This scripts evaluates real hemispherical images to estimate local sky view fraction and below-canopy shortwave radiation if the image is taken inside a forest.
This is a first complete version of HPEval to be references as vs 1.2 | created 2018/08 | last modified on 2019/09/14
T.Jonas1), C.Webster1,2), G.Mazzotti1), J.Malle1,3)
1) WSL Institute for Snow and Avalanche Research SLF, Davos, CH
2) University of Edinburgh, School of GeoSciences, Edinburgh, UK
3) Northumbria University, Newcastle upon Tyne, UK
T.Jonas, C.Webster, G.Mazzotti, and J.Malle; 2019; HPEval: A canopy shortwave transmission model using high resolution hemispherical images; Agricultural and Forest Meteorology, in review
- solar position calculations from NOAA, found online at www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html (accessed 2018/06)
- function to split solar radiation into direct/diffuse component as in the factorial snow model FSM 1.0 by R. Essery available at www.geosci-model-dev.net/8/3867/2015 (accessed 2018/06)
- function to convert UTM into WGS84 coordinates, found online at www.mathworks.com/matlabcentral/fileexchange/44242-utm2lonlat (accessed 2018/08)
- hemispherical images taken around Davos Switzerland and corresponding meta data; by SLF
- shortwave radiation data measured at locations were above images were taken test images; by SLF
- DEM at 100 m spatial resolution, derived from TopoSwiss data available for research; by TopoSwiss
The script has a number of optional outputs, which include
- reclassification of image (canopy, terrain, sky, solar track)
- shortwave radiation (per horizontal, inclined, sun-facing area)
- sky view fraction (for flat and/or hemispherical sensor)
The scripts includes the following steps (some of them being optional):
- grayscale image (according to external input on threshold and thresholding method)
- correct image distortion (using a lens-specific profile)
note: this is an essential feature and should not be skipped - mask surrounding terrain and add to image (based on DEM)
note: helps to avoid misclassification of snow on the ground - calculate sky view fraction
- calculate position of sun (according to formulas by NOAA)
- plot sun disk on image behind canopy / terrain elements to evaluate the transmissivity of direct radiation through the canopy
- split swr data measured at open site into diffuse/direct radiation (according to formulas in FSM 1.0 by R. Essery)
note: potential swr is used if no measurements are available - calculate sub-canopy swr based on diffuse radiation times sky view fraction plus direct radiation times transmissivity
local spherical coordinates are given as [rad,tht,phi], where
- rad is radial distance in [m]
- tht is zenith angle (theta) in degree, i.e.
sun @ zenith is tht = 0;
sun @ horizon is tht = 90 - phi is azimuthal angle (phi) in degree, where
0° is North
90° is East
180° is South
270° is West
note that hemispherical images are taken upwards so that azimuthal angles increase in counterclockwise direction, i.e. South is up if West is left
All settings such as paths to I/O data and model parameters are handled through an external settings file the path/filename of which is the only input argument to this script
run from the Matlab command line: HPEval('HPEval_Settings.m')