Pre-processing of rasters
Begin by masking any values that are outside of the boundaries for the state of Oregon.
state <- maps::map(database = 'state', regions = 'oregon',
fill = TRUE, plot = FALSE)
statePoly <- map2SpatialPolygons(state, IDs = state$names,
proj4string = CRS(proj4string(evi)))
evi_masked <- mask(x = evi, mask = statePoly)
elev_masked <- mask(x = elev, mask = statePoly)
Generate plots to get a sense for the spatial patterns in the data.
# plot maximum growing season EVI for Oregon
rasterVis::levelplot(evi_masked, margin = F, par.settings = eviTheme,
ylab = NULL, xlab = NULL,
main = 'Maximum Growing Season EVI')
# plot elevation (in meters) for Oregon
elevCols <- colorRampPalette(c('grey7', 'grey93'))(100)
elevTheme <- rasterVis::rasterTheme(region = elevCols)
rasterVis::levelplot(elev_masked, margin = F, par.settings = elevTheme,
ylab = NULL, xlab = NULL, main = 'Elevation (m)')
Remove any trends in the data with the ‘remove_plane’ function and take a look at the plots of the images with the trends removed.
evi_masked <- remove_plane(evi_masked)
#> [1] "Order of polynomial that minimizes errors: 1"
elev_masked <- remove_plane(elev_masked) # there was no trend
#> [1] "Order of polynomial that minimizes errors: 0"
# plot again to see what the new raster looks like
rasterVis::levelplot(evi_masked, margin = F, par.settings = eviTheme,
ylab = NULL, xlab = NULL, main = 'EVI without Trend')
rasterVis::levelplot(elev_masked, margin = F, par.settings = elevTheme,
ylab = NULL, xlab = NULL, main = 'Elevation without Trend')
Generate texture images of the state of Oregon
Below we generate a texture image for the state of Oregon using the ‘sa’ metric for elevation. Note that the following step may take some time. We provide the output dataframe files along with this vignette for all metrics included in geodiv for elevation and EVI over ~30km x 30km square moving windows and scaled following calculation for the subsequent analyses in this vignette.
# Calculate elevation sa texture image for state of Oregon
system.time(outrast <- texture_image(elev_masked, window_type = 'square', size = 7,
in_meters = FALSE, metric = 'sa', parallel = TRUE,
ncores = 4, nclumps = 20))
The code below illustrates how to convert the raster generated by the ‘texture_image’ function (i.e., outrast) to a dataframe for subsequent analyses.
# get raster values
vals <- outrast[]
# Convert output raster of sa metrics (outrast) to a dataframe for
# easier use in subsequent analyses
coords <- coordinates(outrast)
sa_data_elev <- data.frame(x = coords[, 1], y = coords[, 2], v = vals)
Generating these data frames can take a while, so we have provided .csv files for all gradient surface metrics calculated for elevation and EVI in case you find them useful for working with this vignette. The below code reads in these provided .csv files by downloading data from figshare.
# The list of figshare files was completed above, so grab the appropriate files
# for the csv's of all texture image outputs for Oregon.
tf <- tempfile()
download.file(url[[2]], destfile = tf, mode = 'wb')
data_evi <- read.csv(tf, stringsAsFactors = FALSE)
unlink(tf)
tf <- tempfile()
download.file(url[[1]], destfile = tf, mode = 'wb')
data_elev <- read.csv(tf, stringsAsFactors = FALSE)
unlink(tf)
Visualization of texture image outputs
Distributions of a few elevation variables:
for (i in c(9, 10, 18, 6)) {
hist(data_elev[, i], breaks = 30, xlab = names(data_elev)[i], main = '')
}
The code below visualizes metrics over the entire state in order to capture different aspects of landscape heterogeneity. Individual metrics primarily distinguish mountainous versus flat terrain, and managed versus more natural areas; however, some metrics are difficult to interpret, or do not show very much variation over the region. The difficulty of interpreting metrics is a known complicating factor for their use. Others have addressed this issue and linked metrics with known ecosystem features or patch metrics (McGarigal et al., 2009; Kedron et al., 2018).
