SRTM 1 second arc elevation data and optimal cycling path from the Cathedral of Christ the Saviour to the Pushkin Square in Moscow

# A proof-of-concept code to calculate
# cycling distance between two points in
# central Moscow that incorporates elevation profiles
#
# By Dmitriy Skougarevskiy, 2016-10-30

# Dependencies
#install.packages(c("gdistance", "RColorBrewer"))

library(gdistance)
library(RColorBrewer)

setwd("your/path")

# Use SRTM V3 elevation raster with 1 arc second (~30 metres)
# resolution from http://earthexplorer.usgs.gov/download/8360/SRTM1N55E037V3/GEOTIFF/EE
# (SRTM1N55E037V3, acquired on 11 February 2000)

# Load the raster of Moscow heights
moscow_area <- raster("n55_e037_1arc_v3.tif")

# Crop to the area within the Garden Ring 
moscow_centre <- crop(moscow_area, extent(37.5860, 37.6556, 55.7339, 55.7734))

# Clean up
rm(moscow_area)

# Ensure that our crop was correct
# plot(moscow_centre)

# Hill shading
hill_slope <- terrain(moscow_centre, opt = "slope")
aspect <- terrain(moscow_centre, opt =  "aspect")
hill <- hillShade(hill_slope, aspect, 40, 270)

# Cycling distance computation between two points
# Builds on Example 1 of vignette("gdistance1")
altDiff <- function(x){x[2] - x[1]}

hd <- transition(moscow_centre, altDiff, 8, symm = F)
slope <- geoCorrection(hd)
adj <- adjacent(moscow_centre, cells=1:ncell(moscow_centre), pairs = T, directions = 8)
speed <- slope

# Tobler's Hiking function
speed[adj] <- 6 * exp(-3.5 * abs(slope[adj] + 0.05))

# Cycling function from P Nourian, F Hoeven, S Rezvani & S Sariylidiz (2015)
# "Easiest paths for walking and cycling: Combining syntactic and geographic
# analyses in studying walking and cycling mobility"
# NB: this function seems to have errors
#speed[adj] <- (85*9.81*sin(abs(slope[adj]))+25)/112

Conductance <- geoCorrection(speed)

# Introduce two coordinates to compute shortest cycling path
## A hypothetical start point (the Cathedral of Christ the Saviour)
start <- c(37.6053, 55.7446)
## A hypothetical end point (Pushkin's square)
finish <- c(37.6063, 55.7651)

start_finish_hiking_path <- shortestPath(Conductance, start, finish, output="SpatialLines")

# Final plot
png(file = "moscow_centre_elevation_path.png", width = 2400, height = 2400, pointsize = 60)
par(mar=c(0,0,0,0))
plot(hill, col = grey(0:100/100), legend = F, main = "")
plot(moscow_centre, col = rev(brewer.pal(11,"RdYlBu")), alpha=0.5, add=TRUE)
lines(start_finish_hiking_path, col="green", lwd = 20)
text(start[1], start[2] - 0.001, "Christ the Saviour", col = "white")
text(finish[1], finish[2] + 0.001, "Pushkin Square", col = "white")
dev.off()