R maps plotting longitude and latitude points

Question

I have a map of the USA and a list of long, lat that I want to plot. Once I get this working, I also want to switch over to the "world" map. The map is generated, but no points appear on the map. The first line of the TSV file contains this header:

LONG{tab}LAT

R appears to be reading in the 'traffic' table OK. What am I doing wrong?

library("maps")

traffic = read.table("C:/temp/traffic_10.40.tsv", header=T, sep="\t")
png(filename="C:/temp/usa.png", width=850, height=600, bg="white")
map('state', plot = TRUE, fill = FALSE, col = palette())
title("Destinations")
points(x=traffic$LONG,y=traffic$LAT,col='red',cex=0.75)
dev.off()

EDIT

> dput(traffic)
structure(list(LONG = c(47.6218, 32.7942, 34.1121, 40.0068, 47.6218,
33.9553, 33.7629, 40.0068, 39.05, 38.1075, 33.7629, 32.769, 37.3857,
29.4576, 34.1674, 38.8147, 32.7942, 31.1429, 40.3254, 30.3059,
38.2248, 47.6218, 33.9553, 38.1075, 27.1943, 29.4576, 30.5175,
38.5668, 42.6657, 40.2982, 32.7539, 40.6698, 47.6742, 32.7942,
47.6218, 35.8219), LAT = c(-122.35, -96.7653, -118.411, -75.1347,
-122.35, -83.3937, -84.4226, -75.1347, -77.4833, -122.264, -84.4226,
-96.5998, -122.026, -98.5054, -84.8014, -77.0647, -96.7653, -81.471,
-78.9195, -97.7505, -85.7412, -122.35, -83.3937, -122.264, -80.2432,
-98.5054, -97.6721, -121.467, -73.799, -111.698, -97.3363, -73.9438,
-122.115, -96.7653, -122.35, -78.6588)), .Names = c("LONG", "LAT"
), class = "data.frame", row.names = c(NA, -36L))

Also, I am a R newbie and have tried finding this on google with limited success because I am not sure what to search for exactly.

Hard to say without access to your data. Any way to post the .tsv or png online? — Jeff Allen
or ... post the data online? the easiest way is to copy and paste the results of dput(traffic); or if it a big data set, try dput(traffic[1:10,] — David LeBauer
Your map of the USA appears to be a PNG file. This has no geographical coordinates associated to it, so there's no way of knowing where a given lat-long point is. It might even be in a funny projection, or Alaska could be tucked down off the coast of California for compactness, and Hawaii in the Gulf. You need a proper georeferenced raster file, or knowledge of the projection and coordinates of the PNG. — Spacedman
It might be worth reposting/moving the post over to gis.stackexchange.com There are a few R and mapping over questions there. — djq
@jftuga, thanks for asking this question. I am new to spatial statistics and was trying for hours to generate a map with plotted points yesterday. Your example is exactly what I was looking for. — Vivek Subramanian

Henry Henry · Accepted Answer · 2012-04-05T21:14:19

The problem is in your data set rather than your later code.

The first point has one co-ordinate 47.6218 and the other -122.35. Latitudes cannot be outside the range [-90,90] degrees so the longitude must be -122.35 and latitude 47.6218, the opposite of your data set. This is slightly north of the Seattle Space needle

x (horizontal) is traditionally longitude or easting; y (vertical) is traditionally latitude or northing

R maps plotting longitude and latitude points

3 Answers