Learn how to process your WingtraOne RedEdge data into a reflectance map in Pix4D.
A reflectance map is a map that stores information about the amount of radiant energy that is being reflected in each pixel and it is used as a base for generating index maps.
A nicely made video tutorial by MicaSense can be found here.
The procedure is also described in the following detailed step by step guide.
Importing the Data
Imagery taken by WingtraOne RedEdge will be automatically recognized since the camera is in the Pix4Dmapper camera database.
- Create a new project in Pix4Dmapper
- Import all of the images for each band: blue, green, red, RedEdge and NIR (near infrared)
- In the "Edit Camera Model" window, ensure that the global shutter model is selected
- Choose the "Ag Multispectral" processing template
Radiometric processing and calibration
On the menu bar, click "Process" and then "Processing Options."
The Processing Options pop-up appears. Press 3. "DSM, Orthomosaic and Index," then select the "Index Calculator" tab.
Under "Radiometric Processing and Calibration," choose the "Camera and Sun Irradiance" option for each band. This means that corrections will be applied for camera parameters that are written in the EXIF metadata and relate to the camera (vignetting, dark current, ISO, etc...), as well as for the sun irradiance information written in the EXIF tag.
The camera used is displayed. Users can calibrate the sensor to perform an illumination adjustment in order to obtain more accurate reflectance values. If there is more than one camera model in the project, all the cameras will be listed in the "Radiometric Processing and Calibration" section.
Using the calibration panel
The "Calibrate" button allows users to apply reflectance corrections based on the information from a radiometric calibration target if such a target was used during the project (MicaSense Reflectance Panel is an example of such target).
If you have a MicaSense Reflectance Panel, you can get the reflectance values for your panel by submitting a request to MicaSense.
This procedure needs to be repeated for each of the five bands, with the corresponding "reflectance factor" value provided for each band. Please note the band order (blue, green, red, NIR, RedEdge) and ensure you are entering the appropriate value for each band.
To do this, click "Calibrate" for a camera model and the "Radiometric Calibration" pop-up will appear.
In "File Name," use the "Browse" button to select the image in which the radiometric calibration target appears.
When an image is browsed, draw a region on the image that will define the radiometric calibration. The "Reset" button resets the area drawn. Use the left mouse button to draw the first three points, then draw the final point with the right mouse button.
Insert the reflectance value that you got from MicaSense for the corresponding band.
Important: The reflectance values range between 0.0 and 1.0.
If a radiometric calibration target is used, make sure that the radiometric target data is taken into account by ensuring the symbol for each spectrum band in the "Processing Options" is a green checkmark.
Processing and generating reflectance maps
After configuring Pix4D to use the calibration data, you can continue to process your data set. You will need to generate a reflectance map. For up-to-date details on processing, please see the Pix4D knowledge base.
On the menu bar, click "View" then "Index Calculator"
On the left part of section 1. "Reflectance Map," there is an arrow that is used to show/hide the section by clicking on it. Expand it and click on the Generate button to start generating the reflectance map if not generated.
Once the reflectance map is generated, this section will also display information for each band of the generated reflectance map(s) for each group of images. The following information is displayed:
- "Band": Name of the band. If more than one group exists and the same band name exists in different groups, the bands are displayed as "Groupname_band."
- "Min": Minimum pixel value per band.
- "Avg": Average pixel value per band.
- "Max": Maximum pixel value per band.
- "Stdev": Standard deviation of pixel values per band.
- "Var": Variance of pixel values per band.
If the reflectance map(s) is not generated, the message "Reflectance map not yet generated" is displayed.
Once you have generated a reflectance map, see How to export to Atlas from Pix4D to export the final output to Atlas for analytics.