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Methods
Cameras, UAVs and Flight Design
The two RGB cameras were the on-board DJI Phantom 3 professional camera and a
Sony ActionCam, which varied in their spectral response curves despite both having
Sony sensors. The 4 k (4 Megapixels) resolution DJI camera provided the highest
resolution images. NIR spectra were recorded using a Mobius ActionCam with a
Rosco RedFire (#19) adapted filter. All three cameras were mounted on the UAV and
collected image data simultaneously for each flight. The images collected were
treated as one survey, pairing the images from the three cameras together so that
environmental conditions were the same for each camera. The cameras took an
image every 2 s, or 5 m in any direction (x, y, z), and each flight survey was taken at
a constant altitude. It was decided that the altitude for flights should be 5 m above the
tallest tree within the flight area. Tree height was measured with an Abney level and
altitude ranged from 19–27 m between flights.
Image Pre-processing: Distortion Removal
The on-board DJI camera was gimbal mounted with automatic orthorectification,
which ensured that the camera was always perpendicular to the ground. Neither the
Sony nor Mobius Action Cams were gimbal-mounted. Both Action-cams were fitted
(as standard) with fisheye lenses, which produced a distortion at the edges of an image
while compressing a wider field of view into the sensor. This had to be reciprocally
distorted to remove the edge misrepresentation and to provide the image with geospatial accuracy. This was achieved by putting batches of images through the image editing software package Adobe© Photoshop, CC 2015 (Photoshop 2016), using the
‘batch processing’ and ‘distortion removal’ functions and inputting the degree of distortion required to be removed for each specific camera. Only every 1 in 3 images was
kept to ensure there was not excessive overlay to confuse mosaicking algorithms.
The Sony and Mobius cameras had smaller sensor sizes and resolutions (1.9 K
and 2.3 K respectively) than the DJI (4 K) and thus a lower resolution at the same
altitude. The Sony and Mobius mosaics, therefore, were interpolated to bring their
pixel size down to match the DJI resolution of 7.8 mm such that the mosaics could
be combined into mosaics into one high-resolution multispectral image. The Sony
and Mobius were interpolated to a factor 1:2.1 and 1:1.7 respectively.
Image Mosaicking
DJI phantom output images were automatically georeferenced to WGS 84/ UTM
Zone 30 N reference system at a pixel size of 7.8 mm/pixel. DJI georeferenced
mosaics were automatically created with high spatial accuracy using Pix4D Mapper
A. Sanders
Methods
Cameras, UAVs and Flight Design
The two RGB cameras were the on-board DJI Phantom 3 professional camera and a
Sony ActionCam, which varied in their spectral response curves despite both having
Sony sensors. The 4 k (4 Megapixels) resolution DJI camera provided the highest
resolution images. NIR spectra were recorded using a Mobius ActionCam with a
Rosco RedFire (#19) adapted filter. All three cameras were mounted on the UAV and
collected image data simultaneously for each flight. The images collected were
treated as one survey, pairing the images from the three cameras together so that
environmental conditions were the same for each camera. The cameras took an
image every 2 s, or 5 m in any direction (x, y, z), and each flight survey was taken at
a constant altitude. It was decided that the altitude for flights should be 5 m above the
tallest tree within the flight area. Tree height was measured with an Abney level and
altitude ranged from 19–27 m between flights.
Image Pre-processing: Distortion Removal
The on-board DJI camera was gimbal mounted with automatic orthorectification,
which ensured that the camera was always perpendicular to the ground. Neither the
Sony nor Mobius Action Cams were gimbal-mounted. Both Action-cams were fitted
(as standard) with fisheye lenses, which produced a distortion at the edges of an image
while compressing a wider field of view into the sensor. This had to be reciprocally
distorted to remove the edge misrepresentation and to provide the image with geospatial accuracy. This was achieved by putting batches of images through the image editing software package Adobe© Photoshop, CC 2015 (Photoshop 2016), using the
‘batch processing’ and ‘distortion removal’ functions and inputting the degree of distortion required to be removed for each specific camera. Only every 1 in 3 images was
kept to ensure there was not excessive overlay to confuse mosaicking algorithms.
The Sony and Mobius cameras had smaller sensor sizes and resolutions (1.9 K
and 2.3 K respectively) than the DJI (4 K) and thus a lower resolution at the same
altitude. The Sony and Mobius mosaics, therefore, were interpolated to bring their
pixel size down to match the DJI resolution of 7.8 mm such that the mosaics could
be combined into mosaics into one high-resolution multispectral image. The Sony
and Mobius were interpolated to a factor 1:2.1 and 1:1.7 respectively.
Image Mosaicking
DJI phantom output images were automatically georeferenced to WGS 84/ UTM
Zone 30 N reference system at a pixel size of 7.8 mm/pixel. DJI georeferenced
mosaics were automatically created with high spatial accuracy using Pix4D Mapper
A. Sanders