Question

I can’t find any documentation of the following problem I’m having with the axis labels in RGoogleMaps:

library(RgoogleMaps)
datas <- structure(list(LAT = c(37.875, 37.925, 37.775, 37.875, 37.875), 
                   LON = c(-122.225, -122.225, -122.075, -122.075, -122.025)), 
                   .Names = c("LAT", "LON"), class = "data.frame", 
                   row.names = c(1L, 2L, 3L, 4L, 5L))
# Get bounding box.
boxt <- qbbox(lat = datas$LAT, lon = datas$LON)
MyMap <- GetMap.bbox(boxt$lonR, boxt$latR, destfile = "Arvin12Map.png", 
maptype = "mobile")
PlotOnStaticMap(MyMap, lat = datas$LAT, lon = datas$LON, 
                axes = TRUE, mar = rep(4, 4))

When I run this on my computer the horizontal axis ranges from 300W to 60E, but the ticks in between aren’t linearly spaced (300W, 200W, 100W, 0, 100E, 160W, 60W).
Also, the vertical axis moves linearly from 300S to 300N. It seems that no matter what data I supply for datas, the axes are always labeled this way.

My question is:
1. Does this problem occur on other machines using this code?
2. Does anyone have an explanation for it?
and
3. Can anybody suggest a way to get the correct axes labels (assuming these are “incorrect”, but maybe i’m somehow misinterpreting the plot!)?

Thank you for your time.

Answer 1

1. Yes

2. As @Andrie suggested, this appears to be a bug. When axes = TRUE, the degAxis() function called by PlotOnStaticMap() extracts the x and y plot coordinates of the pretty tickmarks found by axTicks(). degAxis() expects these coords to be in the coordinate system of the map, but rGoogleMaps returns them as pixel coordinates, calculated from a central origin. With a plot size of 640 x 640, the pretty tickmarks are assigned to -300, -200, -100, 0,100, 200, 300 in both E-W and N-S directions. You end up with 300W, 200W, 100W, 0, 100E, 160W, 60W, because the degreeLabelsEW() function called by degAxis() assumes that, given longitudes must fall within [-180, 180], any longitudes higher than 180 are in fact in the western hemisphere (e.g. 200E is 20 degrees eastward into the western hemisphere, i.e. 160W). Not sure why it doesn’t perform similarly with nonsensical N, S and W coordinates.

3. A quick workaround, continuing with your MyMap object:

   PlotOnStaticMap(MyMap, lat = datas$LAT, lon = datas$LON, 
                   axes = FALSE, mar = rep(4.5, 4))
   
   # x-axis
   xrange <- MyMap$BBOX$ur[2] - MyMap$BBOX$ll[2]
   xticklength <- xrange / (length(axTicks(1)) - 1)
   xticklabs <- seq(MyMap$BBOX$ll[2], MyMap$BBOX$ur[2], xticklength)
   xticklabs <- parse(text = paste(sprintf('%.2f', abs(xticklabs)), 
       ifelse(xticklabs < 0, '*degree*W', '*degree*E'), sep=''))
   axis(1, at=axTicks(1), xticklabs, cex.axis=0.8)
   
   # y-axis
   yrange <- MyMap$BBOX$ur[1] - MyMap$BBOX$ll[1]
   yticklength <- yrange / (length(axTicks(2)) - 1)
   yticklabs <- seq(MyMap$BBOX$ll[1], MyMap$BBOX$ur[1], yticklength)
   yticklabs <- parse(text = paste(sprintf('%.2f', abs(yticklabs)), 
       ifelse(yticklabs < 0, '*degree*S', '*degree*N'), sep=''))
   axis(2, at=axTicks(2), yticklabs, cex.axis=0.8, las=1)