calc_zenith_colrow {rcaiman}

Calculate zenith raster coordinates

Description

Calculate zenith raster coordinates from points digitized with the open-source software package ‘ImageJ’. The zenith is the point on the image that represents the zenith when upward-looking photographs are taken with the optical axis vertically aligned.

Usage

calc_zenith_colrow(path_to_csv)

Arguments

path_to_csv

Character vector. Path to a CSV file created with the point selection tool of ‘ImageJ’ software.

Details

The technique described under the headline ‘Optical center characterization’ of the user manual of the software Can-Eye can be used to acquire the data for determining the zenith coordinates. This technique was used by Pekin and Macfarlane (2009), among others. Briefly, it consists in drilling a small hole in the cap of the fisheye lens (it must be away from the center of the cap), and taking about ten photographs without removing the cap. The cap must be rotated about 30º before taking each photograph.(NOTE: The method implemented here does not support multiple holes).

The point selection tool of ‘ImageJ’ software should be used to manually digitize the white dots and create a CSV file to feed this function. After digitizing the points on the image, use the dropdown menu Analyze>Measure to open the Results window. To obtain the CSV file, use File>Save As...

Another method–only valid when enough of the circle perimeter is depicted in the image– is taking a very bright picture (for example, a picture of the corner of a room with walls painted in light colors) with the lens completely free (do not use any mount). Then, digitize points over the circle perimeter. This was the method used for producing the example file (see Examples). It is worth noting that the perimeter of the circle depicted in a circular hemispherical photograph is not necessarily the horizon.

Value

Numeric vector of length two. Raster coordinates of the zenith, assuming a lens facing up with its optical axis parallel to the vertical line. It is important to note the difference between the raster coordinates and the Cartesian coordinates. In the latter, the vertical axis value decreases downward, but the opposite is true for the raster coordinates, which works like a spreadsheet.

References

Pekin B, Macfarlane C (2009). “Measurement of crown cover and leaf area index using digital cover photography and its application to remote sensing.” Remote Sensing, 1(4), 1298–1320. doi:10.3390/rs1041298.

See Also

Other Lens Functions: azimuth_image(), calc_diameter(), calc_relative_radius(), calibrate_lens(), crosscalibrate_lens(), expand_noncircular(), extract_radiometry(), fisheye_to_equidistant(), fisheye_to_pano(), lens(), test_lens_coef(), zenith_image()

Examples

## Not run: 
path <- system.file("external/points_over_perimeter.csv",
                    package = "rcaiman")
calc_zenith_colrow(path)

## End(Not run)