# New names for plots
plt_names <- data.frame(old = names(data_elev)[3:ncol(data_elev)],
new = c('Shw', 'Srw', 'Srwi', 'Std', 'Stdi', 'S10z',
'Sa', 'Sbi', 'Sci', 'Sdc 50-55%', 'Sdc 80-85%',
'Sdc 0-5%', 'Sdq6', 'Sdr', 'Sds', 'Sfd', 'Sk',
'Sku', 'Smean', 'Sph', 'Spk', 'Sq', 'Ssc',
'Ssk', 'Sv', 'Svi', 'Svk'))
create_maps <- function(df, r, theme) {
maps_list <- list()
for (i in seq(3, ncol(df))) {
temp <- setValues(r, df[, i])
temp[is.na(r)] <- NA
goodname <- as.character(plt_names$new[plt_names$old == names(df)[i]])
maps_list[[i - 2]] <- rasterVis::levelplot(temp, margin = F,
par.settings = theme,
ylab = NULL, xlab = NULL,
main = goodname)
maps_list[[i - 2]]$par.settings$layout.heights[
c( 'bottom.padding',
'top.padding',
'key.sub.padding',
'axis.xlab.padding',
'key.axis.padding',
'main.key.padding') ] <- 1
maps_list[[i - 2]]$aspect.fill <- TRUE
names(maps_list)[i - 2] <- goodname
}
return(maps_list)
}
# Create plots of all possible surface gradient metrics that geodiv calculates
# for elevation and EVI.
elev_maps <- create_maps(data_elev, elev_masked, elevTheme)
evi_maps <- create_maps(data_evi, evi_masked, eviTheme)
# Make sure that order of maps is the same for both EVI and Elevation.
new_order <- match(plt_names$new, names(evi_maps)) # get order according to names table
evi_maps <- evi_maps[new_order]
# Create map panels (3 each for EVI and elevation).
for (l in list(elev_maps, evi_maps)) {
grid.arrange(grobs = l[1:12], nrow = 4, ncol = 3) # 850x800
grid.arrange(grobs = l[13:24], nrow = 4, ncol = 3) # 850x800
grid.arrange(grobs = l[25:27], nrow = 4, ncol = 3) # 850x800
}
Transect analysis
In the code below, we examine an example of local correlation and clustering among the surface gradient metrics by extracting values over a horizontal transect across central Oregon.
First we convert the raw elevation and EVI data from the NASA’s SRTM and MODIS mission, respectively, to a dataframe and add those raw values to the dataframes for EVI and elevation containing the gradient surface metrics we’ve calculated across the state of Oregon.
# Convert the rasters to dataframe format and add value to dataframe with
# metric values.
sp_df <- function(r, df) {
pixdf <- as.data.frame(as(r, "SpatialPixelsDataFrame"))
df$value <- pixdf[, 1]
return(df)
}
data_elev <- sp_df(elev, data_elev)
data_evi <- sp_df(evi, data_evi)
Now we extract the data along a latitudinal transect going across the state of Oregon.
# Create new dataframe of values along a latitudinal transect.
get_transect <- function(r, df) {
# Crop raster to center transect (+/- 7 pixels North or South).
center_row <- round(nrow(r) / 2)
r_crop <- crop(r, extent(r, center_row - 7, center_row + 7, 1, ncol(r)))
# Get 8th latitudinal coordinate (center latitude) from the cropped raster.
central_y <- unique(coordinates(r_crop)[, 2])[8]
# Get the closest latitude in the dataframe to the central raster coordinate.
central_y <- unique(df$y[near(df$y, central_y, 0.01)])[1]
# Extract mean EVI and elevation values by raster column.
r_means <- colMeans(as.matrix(r_crop), na.rm = TRUE)
# Now limit the dataframe to the central row across the transect.
transect_vals <- df[df$y == central_y,]
# Add column means to dataframe.
transect_vals$value <- r_means
return(transect_vals)
}
transect_elev <- get_transect(elev, data_elev)
transect_evi <- get_transect(evi, data_evi)
The code below places standardizes all metrics by placing them on the same scale from 0 - 1.
