image (image taken when the aircraft lands) should be uploaded along with all of the other images.
Tudor - Can you please explain how not including these ground reference images will affaect the end product? What exactly will be adversely affected? I have created several maps where I exlcuded these images thinking they may be detrimental, not useful.
See this link for further discussion:
I recently flew a survey south of 60*S (in Antarctica) using Map Pilot, and processed on MME. The DEM is pretty close to what I expected to get, but I'm just wondering where the reference image sourced an elevation AMSL for calculations from. The shuttle data doesn't extend that far south. Did it just "get lucky" or is there another data set it can access? Airbus WorldDEM (he asked, hopefully)?
Sorry for the delay.
It is embarrassing but we actually use the MapQuest elevation API for the ground reference image offset.
It has good coverage and good terms. We have no idea where they get their data for down there though.
@Tudor @Jos Bol
re: "The ground reference image (image taken when the aircraft lands) should be uploaded along with all of the other images."
I, along with @Jos Bol, much prefer to "hand-catch" my drone slightly above my head at the end of a mission, especially on windy days. Is it required that I literally let the aircraft land on the ground before dis-engaging from the app, or can I dis-engage from the Map Pilot app after the last mapping image is taken, bring it home, hand-catch the drone, stop the rotors (with the battery still on and controller connected), place the drone on the ground at the takeoff point (under my feet) and simply manually snap an image and include it with my upload?? Sorry if this has already been addressed but can't seem to find a concise answer to this.
Also..... what are the effects if I begin the mission if the drone is already at altitude?? (Example 200ft AGL)
The Ground Reference image is now taken when the aircraft takes off on most models (all but M600).
@Zane... Understood. However, I don't take off from ground level. I launch with the drone held over my head. If I manually snap an image while the drone is sitting on the ground before the mission starts (before take off), will that image suffice and meet the definition of a "Ground Reference Image" as it pertains to MME processing?? Thanks in advance...
OK. I thought you were saying that you caught it... I don't know when it is going to take that image then. Probably when the motors are shut off. Regardless, as long as you have an image that is taken at that location that is marked as the 0 altitude point it will work as a Ground Reference image. It may be off by a bit but it will be much better than using the GPS derived numbers.
I am looking for a way to sub in a ground reference image since I apparently threw the originals directly in the garbage.
1. How does the software know which ground reference image is the reference image? Is there a specific tag in the metadata?
I am thinking I might be able to manipulate the metadata from a photo taken from my phone at the takeoff site to have the tag needed... Does that make any sort of sense to you? Thank you!