Loading Rasters

Once you have downloaded a set of eBird Status and Trends products, one of the first tasks is to load one of the abundance, count, or occurrence estimate and plot it. Each estimate is stored in a multi-band GeoTiff file that is most appropriately loaded as a RasterStack object in R. These “cubes” come with areas of predicted and assumed zeroes, such that any cells that are NA represent areas outside of the area of estimation. All cubes have 52 weeks, even if some weeks are all NA (such as those species that winter entirely outside of North America). The following code example shows how to load an estimate raster and assign names.

library(ebirdst)
library(raster)
library(sf)
library(rnaturalearth)
library(ggplot2)
library(viridisLite)
# handle namespace conflicts
extract <- raster::extract

# Currently, example data is available on a public s3 bucket. The following 
# ebirdst_download() function copies the species results to a selected path and 
# returns the full path of the results. In this vignette, we'll use example data 
# for Yellow-bellied Sapsucker
sp_path <- ebirdst_download(species = "example_data")

# load median abundances and label dates
abd <- load_raster(sp_path, "abundance")

# use parse_raster_dates() to get actual date objects for each layer
date_vector <- parse_raster_dates(abd)
print(date_vector)
#>  [1] "2019-01-04" "2019-01-11" "2019-01-18" "2019-01-25" "2019-02-01"
#>  [6] "2019-02-08" "2019-02-15" "2019-02-22" "2019-03-01" "2019-03-08"
#> [11] "2019-03-15" "2019-03-22" "2019-03-29" "2019-04-05" "2019-04-12"
#> [16] "2019-04-19" "2019-04-26" "2019-05-03" "2019-05-10" "2019-05-17"
#> [21] "2019-05-24" "2019-05-31" "2019-06-07" "2019-06-14" "2019-06-21"
#> [26] "2019-06-28" "2019-07-06" "2019-07-13" "2019-07-20" "2019-07-27"
#> [31] "2019-08-03" "2019-08-10" "2019-08-17" "2019-08-24" "2019-08-31"
#> [36] "2019-09-07" "2019-09-14" "2019-09-21" "2019-09-28" "2019-10-05"
#> [41] "2019-10-12" "2019-10-19" "2019-10-26" "2019-11-02" "2019-11-09"
#> [46] "2019-11-16" "2019-11-23" "2019-11-30" "2019-12-07" "2019-12-14"
#> [51] "2019-12-21" "2019-12-28"

All Status and Trends raster predictions made on a standard 2.96 x 2.96 km grid, however, for convenience lower resolution GeoTIFFs are also provided. The three available resolutions are:

  • High resolution (hr): the native 2.96 km resolution data
  • Medium resolution (mr): the hr data aggregated by a factor of 3 in each direction resulting in a resolution of 8.89 km
  • Low resolution (lr): the hr data aggregated by a factor of 9 in each direction resulting in a resolution of 26.7 km

The lower resolution rasters are particularly valuable when performing analyses over large geographic areas because working with the highest resolution data in R requires significant amounts of memory and processing time. To load raster data at a partiular resolution, use the resolution argument to load_raster(), e.g.

abd_lr <- load_raster(sp_path, "abundance", resolution = "lr")
# native resolution
res(abd)
#> [1] 2962.807 2962.809
# low resolution
res(abd_lr)
#> [1] 26665.26 26665.28

In the interest of faster processing time, we’ll work with the medium resolution rasters in this vignette, however, for actual analyses you may want to consider using the high resolution data.

abd <- load_raster(sp_path, "abundance", resolution = "mr")

Mapping Relative Abundance

One of the most common tasks with eBird Status and Trends products is to make maps of relative abundance. Basic maps can easily be produced with limited code, but to make high-quality, presentation-ready maps requires extra work. This section describes some functions in the ebirdst package intended to assist with mapping.

Projections

The sinusoidal projection that NASA provides MODIS data in, while functional because of its equal-area property, is not good for mapping because of the significant distortion of land masses across the globe. Instead, we’ll use the Eckert IV for mapping because of its pleasing conformal properties across the globe, ease of projection configuration within R and its proj4string construction, and the ease of setting central meridians that best display the Western Hemisphere. It is worth noting that while projecting to a more visually appealing projection is ideal for mapping, it is not necessary for quantitative analysis of eBird Status and Trends products, for which purposes the results can be left in the original Sinusoidal projection.

# define an eckert iv projection centered on the western hemisphere
eck <- "+proj=eck4 +lon_0=-90 +x_0=0 +y_0=0 +ellps=WGS84"

# project single layer from stack to mollweide
week38_proj <- projectRaster(abd[[38]], crs = eck, method = "ngb")
week38_proj <- trim(week38_proj)

# optionally, you can project an entire stack, but it takes much longer
# abd_proj <- projectRaster(abund, crs = eck, method = "ngb")

# map single layer with full annual extent
par(mar = c(0, 0, 0, 2))
plot(week38_proj, 
     col = abundance_palette(10, season = "weekly"), 
     axes = FALSE, box = FALSE, 
     maxpixels = ncell(week38_proj))

If working with a full set of data for a species, mapping the layer at the full spatial extent of the analysis area makes for a small map. The raster cube has zeroes for our prediction extent and NAs for the entire Western Hemisphere, unifying the weeks in a cube to the same extent. However, this also means that mapping defaults to the full spatial extent of NAs, the Western Hemisphere. To assist with this, project the RasterStack or RasterLayer to Eckert IV, calculate the full annual spatial extent for the species with the calc_full_extent() function, and then map, showing an extent that works for the entire full-annual cycle of the species.

