Shadow footprint on the ground

Creates a polygonal layer of shadow footprints on the ground, taking into account:

• Obstacles outline (obstacles), given by a polygonal layer with a height attribute (obstacles_height_field)

• Sun position (solar_pos), given by azimuth and elevation angles

The calculation method was inspired by Morel Weisthal's MSc thesis at the Ben-Gurion University of the Negev.

# S4 method for SpatialPolygonsDataFrame
shadowFootprint(
  obstacles,
  obstacles_height_field,
  solar_pos = solarpos2(obstacles, time),
  time = NULL,
  b = 0.01
)

Arguments

obstacles	A SpatialPolygonsDataFrame object specifying the obstacles outline
obstacles_height_field	Name of attribute in obstacles with extrusion height for each feature
solar_pos	A matrix with one row and two columns; first column is the solar azimuth (in decimal degrees from North), second column is sun elevation (in decimal degrees)
time	When solar_pos is unspecified, time can be passed to automatically calculate solar_pos based on the time and the centroid of obstacles, using function maptools::solarpos. In such case obstacles must have a defined CRS (not NA). The time value must be a POSIXct or POSIXlt object
b	Buffer size for shadow footprints of individual segments of a given polygon; used to eliminate minor internal holes in the resulting shadow polygon.

Value

A SpatialPolygonsDataFrame object representing shadow footprint, plus buildings outline. Object length is the same as that of the input obstacles, with an individual footprint feature for each obstacle.

References

Weisthal, M. (2014). Assessment of potential energy savings in Israel through climate-aware residential building design (MSc Thesis, Ben-Gurion University of the Negev). https://www.dropbox.com/s/bztnh1fi9znmswj/Thesis_Morel_Weisthal.pdf?dl=1

Examples

time = as.POSIXct("2004-12-24 13:30:00", tz = "Asia/Jerusalem")
proj4string(build) = CRS("+init=epsg:32636")
location_geo = matrix(c(34.7767978098526, 31.9665936050395), ncol = 2)
solar_pos = maptools::solarpos(location_geo, time)
footprint1 =               ## Using 'solar_pos'
  shadowFootprint(
    obstacles = build,
    obstacles_height_field = "BLDG_HT",
    solar_pos = solar_pos
    )
#> Warning: CRS object has comment, which is lost in output
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
footprint2 =               ## Using 'time'
  shadowFootprint(
    obstacles = build,
    obstacles_height_field = "BLDG_HT",
    time = time
    )
#> Warning: CRS object has comment, which is lost in output
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
#> Warning: CRS object has comment, which is lost in output
#> Warning: spgeom1 and spgeom2 have different proj4 strings
all.equal(footprint1, footprint2)
#> [1] TRUE
footprint = footprint1
plot(footprint, col = adjustcolor("lightgrey", alpha.f = 0.5))
plot(build, add = TRUE, col = "darkgrey")