# Get all metrics on same scale (0-1).
scale_mets <- function(df) {
for (i in 3:ncol(df)) {
df[,i] <- (df[, i] - min(df[, i], na.rm = TRUE)) /
(max(df[, i], na.rm = TRUE) - min(df[, i], na.rm = TRUE))
}
return(df)
}
transect_elev <- scale_mets(transect_elev)
transect_evi <- scale_mets(transect_evi)
Clustering analysis over transect
For the transect analysis, we will perform hierarchical clustering on metric values using the function ‘eclust’ in the package factoextra (Kassambara & Mundt, 2020). First, some additional data wrangling is required to prepare the data for the clustering analysis below.
# Remove NA values from the metric columns.
rm_nas <- function(df) {
for (i in 3:ncol(df)) {
df <- df[!is.na(df[, i]),]
}
return(df)
}
transect_elev <- rm_nas(transect_elev)
transect_evi <- rm_nas(transect_evi)
The code below performs the clustering analysis on the surface gradient metrics. We first determine the optimal number of clusters by examining the gap statistic, and then plot the clustered variables to see the relationships among them.
### Plot optimal number of clusters
plot_gap <- function(df) {
# enhanced k-means clustering
res.km <- clusGap(t(df)[3:(ncol(df) - 1), ], stats::kmeans, K.max = 10,
B = 100, nstart = 25)
# gap statistic plot
factoextra::fviz_gap_stat(res.km)
}
plot_gap(transect_evi)
### Dendrogram and scatterplot of clusters
get_clusters <- function(df, nclust) {
# Enhanced hierarchical clustering using optimal # of clusters.
res.hc <- factoextra::eclust(t(df)[3:(ncol(df) - 1),],
"hclust", k = nclust)
return(res.hc)
}
plot_dendrogram <- function(res.hc, nclust){
# Plot colors
plt_cols <- c('lightgoldenrod1', 'lightblue', 'grey', 'lightsteelblue4')
# Dendrogram plot
fviz_dend(res.hc, rect = FALSE, k_colors = plt_cols[1:nclust],
lwd = 1, label_cols = 'black', cex = 0.8, main = "", ylab = "",
type = 'rectangle', horiz = TRUE, labels_track_height = 14) +
theme(axis.text.y = element_blank(), axis.ticks = element_blank())
}
plot_scatter <- function(res.hc) {
# Scatterplot
fviz_cluster(res.hc)
}
res.hc_elev <- get_clusters(transect_elev, nclust = 4)
res.hc_evi <- get_clusters(transect_evi, nclust = 3)
plot_dendrogram(res.hc_elev, nclust = 4)
plot_dendrogram(res.hc_evi, nclust = 3)
plot_scatter(res.hc_elev)
Now we generate plots that show the EVI and elevation surface gradient metrics along the Oregon state transect.
First, the data have to be gathered into a longer format for this visualization.
# Create gathered (long) version of dataframe for the clustering analysis.
gather_data <- function(df) {
df <- df %>% tidyr::gather(key = 'var', value = 'value',
names(df[, seq(3, ncol(df))]))
# Order variables.
df <- df[order(df$var),]
return(df)
}
gathered_elev <- gather_data(transect_elev)
gathered_evi <- gather_data(transect_evi)
Now we can plot the metrics along the transect, labeling the cluster.
