mapi (version 1.1.3)

MAPI_Varicell: Function MAPI_Varicell

Description

Alter grid by changing polygons size according to samples density.

Usage

MAPI_Varicell(grid, samples, buf = 0, var.coef = 50)

Value

a spatial object of class 'sf' including cell area, the x and y coordinates of cell centroids, cell geometry (polygons) and cell id (gid) plus the computed densities and weights.

Arguments

grid

a spatial object of class 'sf' with the geometry of each cell. When using your own grid, please check that the object structure is the same as returned by MAPI_GridAuto or MAPI_GridHexagonal. Note that the computation of the cartogram may shrink the grid close to samples; therefore the grid should be built with a very large buffer around the sampling points.

samples

a data.frame with names and geographical coordinates of samples. Column names must be: 'ind', 'x', 'y'. Optional column 'errRad' with an error radius for sample locations (eg. GPS uncertainty). Coordinates must be projected (not latitude/longitude) and in the same projection as the grid.

buf

This parameter allows to clip the grid around samples by a number of units in the same reference system as the grid geographical coordinates (0 by default). A value 3-5 times smaller than the size used to build the grid may be OK.

var.coef

optional, default value=50. The expected ratio of size between largest and smallest cells in final grid.

Examples

if (FALSE) {
# Dummy example!
require(data.table, sf, mapi, spatstat.geom, spatstat.core)
data("samples")
keep <- c(2,3,6,9,10,11,16,18,19,20,21,23,26,27,29,31,33,34,38,41,46,50,54,58,61,63,65,71,72,73,
76,78,79,81,85,92,93,94,97,98,99,101,103,113,115,119,120,121,124,127,130,134,142,143,151,152,159,
160,176,185,189,191,195,196,197,198,199)
samples2 <- samples[keep, ] # keep only one third in order to create discontinuities
set.seed(1234)
# Builds a grid of hexagonal cells according to samples coordinates (columns x and y) 
# using the EPSG:3857 projection and an halfwidth cell value of hw=250m.
grid <- MAPI_GridHexagonal(samples2, crs=3857, hw=250, buf=1500)
grid.var <- MAPI_Varicell(grid, samples2, buf=250, var.coef=20)
ggplot() + 
    geom_sf(data=grid.var, aes(fill=dens), size=0.1) + 
    geom_sf(data=st_as_sf(samples2, coords=c("x","y"), crs=3857), aes())
}