Taking the Calibration Pictures
Prepare by laying out the special calibration target on a flat surface, preferably pavement. Make sure it does not move during this entire process.
Open DJI Go and check camera settings:
Manual Focus (MF): The camera should be set to manual focus, and the slider moved to the lower endpoint, which represents focus set to infinity. All mapping fligts should be conducted with this setting as well.
- Verify that manual focus is selected and set to infinity
- Tap to focus mode should be selected
Exposure: Set to Manual (M). Set shutter speed to no less than 1/80 second. This is important when the drone is handheld. Since the drone is focused on infinity, a good depth-of-field ensures good focus. f/8 or higher is recommended to achieve this. Finally, ISO can be adjusted to get an Exposure Value (EV) of 0.0. The exposure adjustments should be made with the target filling all or most of the display.
Open the Camera Settings (Yellow Box) and set them as shown below:
- Image Size: 3.2
- Image Format: JPG
- White Balance: Either Sunny or Cloudy; Auto (AWB) is not recommended
- Lock Gimbal during Capture: ON (only available on Phantom)
- Mechanical Shutter: ON (this should always be ON!)
Photographing the Target
NOTE: before taking photos, power on the aircraft and wait for it to acquire satellite signal!
The process of capturing the photos is best done with two people: one for holding the drone, and a second for using the controller to adjust the gimbal and capture the photos. The photos should be captured by circling the target with the drone, with the camera pointed (pitched) to the center of the target. Several concentric circles should be done, but with the radius decreasing and the height increasing each time, effectively photographing from a half-sphere over the target. Distance should be close enough that the target occupies as much of the display as possible, but while maintaining at least a distance of 2.5-3 feet, so the target stays in reasonable focus. The goal is to expose all parts of the sensor to the target, and do so at many different scales.
A good combination is three circles, with sixteen photos each in the low and middle circles and eight in the highest. Follow that by two more circles of sixteen with the camera inverted. This provides a total of 72 photos of the target. For the Phantom 4 Pro, the gimbal lock can be left in place so that you can invert the drone and directly point the camera. The included gimbal lock will have to be modified, as it obscures the camera's view.
The next two photos show a typical geometry of collection, and the thumbnails of a typical set of photos.
Processing the Photos
- Create a project and click Next
- Load Images (if you did the calibration outside, you should have georeferenced images)
- Take the defaults for image properties and coordinate systems (we're not processing data, no need to set output parameters)
- Make sure that "Select Camera Model" field does not have a random string of characters after it. If it does, you have already calibrated this drone and have saved the values.
- Pick the usual 3D Maps type project
- Check "1. Initial Processing" Uncheck the other 2 options
- Click "Processing Options" icon. Check the "Advanced" Box. Navigate to the calibration tab:
- "Standard" calibration method
- "All" for the two optimization choices
- "Automatic" rematch
- Click OK, then under the main screen click Start to start the project.
- When your project is finished, from the Project menu, open "image Properties Editor" and Click Edit.
- Click Edit on the Edit Camera Model dialog.
- Click the "load Optimized Parameters" button. This loads the calibrated parameters into the dialog
- Notice the values have changed
- Click the "Save to DB" button to save these and select "Yes" in the dialog. The parameters will be saved with the associated camera serial number.
- The Camera Model Name will now be changed, and it will reflect that specific drone. Anytime going forward, pictures loaded into Pix4D will choose this camera model based on the exif serial number. Click OK.
Importing/Exporting these Settings
Take a screenshot of the final optimized camera parameters and save them wherever you store your data. They should be labeled with the Drone ID or serial number, both is ideal. Once you start a project with a new camera (drone), import these values manually and save them, and they will be there forever.
In this example, a filename of FIC999-ee869ab0339c49cf2282550a1269acdb.jpg
There will also be a master excel spreadsheet of values as well. Click edit and copy and paste the values in teh spreadsheet with the drone number and serial number. Make sure any negative values include the minus when copying/pasting.
Upon receiving photos from a calibrated source, but when your Pix4D doesn't have the calibrated camera in its DB, before step 1 in processing, go to Project > Edit Camera Model. The EXIF ID will be at the top, and you can check your database for the serial number screenshot/excel sheet. (See above for how to export)
Click Edit and copy/paste the values or type them in manually.
Click Edit and Copy/Paste the values or type them in manually.
Click OK and Save to DB. Say Yes and Save under the serial number to have the values imported into your DB forever.