How to draw grid contours within a map

fcst$lon[121:125]
[1] -60.0 -57.5 -55.0 -52.5 -50.0
fcstn$lat[23:27]
[1] -35.0 -32.5 -30.0 -27.5 -25.0

From: Lyndon Estes <lestes at princeton.edu>
Subject: Re: [R-sig-Geo] How to draw grid contours within a map
To: "Thiago Veloso" <thi_veloso at yahoo.com.br>
Cc: "R SIG list" <r-sig-geo at stat.math.ethz.ch>
Date: Tuesday, 15 February, 2011, 12:43
Hi Thiago,

I was doing something a bit similar to what you are doing
just
recently.? Instead of creating a raster of the grids,
I created
polygon grids, which is then quite simple to overlay on
other
polygons.

Here's a function I used to create the polygon grids:

createPolyGrid <- function(x, y, h, w, size, prj) {
# Creates a polygon grid in meters
# Args:
#???x: X coordinate of lower left corner of
lower left of polygon, in meters
#???y: Y coordinate of lower left corner of
lower left of polygon, in meters
#???h: The desired North-South extent of the
grid
#???w: The desired East-West extent of the
grid
#???size: Grid polygon cell size in meters
#???prj: Object holding the crs string for
the desired projection (optional)
# Returns:
#???A SpatialPolygonsDataFrame with square
grids, with North-South and
East-West distances rounded to
#???avoid fractionating grid cells. An
attribute data frame of 1
column holding polygon identifiers is created

? if(missing(x)) {
? ? stop("missing x")
? } else if(missing(y)) {
? ? stop("missing y")
? } else if(missing(h)) {
? ? stop("missing h")
? } else if(missing(w)) {
? ? stop("missing w")
? } else if(missing(size)) {
? ? stop("missing size")
? }

? # Define numbers of row and columns in grid,
rounding to nearest whole numbers
? xrow <- round(w / size, 0)
? yrow <- round(h / size, 0)

? # Define coordinates for center of lower left cell
? xcen <- x + size / 2
? ycen <- y + size / 2

? # Create grid topology
? grd <- GridTopology(c(xcen, ycen), c(size, size),
c(xrow, yrow))

? # Turn it into polygons
? if(missing(prj)) {
? ? grd.shp <-
as.SpatialPolygons.GridTopology(grd)
? } else if(!missing(prj)) {
? ? grd.shp <-
as.SpatialPolygons.GridTopology(grd, prj)
? }

? # Create data frame and then SpatialPolygons into
SpatialPolygonsDataFrame
? grd.dat <-
as.data.frame(matrix(1:length(grd.shp), ncol = 1))
? colnames(grd.dat) <- "polID"
? row.names(grd.dat) <-
as.character(grd.dat$polID)
? grd.out.shp <- SpatialPolygonsDataFrame(grd.shp,
grd.dat, match.ID = FALSE)

? return(grd.out.shp)
}


# Here's an example of how to make the grid with the
function above
and plot it over another polygon, based on
# the meuse dataset (meuse.sr example taken from
http://r-spatial.sourceforge.net/gallery/)

library(gstat)

data(meuse.riv)
meuse.sr =
SpatialPolygons(list(Polygons(list(Polygon(meuse.riv)),"meuse.riv")))
? # Polygon of river

# Find extent of river shape and use the coordinates to
create polygon grid
ex <- bbox(meuse.sr)
gr <- createPolyGrid(x = ex[1, 1], y = ex[2, 1], h =
ex[2, 2] - ex[2,
1], w = ex[1, 2] - ex[1, 1], size = 500)

# Plot
plot(meuse.sr)
plot(gr, add = T)

Hope this helps.

Cheers, Lyndon
















On Tue, Feb 15, 2011 at 7:22 AM, Thiago Veloso <thi_veloso at yahoo.com.br>
wrote:

? Dear R-Siggers,

? In order to illustrate the spatial domain of the

General Circulation Model (GCM) output data I am using, I
need to display some grids within a map. They need to be
2.5?x2.5? lat-lon over specific coordinates. Actually, I
am trying to partially reproduce Figure 1 inside Challinor
et al., 2005 (image attached).

? To load and display the shapefile I use the

following code:

library(maptools)

southern=readShapePoly("D:/Maps/papers-in-progress/regiao_sul.shp")

plot(southern)

? Following, I create the "empty" grids to draw:

library(raster)
rprecip<-raster(xmn=-60,xmx=-50,ymn=-35,ymx=-25)
res(rprecip)<-2.5

? But I am stuck on the task of displaying those

grids on the map.

? Could anybody please point any directions?

? Best,

? Thiago.

-- 
Lyndon Estes
Research Associate
Woodrow Wilson School
Princeton University
+1-609-258-2392 (o)
+1-609-258-6082 (f)
+1-202-431-0496 (m)
lestes at princeton.edu