MapGAM (version 0.6-2)

predgrid: Create a Grid and Clip it to a Map and Data Bounds

Description

Creates a data frame containing a rectangular grid of points to cover the range of X and Y coordinates provided in a data set, and trims the grid so that the points do not extend beyond the boundaries shown on a map.

Usage

predgrid(dataXY, map = NULL, nrow = 100, ncol = 100)

Arguments

dataXY

A data frame with X and Y coordinates (required). If the data frame has more than 2 columns, the X and Y coordinates must be in the 2nd and 3rd columns, respectively (the format required for the modgam function). If the data frame has only

map

A map for clipping the grid, provided in a recognized format. Supported classes include "map", the format produced by the maps package, and "SpatialPolygonsDataFrame", which can be produced from an ESRI shapefile using the readShapePoly function in the m

nrow

number of rows in the grid (default=100)

ncol

number of columns in the grid (default=100)

Value

• A data frame with X and Y coordinates in the first two columns. The column names from dataXY are used for the output. If the columns of dataXY are unnamed then the names "X" and "Y" are assigned to the data frame.

Details

predgrid creates a grid of dimensions nrow*ncol using the range of X and Y coordinates (e.g., longitude and latitude) in the data frame supplied as dataXY. If the map argument is used, the function trimdata is used to clip the grid. Users should be sure to use the same projection for the map and the data; putting both on the same plot can help reveal differing projections. If the map centroid is not in the range of the data a warning message is printed; this might indicate differing projections but can occur naturally when the data were not sampled from the entire extent of the map or when map boundaries are concave.

See Also

trimdata, optspan, modgam, colormap.

Examples

# define a rectangular 100x100 grid covering the MA data 
data(MAdata)
gamgrid <- predgrid(MAdata)
# plot the grid points
plot(gamgrid$Xcoord, gamgrid$Ycoord, cex=0.1, col="red")
# and the data locations
points(MAdata$Xcoord,MAdata$Ycoord)

# But that grid extends beyond the state boundaries and into the ocean!
# Better to also clip the grid to a map of MA using the following code
# which requires the PBSmapping package:
if (require(PBSmapping)) {
# Clip a 50x50 grid covering the MA data to a map of MA 
data(MAmap)
gamgrid2 <- predgrid(MAdata, map=MAmap, nrow=50, ncol=50)
# plot the MA map and grid points
plot(MAmap)
points(gamgrid2$Xcoord, gamgrid2$Ycoord, cex=0.1, col="red")
}