Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery
Abstract
:1. Introduction
1.1. Vegetation Mapping with Unmanned Aerial Vehicles
1.2. From Imagery to Reflectance Measurements
1.3. Problems in Radiometric Calibration
1.4. Effects of Illumination Geometry
1.5. Flying Height
1.6. Objectives and Research Questions
- How sensitive to changing solar elevation and azimuth are reflectance and NDVI measured by the Sequoia in individual images and orthomosaics?
- Does flying height influence surface reflectance in Sequoia images and orthomosaics?
- How consistent is reflectance measured by the Sequoia with ground measurements from a field spectrometer?
2. Materials and Methods
2.1. Data Collection
2.1.1. Study Site
2.1.2. UAS Platform and Sensor
2.1.3. Image Acquisition
2.1.4. Ground Reference Data
2.2. Image Processing
2.2.1. Correction and Calibration of Individual Images
2.2.2. Reflectance and NDVI Maps
2.3. Analysis and Visualisation
2.3.1. Illumination Geometry
2.3.2. Flying Height
2.3.3. Comparison between UAS and Ground Reference Data
- Mean reflectance in each Sequoia band from images collected above 25 m in vertical profile flights.
- Mean reflectance in each band in the six ROIs for reflectance maps of the 25-m flight and the 13:40 10-m flight on 14 May.
3. Results
3.1. Effects of Illumination Geometry
3.2. Effects of Flying Height
3.3. Comparison between Sequoia and Ground Reference Data
4. Discussion
4.1. Anisotropic Reflectance
4.2. Flight Planning
4.3. Flying Height
4.4. Radiometric Calibration
5. Conclusions
Supplementary Materials
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Band | Centre Wavelength (nm) | Band Width (nm) | Focal Length (mm) | Image Size (pixels) | Field of View |
---|---|---|---|---|---|
Green | 550 | 40 | 3.98 | 1280 × 960 | Horizontal: 61.9° Vertical: 48.5° Diagonal: 73.7° |
Red | 660 | 40 | |||
Red Edge | 735 | 10 | |||
NIR | 790 | 40 |
Date | Target | Time of Measurements (UTC+0100) | Number of Measurements |
---|---|---|---|
27 April | Tarp | 11:29–11:31 | 4 |
Vegetation | 11:35–12:05 | 20 | |
14 May | Vegetation | 12:10–12:35 | 14 |
21 May | Tarp | 11:00–11:04 | 10 |
Vegetation | 12:15–12:50 | 22 | |
7 June | Tarp | 13:58 | 9 |
14:56 | 10 | ||
15:53 | 10 | ||
Vegetation | 16:04–16:16 | 10 |
Flight Time | Flying Height (m) | Average GSD (cm) |
---|---|---|
11:00 | 10 | 1.15 |
11:50 | 10 | 1.29 |
13:40 | 10 | 1.05 |
14:50 | 10 | 1.09 |
16:00 | 10 | 0.85 |
12:55 | 25 | 2.92 |
Flight Time | Mean Absolute NDVI Difference |
---|---|
11:00 | 0.0572 |
11:50 | 0.0484 |
14:50 | 0.0432 |
16:00 | 0.0445 |
Band | 10 m vs. 25 m | Sequoia vs. ASD | |||
---|---|---|---|---|---|
Vegetation | Tarp | ||||
10-m Flights | 25-m Flights | All Flights | All Flights | ||
Green | 0.0116 | 0.0315 | 0.0208 | 0.0267 | 0.0149 |
Red | 0.0068 | 0.0114 | 0.0089 | 0.0103 | 0.0076 |
Red edge | 0.0083 | 0.0642 | 0.0600 | 0.0621 | 0.0123 |
NIR | 0.0098 | 0.0439 | 0.0444 | 0.0441 | 0.0176 |
NDVI | 0.0193 | 0.0789 | 0.0621 | 0.0710 | 0.1394 |
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Share and Cite
Stow, D.; Nichol, C.J.; Wade, T.; Assmann, J.J.; Simpson, G.; Helfter, C. Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery. Drones 2019, 3, 55. https://doi.org/10.3390/drones3030055
Stow D, Nichol CJ, Wade T, Assmann JJ, Simpson G, Helfter C. Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery. Drones. 2019; 3(3):55. https://doi.org/10.3390/drones3030055
Chicago/Turabian StyleStow, Daniel, Caroline J. Nichol, Tom Wade, Jakob J. Assmann, Gillian Simpson, and Carole Helfter. 2019. "Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery" Drones 3, no. 3: 55. https://doi.org/10.3390/drones3030055