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.