plotrix (version 3.8-4)

densityGrid: Display a matrix of cell values as symbols.

Description

Displays a matrix of values as symbols on an existing image.

Usage

densityGrid(x,z=NULL,xrange=NA,zrange=NA,range.cex=c(1,10),
 xlim=c(-180,180),ylim=c(-90,90),red=c(0,1),green=c(0,1),blue=c(0,1),alpha=1,
 pch=".",geocoord=TRUE)

Value

nil. Displays a grid of symbols on an existing plot device.

Arguments

x

Matrix of values representing counts in cells (usually locations).

z

Optional matrix of values attached to the cells in x.

xrange,zrange

Explicit ranges for the counts in x and z. Used to define a "pretty" set of values to label legends.

range.cex

The range of expansion for the symbols when this is used to indicate the number of counts in the cells.

xlim

The extreme coordinates in the horizontal direction (see Details).

ylim

The extreme coordinates in the vertical direction (see Details).

red,green,blue

Values in an RGB colorspace to use in transforming the cell values into colors.

alpha

Transparency of the colors.

pch

The symbol to use in displaying the observation density. Either "." or 15 seem to work well depending upon the resolution of the grid.

geocoord

Whether the size of the symbols that indicate density when there are intensity values should be corrected for a Mercator projection.

Author

Jim Lemon

Details

densityGrid expects one matrix or a list of two matrices containing values that will be transformed into colors or sizes of the symbols displayed. The two matrices may be passed as a list. If only one matrix is present, the color of the symbols is determined by the values (counts) in the matrix. If a second matrix is passed, The values in that matrix will be used to determine the colors, and the size of the symbols will be proportional to the values in the first matrix. In the case of only one matrix, the user should set the first value of range.cex to the desired expansion of the symbols.

Currently densityGrid does not display anything in grid cells that have zero count values.

densityGrid was developed to allow very large numbers of coordinate locations to be accumulated in a matrix for display on a geographic map. Thus the default limits refer to coordinates as latitude/longitude for the earth. Because some geographic data are so numerous that memory limits are exceeded, the underlying makeDensityMatrix function can be run on small sections of the entire data set and the resulting matrices added as long as the initial coordinate limits are used throughout. Note that if the values for counts (with one matrix) or for intensity (with two matrices) cover a very large range, it may be necessary to trim extreme values (noting this on any legends) and transform the data (usually log10) to get good color separation.

See Also

makeDensityMatrix,color.scale

Examples

 if (FALSE) {
  data(l2010)
  # log10 transform both density and intensity
  l2010[[1]]<-log10(l2010[[1]])
  l2010[[2]]<-log10(l2010[[2]])
  library(maps)
  x11(width=10)
  par(mar=c(7,3,2,3))
  plot(0,xlim=c(-180,180),ylim=c(-90,90),type="n",axes=FALSE,xlab="",ylab="")
  densityGrid(l2010,pch=".",xrange=c(0,6),zrange=c(2,8),range.cex=c(2,8),
   red=c(0,0.5,1),green=c(0,1,0),blue=c(1,0,0),alpha=1)
  color.legend(-60,-107,60,-97,c("2","3","4","5","6","7","8"),
   rect.col=color.scale(1:7,cs1=c(0,0.5,1),cs2=c(0,1,0),cs3=c(1,0,0),alpha=1),
   cex=0.5)
  par(xpd=TRUE)
  text(0,95,"Lightning strikes 2010")
  text(0,-114,"Mean intensity kVA (10^n)",cex=0.5)
  points(x=seq(-60,60,20),y=rep(-125,7),pch=".",cex=1:7)
  text(x=seq(-60,60,20),y=rep(-132,7),c("<=1","2","3","4","5","6",">6"),cex=0.5)
  text(0,-142,"Cell frequency (10^n)",cex=0.5)
  par(xpd=FALSE)
  map("world",mar=c(7,3,2,3),add=TRUE)
  dev.off()
  # now only Australia
  par(mar=c(7,3,2,3))
  plot(0,xlim=c(112,154),ylim=c(-43.8,-11.1),type="n",axes=FALSE,xlab="",ylab="")
  densityGrid(l2010,pch=".",xrange=c(0,6),zrange=c(2,8),range.cex=c(2,8),
   xlim=c(112,154),ylim=c(-43.8,-11.1),red=c(0,0.5,1),green=c(0,1,0),
   blue=c(1,0,0),alpha=1)
  color.legend(120,-47,146,-45,c("2","3","4","5","6","7","8"),
   rect.col=color.scale(1:7,cs1=c(0,0.5,1),cs2=c(0,1,0),cs3=c(1,0,0),alpha=1),
   cex=0.5)
  par(xpd=TRUE)
  text(133,-9,"Lightning strikes 2010 (Australia)")
  text(133,-48.2,"Mean intensity kVA (10^n)",cex=0.5)
  points(x=seq(121,145,4),y=rep(-50,7),pch=".",cex=1:7)
  text(x=seq(121,145,4),y=rep(-51,7),c("<=1","2","3","4","5","6",">6"),cex=0.5)
  text(133,-52,"Cell frequency (10^n)",cex=0.5)
  par(xpd=FALSE)
  map("world",mar=c(7,3,2,3),xlim=c(112,154),ylim=c(-43.8,-11.1),add=TRUE)
 }