quickStewart {SpatialPosition}

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"
)

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.

Value

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

See Also

stewart, isopoly

Examples

# 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")
}