quickStewart: Create Polygons of Potentials Contours

Description

This function is a wrapper around stewart, and isopoly functions. Providing only the main parameters of these functions, it simplifies a lot the computation of potentials. This function creates polygons of potential values. It also allows to compute directly the ratio between the potentials of two variables.

Usage

quickStewart(
  x,
  spdf,
  df,
  spdfid = NULL,
  dfid = NULL,
  var,
  var2,
  typefct = "exponential",
  span,
  beta,
  resolution,
  mask,
  nclass = 8,
  breaks,
  bypassctrl = FALSE,
  returnclass = "sp"
)

Value

A polyfon object is returned ("sp" or "sf", see isopoly Value).

Arguments

x: sp or sf object; this is the set of known observations to estimate the potentials from.
spdf: a SpatialPolygonsDataFrame.
df: a data frame that contains the values to compute
spdfid: name of the identifier field in spdf, default to the first column of the spdf data frame. (optional)
dfid: name of the identifier field in df, default to the first column of df. (optional)
var: name of the numeric field in df used to compute potentials.
var2: name of the numeric field in df used to compute potentials. This field is used for ratio computation (see Details).
typefct: character; spatial interaction function. Options are "pareto" (means power law) or "exponential". If "pareto" the interaction is defined as: (1 + alpha * mDistance) ^ (-beta). If "exponential" the interaction is defined as: exp(- alpha * mDistance ^ beta). The alpha parameter is computed from parameters given by the user (beta and span).
span: numeric; distance where the density of probability of the spatial interaction function equals 0.5.
beta: numeric; impedance factor for the spatial interaction function.
resolution: numeric; resolution of the output SpatialPointsDataFrame (in map units). If resolution is not set, the grid will contain around 7250 points. (optional)
mask: sp or sf object; the spatial extent of this object is used to create the regularly spaced points output. (optional)
nclass: numeric; a targeted number of classes (default to 8). Not used if breaks is set.
breaks: numeric; a vector of values used to discretize the potentials.
bypassctrl: logical; bypass the distance matrix size control (see CreateDistMatrix Details).
returnclass: "sp" or "sf"; class of the returned object.

Details

If var2 is provided, the ratio between the potentials of var (numerator) and var2 (denominator) is computed.

Examples

Run this code

# load data
data("hospital")
# Compute potentials
pot <- quickStewart(x = hospital,
                    var = "capacity",
                    span = 1000,
                    beta = 2, mask = paris, 
                    returnclass = "sf")
# cartography
if(require("cartography")){
  breaks <- sort(c(unique(pot$min), max(pot$max)), decreasing = FALSE)
  choroLayer(x = pot,
             var = "center", breaks = breaks,
             legend.pos = "topleft",
             legend.title.txt = "Nb. of Beds")
}

# Compute a ratio of potentials
hospital$dummy <- hospital$capacity + c(rep(50, 18))
pot2 <- quickStewart(x = hospital,
                     var = "capacity",
                     var2 = "dummy",
                     span = 1000,
                     beta = 2, 
                     mask = paris, 
                     returnclass = "sf")
# cartography
if(require("cartography")){
  breaks <- sort(c(unique(pot2$min), max(pot2$max)), decreasing = FALSE)
  choroLayer(x = pot2,
             var = "center", breaks = breaks,
             legend.pos = "topleft",legend.values.rnd = 3,
             legend.title.txt = "Nb. of DummyBeds")
}