Create a composite of multiple plan position indicators (ppi)

Combines multiple plan position indicators (ppi) into a single ppi. Can be used to make a composite of ppi's from multiple radars.

composite_ppi(
  x,
  param = "all",
  nx = 100,
  ny = 100,
  xlim,
  ylim,
  res,
  crs,
  raster = NA,
  method = "max",
  idp = 2,
  idw_max_distance = NA,
  coverage = FALSE
)

Arguments

x: A list of ppi objects.
param: Character (vector). One or more parameter name(s) to composite. To composite all available scan parameters use all (default).
nx: number of raster pixels in the x (longitude) dimension
ny: number of raster pixels in the y (latitude) dimension
xlim: x (longitude) range
ylim: y (latitude) range
res: numeric vector of length 1 or 2 to set the resolution of the raster (see res). If this argument is used, arguments nx and ny are ignored. Unit is identical to xlim and ylim.
crs: character or object of class CRS. PROJ.4 type description of a Coordinate Reference System (map projection). When 'NA' (default), an azimuthal equidistant projection with origin at the radar location is used. To use a WSG84 (lat,lon) projection, use crs="+proj=longlat +datum=WGS84"
raster: (optional) RasterLayer with a CRS. When specified this raster topology is used for the output, and nx, ny, res arguments are ignored.
method: Character (vector). Compositing method(s), either mean, min, max or idw. To apply different methods for each of the parameters, provide a vector with the same length as param.
idp: Numeric. Inverse distance weighting power.
idw_max_distance: Numeric. Maximum distance from the radar to consider in inverse distance weighting. Measurements beyond this distance will have a weighting factor of zero.
coverage: Logical. When TRUE, adds an additional coverage parameter to the ppi indicating the number of ppis covering a single composite pixel.

Value

A ppi object.

Details

The function can combine multiple ppis of different scan elevations of the same radar or ppis of different radars. The coordinates of the returned ppi object are in the WGS84 datum, unless a different crs is provided. If only res is provided, but no crs is set, res is in meters and the origin of the composite ppi is set to the mean(lat, lon) location.

The method parameter determines how values of different ppis at the same geographic location are combined:

mean: Compute the average value.
max: Compute the maximum value. If ppis are of the same radar and the same polar volume, this computes a max product, showing the maximum detected signal at that geographic location.
min: Compute the minimum value.
idw: This option is useful primarily when compositing ppis of multiple radars. Performs an inverse distance weighting, where values are weighted according to 1/(distance from the radar)^idp.

Argument method determines how values of different ppi's at the same geographic location are combined.

mean: Compute the average value
max: Compute the maximum value. If ppi's are of the same radar and the same polar volume, this computes a max product, showing the maximum detected signal at that geographic location.
min: Compute the minimum value
idw: This option is useful primarily when compositing ppi's of multiple radars. Performs an inverse distance weighting, where values are weighted according to 1/(distance from the radar)^idp

The coordinates system of the returned ppi is a WGS84 (lat, lon) datum, unless a different crs is provided. If only res is provided, but no crs is set, res is in meter units and the origin of the composite ppi is set to the mean (lat, lon) location.

This function is a prototype and under active development

Examples

# Locate and read the polar volume example file
# \donttest{
pvolfile <- system.file("extdata", "volume.h5", package = "bioRad")
pvol <- read_pvolfile(pvolfile)

# Calculate a ppi for each elevation scan, 1000m grid resolution
ppis <- lapply(pvol$scans, project_as_ppi, grid_size=1000)

# Overlay the ppis, calculating the maximum value observed
# across the available scans at each geographic location
composite <- composite_ppi(ppis, method = "max", res=1000)

# Plot the calculated max product on the basemap
if (all(sapply(c("ggspatial","prettymapr", "rosm"), requireNamespace, quietly = TRUE))) {
map(composite)
}
#> Zoom: 8
#> Fetching 4 missing tiles
#> 
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#> ...complete!

# }