# Plot metrics along transect, with cluster labeled.
plot_transect_mets <- function(df, res.hc, varname) {
# Map colors to cluster or variable names.
col_map <- c("1" = "lightgoldenrod1", "2" = "lightblue", "3" = "grey",
"4" = "lightsteelblue4", "EVI" = "white", "Elev" = "white")
# Create a dataframe to match variable names with cluster number.
clust_df <- data.frame(var = res.hc$labels, clust = res.hc$cluster)
clust_df <- clust_df[order(clust_df$clust),]
# Convert var to character.
clust_df$var <- as.character(clust_df$var)
# Join cluster number with main dataframe to get cluster labels for plotting.
df <- left_join(df, clust_df, by = 'var')
# Anything not labeled with a cluster (i.e., the actual value) gets labeled.
df$clust[is.na(df$clust)] <- varname
# Change 'value' label to actual variable name.
df$var[df$var == 'value'] <- varname
# Convert cluster names to factors and match with colors.
df$clust <- as.factor(df$clust)
df$var <- factor(df$var, levels = c(clust_df$var, varname))
cols_to_use <- col_map[names(col_map) %in% df$clust]
ggplot(df, aes(x = x, y = value)) +
geom_rect(aes(xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf,
fill = clust)) +
geom_line(lwd = 0.7) +
xlab('Longitude') +
facet_grid(var~., switch = 'y') +
scale_fill_manual(values = cols_to_use, name = 'Cluster') +
theme_bw() +
theme(axis.title.y = element_blank(),
axis.text.y = element_blank(),
strip.text.y.left = element_text(face = 'bold', size = 11, angle = 0),
legend.position = 'none',
axis.title.x = element_text(face = 'bold', size = 11))
}
plot_transect_mets(gathered_elev, res.hc_elev, "Elev")
plot_transect_mets(gathered_evi, res.hc_evi, "EVI")
Summary of transect cluster analysis
Overall trends along the transect were similar among metrics, with more variation at smaller intervals. Using hierarchical clustering, we found four clusters of metrics for elevation, and three for EVI. The metrics fell into different combinations based on the variable considered (elevation or EVI). For example, Sdq6 and S10z grouped together for both variables, but Std and Srw were in the same group for elevation, and different groups for EVI.
Principal Components Analysis (PCA)
Next, we will determine the statewide elevation and EVI variance explained by metrics using PCA. First, we need to get the data ready for the PCA. In the code below, we remove several variables due to their large number of NA values, caused either by windows containing too few values, or windows lacking ‘peaks’ or ‘valleys’ (pixels surrounded by lower or higher values, respectively). After cleaning the data, 21 metrics remain in the analysis.
# Get data ready for PCA by removing NA values.
clean_data <- function(df) {
# Remove columns with very large numbers of NAs.
NAs <- sapply(df, function(x) sum(is.na(x)))
rm_cols <- which(NAs >= 20000)
df <- df[, -rm_cols]
# Remove NAs from remaining columns.
df <- na.omit(df)
return(df)
}
data_elev_noNA <- clean_data(data_elev)
data_evi_noNA <- clean_data(data_evi)
In the code below, the PCA is performed with the remaining metrics using the ‘prcomp’ function in the stats package.
