Hi there Ximena Tagle!

What are you doing at the moment?

I am working on my MSc thesis. It is about radiometric calibration of UAV images for vegetation mapping.

My supervisor is Lars Eklundh. I work in collaboration with Per-Ola Olsson, Hongxiao Jin and Stephania Zabala. We are part of the Remote Sensing group at the Department of Physical Geography and Ecosystem Science (INES) in Lund University.
Fig. 1.  Stephania (left) and Ximena (right) with all the equipment, ready to go to the field. Photo by Miguel Castro.
Fig. 1. Stephania (left) and Ximena (right) with all the equipment, ready to go to the field. Photo by Miguel Castro.
Fig 2. The camera calibration starts on the roof of the department. Close pictures of reflective targets and other features, as vegetation, are taken. Right after the picture, the reflectance is measured with the spectrometer. Photo by Ximena Tagle.
Fig 2. The camera calibration starts on the roof of the department. Close pictures of reflective targets and other features, as vegetation, are taken. Right after the picture, the reflectance is measured with the spectrometer. Photo by Ximena Tagle.

What exactly are you doing when calibrating?

I am trying to reduce radiometric issues when processing UAV imagery from a multispectral camera. I chose to work with the Micasense Redge camera. The aim is to obtain quite homogeneous orthomosaics in relation to the light variation. In addition, I am studying the relationship between reflectance values at the ground, and the Digital Numbers (DN) from the camera in order to provide orthomosaics with reflectance values.

For this purpose, on each mission we get data from a downwelling light sensor (DLS) that provides irradiance at the sensor; irradiance measurements from a spectrometer located on the ground; and PAR data from the Eddie covariance tower located at the site. With this data, I am trying to remove light variation among images. After this task, I am performing a normalization of the images (histogram matching).

Regarding the reflectance relationship, I use an ASD spectroradiometer to measure the reflectance of three near Lambertian reflective targets and other features as bare soil, vegetation and water. The reflective targets are used to calibrate the model, and the other features are used for the validation of the model.

Once the images are calibrated, the orthomosaic is produced. Orthomosaics are generated from several images that overlap, using the Structure from Motion (SfM) technique.
Fig. 3. Calibration of the Downwelling Light sensor at Lönnstorp. Per-Ola had to rotate the UAV in different ways according to the Micasense application (connected via WIFi to the cellphone). Photo by Ximena Tagle.
Fig. 3. Calibration of the Downwelling Light sensor at Lönnstorp. Per-Ola had to rotate the UAV in different ways according to the Micasense application (connected via WIFi to the cellphone). Photo by Ximena Tagle.

How should this information be used further on?

The Explorian UAV (the biggest UAV we have now), has a Sony RGB camera, a multispectral camera (Micasense Rededge) and a thermal camera (FLIR). They will be used together to study the influence of spatial heterogeneity and temporal variations of vegetation and land surface on ecosystem carbon fluxes. The idea is to upscale data from the flux towers to a landscape level using spectral indices derived from remotely sensed data. In order to do this, it is recommended to work with homogeneous orthomosaics adjusted with the reflectance values, the tasks I am working with at the moment.
Fig. 4. a) UAV flying over the reflective targets at Lönnstorp. b) Picture of the targets from the UAV. Photo by Ximena Tagle.
Fig. 4. a) UAV flying over the reflective targets at Lönnstorp. b) Picture of the targets from the UAV. Photo by Ximena Tagle.

Tell us about yourself!

I am Ximena Tagle. I work at the Research Institute of the Peruvian Amazon, at the program of Forest management and environmental services. I am doing my MSc thesis to obtain a degree in Geo-information Science and Earth Observation for Environmental Modelling and Management (GEM).
Fig. 5. (Me) connecting the spectrometer for its calibration with the spectralon (white circle in the picture) before the measurements. Behind it can be seen the Explorian UAV. Photo by Miguel Castro.
Fig. 5. (Me) connecting the spectrometer for its calibration with the spectralon (white circle in the picture) before the measurements. Behind it can be seen the Explorian UAV. Photo by Miguel Castro.

Finally, anything more to add?

This task is very interesting because it is a new topic, so we are having challenges most of the time. We have to be creative to solve different issues. We have to be persistent as well. I find inspiration when we attend conferences or workshops, and there are people who are studying similar issues and we can discuss new solutions; we are learning all the time! This field is developing so fast that you must try to keep track of the innovations.
Fig. 6. The Explorian 8 UAV. Photo by Ximena Tagle.
Fig. 6. The Explorian 8 UAV. Photo by Ximena Tagle.

Thanks for this insight Ximena and SITES wish you good luck with the MSc thesis!

Stephania Zabala and Ximena Tagles MSc projects are part of the integrating facility SITES Spectral.

Both Ximena and Stephanias MSc Projects are part of the EU Erasmus program in Environmental Modelling and Management (GEM).

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