Hi,
I'm hoping someone can answer a couple of questions regarding the geo referencing function of mission planner. I have a fairly large dataset of some vertical imagery taken recently which I'm having trouble accurately geo tagging.
The main reason for this is there is a discrepancy between the number of images vs the number of trigger events. In all of the flights there are more trigger events than images, which means that geo tagging via the CAM event function of mission planner always fails. I can load the pixhawk log file straight into the processing software (Pix4d) but it assumes that the cam events match the number of photos. They have confirmed that if there are consecutive photos missing from the middle of the batch for example, that it will simply assign the Cam event to the photo in the line, thereby giving it the incorrect co-ordinates.
My second problem is that I deleted the first few images which were of the aircraft on the ground, and this means the time offset estimate will be slightly out as well.
A couple of questions.
1. I have calculated the offset between the camera internal clock, and the GPS time by taking a picture of GPS time off an Iphone app for this purpose. Is it possible to then apply this offset in mission planner?
For example, if an image was taken at X time and the Offset calculated to be Y number of seconds, go back through the log and get the GPS time or the nearest CAM message time and location based on this offset? I can do this manually however it is a tedious task.
I've tried inputting this offset and running it and it looks ok from Mission planner, but it seems to be out when Pix4d processes it.
2. Currently, my understanding is the time offset function looks at the time the first image was taken, and at the time the log was first created. Is this when the pixhawk was turned on and started logging? Is it from when the aircraft was armed? Or is is the from the time it first received a valid GPS signal.
3. Does anyone have a better way of currently doing this?
Thanks,
Baz
Replies
1. yes, this is the time offset mode.
2. correct.
3, this is where stuff like deleteing photo's etc, makes this hard.
the easiest way to fix this would be to make the camera time match the gps time. then the offset would be 0.
Michael, just another couple of questions if I may as I still seem to be getting some problems geo-referencing
From the tutorial on geo-tagging images -
http://copter.ardupilot.com/wiki/common-geotagging-images-with-miss...
For arguments sake and to make it simple for me the first GPS logged time is 00:00:00 and the first picture time is 30 seconds later at 00:00:30
Based on that would the offset then be 0? Or would the offset be 30?
I've calculated the difference between my cameras internal clock and GPS time to be 34409 seconds.
Mission planner picks it up on one flight as being 34462.4
I'm guessing this means that the first picture was taken 53.4 seconds after the first logged GPS time?
One problem I have is that we used a servo camera trigger which fired a photo as it was being powered up. Unfortunately my other downfall was also not understanding the system and so I've been through and deleted the images on the ground which weren't required. Unfortunately this has made my job of geo-referencing 1300 images quite difficult.
Thanks again.
the thing to remember is, that gps time is in its own time, which is close to utc. so setting the camera time to local time will still need to be offset.
eg gmt +8 would be an offset of 28800 seconds
im gana guess you are +9.5? so South Aus?
in your example it would need to be 30
Thanks Michael.
In that case it appears that I have calculated the offset incorrectly from the camera. I'll try it again and see what result I get.