# Calculate the principal components.
elev_prc <- prcomp(data_elev_noNA[,3:22], center = TRUE, scale = TRUE)
evi_prc <- prcomp(data_evi_noNA[,3:22], center = TRUE, scale = TRUE)
summary(elev_prc)
#> Importance of components:
#> PC1 PC2 PC3 PC4 PC5 PC6 PC7
#> Standard deviation 2.3592 1.7964 1.5836 1.28897 1.12823 1.02005 0.99978
#> Proportion of Variance 0.2783 0.1613 0.1254 0.08307 0.06365 0.05203 0.04998
#> Cumulative Proportion 0.2783 0.4396 0.5650 0.64810 0.71174 0.76377 0.81374
#> PC8 PC9 PC10 PC11 PC12 PC13 PC14
#> Standard deviation 0.91966 0.79134 0.72898 0.60892 0.58858 0.52619 0.50061
#> Proportion of Variance 0.04229 0.03131 0.02657 0.01854 0.01732 0.01384 0.01253
#> Cumulative Proportion 0.85603 0.88734 0.91391 0.93245 0.94978 0.96362 0.97615
#> PC15 PC16 PC17 PC18 PC19 PC20
#> Standard deviation 0.42315 0.34368 0.29564 0.27348 0.11043 0.07380
#> Proportion of Variance 0.00895 0.00591 0.00437 0.00374 0.00061 0.00027
#> Cumulative Proportion 0.98510 0.99101 0.99538 0.99912 0.99973 1.00000
summary(evi_prc)
#> Importance of components:
#> PC1 PC2 PC3 PC4 PC5 PC6 PC7
#> Standard deviation 2.5246 1.9870 1.37645 1.25486 1.09009 0.99925 0.96783
#> Proportion of Variance 0.3187 0.1974 0.09473 0.07873 0.05941 0.04992 0.04683
#> Cumulative Proportion 0.3187 0.5161 0.61081 0.68955 0.74896 0.79889 0.84572
#> PC8 PC9 PC10 PC11 PC12 PC13 PC14
#> Standard deviation 0.82448 0.79386 0.70303 0.53013 0.50317 0.47567 0.40850
#> Proportion of Variance 0.03399 0.03151 0.02471 0.01405 0.01266 0.01131 0.00834
#> Cumulative Proportion 0.87971 0.91122 0.93593 0.94998 0.96264 0.97396 0.98230
#> PC15 PC16 PC17 PC18 PC19 PC20
#> Standard deviation 0.38320 0.30381 0.2531 0.1547 0.14751 0.07128
#> Proportion of Variance 0.00734 0.00461 0.0032 0.0012 0.00109 0.00025
#> Cumulative Proportion 0.98964 0.99426 0.9975 0.9987 0.99975 1.00000
Now let’s look at some diagnostic plots for the principal components. Scree plots indicate the importance of the principal components with a broken stick criterion. The point at which the scree plot curve crosses the broken stick model distribution, which we will plot in red, is considered to indicate the maximum number of components to retain.
# Take a look at the components using a screeplot.
plot_scree <- function(pc_dat) {
screeplot(pc_dat, type = "l", npcs = 15,
main = "Screeplot of the first 10 PCs")
abline(h = 1, col = "red", lty = 5)
legend("topright", legend = c("Eigenvalue = 1"),
col = c("red"), lty = 5, cex = 0.6)
}
plot_scree(elev_prc)
We can also take a look at the components for elevation and EVI to see how much variance the surface metrics explain with cumulative variance plots.
# Look at how much variance is explained using a cumulative variance plot.
plot_cvar <- function(pc_dat) {
# Get cumulative variance explained.
cumpro <- summary(pc_dat)$importance[3, ][1:16]
# Create plot of cumulative variance, marking the 5th component as the cutoff.
plot(cumpro, xlab = "PC #", ylab = "Amount of explained variance",
main = "Cumulative variance plot")
abline(v = 5, col = "blue", lty = 5)
abline(h = cumpro[5], col = "blue", lty = 5)
legend("topleft", legend = c("Cut-off @ PC5"),
col = c("blue"), lty = 5, cex = 0.6)
}
plot_cvar(elev_prc)
For both elevation and EVI, the first 5 principal components explained >70% of the variation. Now, let’s plot the first components for elevation to examine their relationships.
# Create scatterplots to look at relationships among principal components.
plot(elev_prc$x[, 1], elev_prc$x[, 2], xlab = "PC1 (27.83%)",
ylab = "PC2 (16.13%)", main = "PC1 / PC2 - plot")
plot(elev_prc$x[, 1], elev_prc$x[, 3], xlab = "PC1 (27.83%)",
ylab = "PC3 (12.54%)", main = "PC1 / PC3 - plot")
plot(elev_prc$x[, 2], elev_prc$x[, 3], xlab = "PC2 (16.13%)",
ylab = "PC3 (12.54%)", main = "PC2 / PC3 - plot")
In the code below, we map the components to see if there are any spatial patterns readily identifiable.
