[R-sig-Geo] using Albers equal area conic projection

Sarah A Spaulding Sarah.Spaulding at Colorado.EDU
Tue May 19 18:48:21 CEST 2009

I working to generate a map of diatom species presence, abundance and relative abundance for the western US. I would like to use an Albers equal area conic projection, with standard parallels of 29.5, 45.5, and central meridion of 96. The map that I generate does not match the one that (at least I think) is an Albers projection.

Is it correct to specify the standard parallels in the parameter statement? For example,
map("world", projection ="albers", parameter =c(29.5,45.5), fill=FALSE, xlim=c(-125, -95), ylim = c(30,50))

How is the central meridian specified? I have tried using the "orientation" argument, but I don't think that is quite right, as I get rotated maps.

I appreciate suggestions on what I might be missing.

# make a map of the US distribution

# read from one file that contains lat & lon sites for species
tax1 <- read.csv("EMAPweblist_2008_01_08.csv", header = T)

# specify the taxon column number in tax1
tax <- 217
library (maps)

# create a map using the Albers equal area conic projection  
# the standard parallels are 29.5 and 45.5, is it correct to place these in "parameters = c(29.5,45.5)"?
# the central meridian is 96, how is that designated? 

# create the map
map("world", projection ="albers", parameter =c(29.5,45.5), fill=FALSE, xlim=c(-125, -95), ylim = c(30,50))
map("state", projection ="", par=c(29.5,45.5), boundary = FALSE, lty = 2, add = TRUE)


# plot points on the map for species absence, presence and relative abundance
points (mapproject (tax1$LON_DD, tax1$LAT_DD, projection = "albers", par=c(29.5,45.5)), cex = .6, pch = 20)
points (mapproject (tax1$LON_DD, tax1$LAT_DD, projection = "albers", par=c(29.5,45.5)), lwd = 5, cex = (tax1$dummy_20 * 20), col = "red")

Sarah A. Spaulding
Ecologist, US Geological Survey

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