Scalebar for ggplot2 spatial maps

ggScalebar.R

##
## Example for an entirely non-smart scalebar, which can be added to a ggplot2 object
## Author: Benjamin Leutner (@bleutner)
##
ggScalebar <- function(origin, ## raster object  or vector in the form of c(xmin, ymin)
        breaks = seq(0, 10000, 1000),  ## breaks for the scale bar 
        yoff = 1000, xoff = 1000,  ## position offset from origin in x and y direction (in map units)
        sdist = 1000, ## length of major ticks (in map units)
        tdist = 1000, ## distance labels to ticks (in map units)
        size = 1,  ## linewidth of scalebar
        alpha = 1,  ## transparency of scalebar
        distUnit = "km",  ## unit label
        scaleUnit = 1000,  ## scale factor for units (i.e. if map units = m, a scaleUnit of 1000 will result in km )
        kmsep=" ", ## character separating last digit and unit 5<kmsep>km
        tsize = 3, ## size of labels
        col = "black", ## color of scalebar and labels
        tinyFact = 0.6, ## size of minor (non-labelled) ticks, relative to major (labelled) ticks
        inherit.aes = FALSE ## do not use global aestetics (default=FALSE)
){
    breaks <- sort(breaks)
    if(inherits(origin, "Raster")){
        x <- xmin(origin) + xoff
        y <- ymin(origin) + yoff
    } else {
        x<-origin[1] + xoff
        y<-origin[2] + yoff
    }
    xend <- x + max(breaks)
    brx <- x + breaks
    kPad = paste0(rep(" ", nchar(distUnit)), collapse = "")
    line  <- data.frame(x = c(x,xend), y=rep(y,2))
    ticks <- data.frame(x = brx, xend = brx, y = c(y-sdist), yend = y) 
    text  <- data.frame(x = brx, y = y - sdist - tdist, lab = paste0(c(rep("", 
                                    length(breaks)-1),kPad), 
                    breaks/scaleUnit, 
                    c(rep("", length(breaks)-1),
                            paste0(kmsep, distUnit)))) 
    
    tinyticks <- data.frame(          
            x = ticks$x[-nrow(ticks)] + diff(ticks$x)/2,
            xend = ticks$xend[-nrow(ticks)] + diff(ticks$xend)/2,
            y = ticks$y[-1] + sdist*tinyFact ,
            yend = y
    )
    
    list(geom_line(data = line, aes(x = x, y = y), inherit.aes=inherit.aes, alpha = alpha, colour = col, size = size) ,
            geom_segment(data = ticks, aes(x = x, xend = xend, y = y, yend = yend),inherit.aes = inherit.aes, alpha = alpha, colour = col, size = size) ,
            geom_segment(data = tinyticks, aes(x = x, xend = xend, y = y, yend = yend), inherit.aes = inherit.aes, alpha = alpha, colour = col, size = size) ,
            geom_text(data = text, aes(x = x, y = y, label = lab), col = col, size = tsize,alpha = alpha, inherit.aes = inherit.aes))
    
}

## Example usage
library(RStoolbox)
data(lsat)
ggR(lsat, stretch = "lin") + ggScalebar(lsat, xoff=300, breaks = c(0,5000, 2500), tsize = 4,
        sdist = 300, col = "yellow", tdist = 200, size = 1)