# Map components across state.
map_comps <- function(pc_dat, noNA_df, full_df, r, theme) {
# Add pc values to no-NA dataframe.
for (i in 1:5) {
colname <- paste0('prc', i)
noNA_df[, colname] <- pc_dat$x[, i]
}
# Add PCA results back to full raster dataframe.
full_dat <- full_df %>% left_join(noNA_df)
# Cut to only the prc columns.
full_dat <- full_dat[, grep('prc', names(full_dat))]
# Create rasters and maps with principle component values.
out_maps <- list()
for (i in 1:5) {
new_rast <- setValues(r, full_dat[, i])
pc_map <- rasterVis::levelplot(new_rast, margin = F,
par.settings = theme,
ylab = NULL, xlab = NULL,
main = paste0('PC', i))
pc_map$par.settings$layout.heights[c( 'bottom.padding',
'top.padding',
'key.sub.padding',
'axis.xlab.padding',
'key.axis.padding',
'main.key.padding') ] <- 1
pc_map$aspect.fill <- TRUE
out_maps[[i]] <- pc_map
}
# Plot in a grid.
grid.arrange(grobs = out_maps, nrow = 2, ncol = 3)
}
map_comps(elev_prc, data_elev_noNA, data_elev, elev, elevTheme)
#> Joining, by = c("x", "y", "std", "s10z", "sa", "sbi", "sci", "sdc05055", "sdc08085", "sdc0005", "sdq6", "sdr", "sds", "sk", "sku", "sph", "spk", "sq", "ssk", "sv", "svi", "svk", "value")
map_comps(evi_prc, data_evi_noNA, data_evi, evi, eviTheme)
#> Joining, by = c("x", "y", "s10z", "sa", "sbi", "sci", "sdc05055", "sdc08085", "sdc0005", "sdq6", "sdr", "sds", "sk", "sku", "sph", "spk", "sq", "ssk", "std", "sv", "svi", "svk", "value")
What are the principal component loadings for elevation?
# Plot principal component loadings.
plot_loadings <- function(pc_dat) {
# Get rotation for top 5 components.
loadings <- pc_dat$rotation[, 1:5]
# Figure out the relative loadings.
aload <- abs(loadings)
rel <- sweep(aload, 2, colSums(aload), "/")
# Convert relative loadings to dataframe.
rel <- as.data.frame(rel)
# Get good variable names (from dataframe created earlier).
rel$var <- plt_names$new[match(rownames(rel), plt_names$old)]
# Create importance plots.
imp_plts <- list()
for (i in 1:5) {
temp <- rel
# Determine whether component loading is postive or negative.
temp$sign <- factor(sapply(loadings[, i], FUN = function(x) x / abs(x)),
levels = c(-1, 1))
# Order loadings by value.
temp <- temp[order(temp[, i]),]
temp$var <- factor(temp$var, levels = temp$var)
temp_plt <- ggplot(temp, aes(x = temp[, i], y = var)) +
geom_point(size = 3, aes(pch = sign)) +
scale_shape_manual(name = element_blank(),
breaks = c(1, -1),
values = c(19, 8),
labels = c("Positive", "Negative")) +
xlab(paste0('PC', i)) +
ylab('Metric') +
theme_bw() +
theme(panel.grid.minor = element_blank(),
legend.justification = c(1, 0),
legend.position = c(1, 0),
legend.background = element_blank(),
legend.text = element_text(size = 12),
axis.title = element_text(size = 12))
imp_plts[[i]] <- temp_plt
}
# Return grid of first three components.
grid.arrange(grobs = imp_plts[1:3], ncol = 3)
}
plot_loadings(elev_prc)
What are the principal component loadings for EVI?
Summary of PCA
Looking at the PCA components and loadings, the first component for both elevation and EVI described general surface heterogeneity, whereas the second was related to the shape of the regional value distribution. The metric groupings observed match previous findings, and the first two components represent the same groupings (roughness and distribution) found by McGarigal et al. (2009).
