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2. Topographic maps

Source: vignettes/topographies.Rmd
topographies.Rmd

1. Topography matrices

The built-in volcano topographic data.

data(volcano)

Attach the package.

Create a color ramp

Specify tiepoints for the color ramp calibration.

tiepoints <- data.frame(
    z = c(0, 100, 120, 160,220), 
    color = c("#46AF64", "#96C869", "#E1D791", "#CDB991", "#7D695A")
)

Construct an actual color ramp from the tiepoints:

# expand
ramp <- expand(tiepoints, n=256) 
str(ramp)
#> List of 3
#>  $ col   : chr [1:256] "#46AF64" "#46AF64" "#47AF64" "#48AF64" ...
#>  $ breaks: num [1:257] -0.431 0.431 1.294 2.157 3.02 ...
#>  $ mid   : num [1:256] 0 0.863 1.725 2.588 3.451 ...
#>  - attr(*, "class")= chr "calibramp"

Visualize the color ramp.

plot(ramp)

Plotting with image

Use the color ramp to plot the data.

image(volcano, breaks=ramp$breaks, col=ramp$col, axes=FALSE)

Use the ramplegend function to draw a legend:

image(volcano, breaks=ramp$breaks, col=ramp$col, axes=FALSE)
ramplegend("topright", ramp=ramp, cex=0.7, box=list(border=NA, col="#ffffff55"))

Different shading 

tiepoints2 <- data.frame(
    z = c(0, 100, 110, 130,200), 
    color = c("#46AF64", "#96C869", "#E1D791", "#CDB991", "#7D695A")
)
ramp2 <- expand(tiepoints2, n=512)

The same topography, same palette, with different tiepoints:

image(volcano, breaks=ramp2$breaks, col=ramp2$col, axes=FALSE)
ramplegend("topright", ramp=ramp2, cex=0.7, box=list(border=NA, col="#ffffff55"))

Changes

A hypothetical succession assuming a 5 meter increase:

par(mfrow=c(3,2), mar=c(1,1,1,1))
image(volcano-10, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="-10 m")
image(volcano-5, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="-5 m")
image(volcano, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="+0 m")
image(volcano+5, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="+5 m")
image(volcano+10, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="+10 m")
image(volcano+15, breaks=ramp$breaks, col=ramp$col, axes=FALSE, main="+15 m")
ramplegend("bottomright", ramp=ramp, cex=0.7, box=list(border=NA, col="#ffffff55"))

With imagePlot

A similar effect can be reached using the imagePlot function from fields extension package, which will display the legend for color ramp automatically.

library(fields)
#> Loading required package: spam
#> Spam version 2.10-0 (2023-10-23) is loaded.
#> Type 'help( Spam)' or 'demo( spam)' for a short introduction 
#> and overview of this package.
#> Help for individual functions is also obtained by adding the
#> suffix '.spam' to the function name, e.g. 'help( chol.spam)'.
#> 
#> Attaching package: 'spam'
#> The following objects are masked from 'package:base':
#> 
#>     backsolve, forwardsolve
#> Loading required package: viridisLite
#> 
#> Try help(fields) to get started.

# the function call
imagePlot(volcano, breaks=ramp$breaks, col=ramp$col, axes=FALSE)

2. Georeferenced raster topography

Built-in objects for topographies

The rampage package includes a built-in set of topogoraphic color schemes (topos). These can be accessed with the data function.

data(topos)

This object is a list of data.frames that are potential inputs to the expand function. For instance, if you want to generate 256 colors from the 'zagreb' theme, you can do that with

topoRamp <- expand(topos$zagreb, n=256)

This ramp can be visualized with the rampplot function:

rampplot(topoRamp)

This shows the exact values that are rendered to the specific interals of height.

Here are the currently accessible ramps:

par(mfrow=c(2,3))
for(i in 1:length(topos)){
        # the current color map
        current<- expand(topos[[i]], n=100)
        rampplot(current, xlab=names(topos[i]), breaklabs=FALSE)
}

Topographies with terra

The terra extension is an ideal tool to visualize georeference topographic data, such as the ETOPO global topographic relief model. To provide an example, a downscaled 0.1x0.1 degree-resolution version of this ETOPO1 version of this model is deposited on the package’s website, which you can read in with the following line of code:

library(terra)
#> terra 1.8.54
#> 
#> Attaching package: 'terra'
#> The following object is masked from 'package:fields':
#> 
#>     describe
etopo <- rast("http://adamtkocsis.com/rampage/etopo1_Ice_c_gdal_0.1.nc")
etopo
#> class       : SpatRaster 
#> size        : 1800, 3600, 1  (nrow, ncol, nlyr)
#> resolution  : 0.1, 0.1  (x, y)
#> extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#> coord. ref. : lon/lat WGS 84 
#> source      : etopo1_Ice_c_gdal_0.1.nc 
#> name        : etopo1_Ice_c_gdal_0.1

This can be visualized with the default viridis palette using the plot function:

plot(etopo)

We can get this to work with the our topographic ramp, by referring to the col abd breaks arguments of the plot, as we did before.

plot(etopo, col=topoRamp$col, breaks=topoRamp$breaks)

Note that by default this will use a categorical legend, which is not ideal. Setting type="continuous" will reposition the color values, making it difficult to assert the correctness of the legend-to-values relationship. If a legend is desired, it can be plotted with the ramplegend function:

plot(etopo, col=topoRamp$col, breaks=topoRamp$breaks, legend=FALSE)
ramplegend(col=topoRamp$col,breaks=topoRamp$breaks, cex=0.3, x=-160, y=20)

Which leaves a much better visual impression.