Georeferencing and Projecting
Mavis provides four georeferencing options:
I) Data flash log file and camera log file
For precise geotagging both the UAV dataflash .log file and the camera .cam log file are required to compensate the trigger delay between the Autopilot signal and the time it takes until the camera takes the picture. If images have been taken before arming – for example to test if the camera triggers – the number of these images has to be set in Mavis. Mavis checks if there is some trigger timing inconsistency between two images. If a trigger inconsistency is detected for example between the 2nd and the 3rd image, „Images triggered before arming“ should be set to 2.
II) Data flash log file only
If no .cam file is available, georeferencing is possible using the CAM messages of the UAV .log file only. In this case care has to be taken that flight speed is not too fast for the minimum trigger interval of the camera. If some images have been removed from the dataset before processing in Mavis, the first (remaining) image of the dataset has to be aligned with the corresponding trigger message in the log file. This is possible using the „Images triggered before arming“ option.
III) Data flash log file and image time stamps
This is similar to the first georeferencing option where a .cam log file is available. In this case the TimeTag.cam file whoch is generated automatically for each image set can be used, to account for missing (not triggered) images. Compared to using a camera log file this option is less precise because the precision is generally 1s only. Additionally the first image in the workspace must correlate with the first CAM message in the log file. Same as for the previously described option it is possible to set a global time offset, which can be used to account for a shutter delay and thus to adjust the position.
IV) Geotagged images
Images which are already geotagged can be used without using any additional log file. However, some analysis options are limited. The following geotags are required:
- GPSLatitudeRef (N = north latitude, S = south latitude)
- GPSLongitudeRef (E = east longitude, W = west longitude)
- GPSAltitude (Above sea level)
All data is projected to UTM. The UTM zone is automatically determined by Mavis from the WGS84 coordinates provided in the .log file or the previously geotagged images. However, the zone can be adjusted manually if required.
Images with only a few neighboring images or images taken at different altitudes might cause problems in the camera calibration process. To reduce this problem Mavis automatically removes images, which were taken between take-off and the first waypoint as well as images after the last waypoint. These images are moved to /Mavis/Deleted/Georeferencing/. If geotagged images are used, such images should be removed manually.
Mavis allows to set a Yaw offset and a Time offset. This helps to manually adjust georeferencing for direct stitching in the Quality Control section. There is often a small Yaw offset of +- 2°.