Mapping Occurrence

Most examples in this vignette focus primarily on relative abundance estimates, as they’re the most novel and informative. However, we also provide estimates for the probability of occurrence. These are much simpler to map than abundance in terms of color scales and binning, as values range between 0 and 1 throughout the year.

occ <- load_raster(sp_path, "occurrence", resolution = "mr")

# select a week in the summer
occ_week26 <- occ[[26]]

# create breaks every 0.1 from 0 to 1
occ_bins <- seq(0, 1, by = 0.1)
occ_proj <- projectRaster(occ_week26, crs = eck, method = "ngb")
occ_proj <- trim(occ_proj)

par(mar = c(0, 0, 0, 2), cex = 0.9)
plot(occ_proj, 
     breaks = occ_bins, 
     col = abundance_palette(length(occ_bins) - 1, season = "weekly"),
     axes = FALSE, box = FALSE, 
     maxpixels = ncell(occ_proj),
     legend.width = 2, legend.shrink = 0.97)

Calculating Abundance Bins

Mapping relative abundance estimates across the full-annual cycle presents a challenge, in that patterns of concentration and dispersal in abundance change throughout the year, making it difficult to define color bins that suit all seasons and accurately reflect the detail of abundance predictions. To address this, when mapping the relative abundance data, we recommend using quantile bins based on the underlying count distribution, adjusted according to the relative abundance distribution. The function calc_bins() will generate bins using this method.

To compare, we first scale the colors linearly, based on the maximum from the entire year. Since the max is quite high, times of the year with low concentration appear flat.

year_max <- max(maxValue(abd), na.rm = TRUE)

week14_proj <- projectRaster(abd[[14]], crs = eck, method = "ngb")
week14_proj <- trim(week14_proj)

# set graphical params
par(mfrow = c(1, 2), mar = c(0, 0, 0, 4))

# use raster bounding box to set the spatial extent for the plot
bb <- st_as_sfc(st_bbox(week14_proj))
plot(bb, col = "white", border = "white")
# plot the abundance
plot(week38_proj, zlim = c(0, year_max), 
     col = abundance_palette(20, season = "weekly"), 
     maxpixels = ncell(week38_proj),
     axes = FALSE, box = FALSE, legend = FALSE, add = TRUE)

# do the same for week 14
par(mar = c(0, 0, 0, 4))
plot(bb, col = "white", border = "white")
plot(week14_proj, zlim = c(0, year_max),
     col = abundance_palette(20, season = "weekly"), 
     maxpixels = ncell(week14_proj),
     axes = FALSE, box = FALSE, legend.shrink = 0.97, add = TRUE)

We can compare this with maps made using the calc_bins() method.

# calculate ideal color bins for relative abundance
count <- load_raster(sp_path, "count", resolution = "mr")
year_bins <- calc_bins(abundance = abd, count = count)

# plot
par(mfrow = c(1, 2), mar = c(0, 0, 0, 6))
plot(bb, col = "white", border = "white")
plot(week38_proj, 
     breaks = year_bins, 
     col = abundance_palette(length(year_bins) - 1, season = "weekly"), 
     maxpixels = ncell(week38_proj), 
     axes = FALSE, box = FALSE, legend = FALSE, add = TRUE)
par(mar = c(0, 0, 0, 6))
plot(bb, col = "white", border = "white")
plot(week14_proj, 
     breaks = year_bins, 
     col = abundance_palette(length(year_bins) - 1, season = "weekly"), 
     maxpixels = ncell(week14_proj), 
     axes = FALSE, box = FALSE, legend = FALSE, add = TRUE)

# plot legend
lbls <- attr(year_bins, "labels")
plot(week14_proj, zlim = c(0, 1), legend.only = TRUE, 
     col = abundance_palette(length(year_bins) - 1, season = "weekly"), 
     breaks = seq(0, 1, length.out = length(year_bins)), 
     legend.shrink = 0.97, legend.width = 2,  
     axis.args = list(at = seq(0, 1, length.out = length(lbls)), 
                      labels = signif(lbls, 3),
                      col.axis = "black", fg = NA,
                      cex.axis = 0.9, lwd.ticks = 0,
                      line = -0.5))

Mapping Abundance

As seen in the map above, the calc_bins method excludes zeroes. However, with color bins that accurately represent the data distribution, we can add in details about zeroes, tweak the legend, and add some reference data to make a complete map. Like the quick start guide, this will show you how to download example data and plot abundance values similar to how they are plotted for the eBird Status and Trends Abundance animations.

# to add context, let's pull in some reference data to add
wh_states <- ne_states(country = c("United States of America", "Canada"),
                    returnclass = "sf") %>% 
  st_transform(crs = eck) %>% 
  st_geometry()

# we'll plot a week in the middle of summer
week26_proj <- projectRaster(abd[[26]], crs = eck, method = "ngb")
week26_proj <- trim(week26_proj)

# set graphics params
par(mfrow = c(1, 1), mar = c(0, 0, 0, 6))

# use the extent object to set the spatial extent for the plot
plot(st_as_sfc(st_bbox(week26_proj)), col = "white", border = "white")

# add background spatial context
plot(wh_states, col = "#cfcfcf", border = NA, add = TRUE)

# plot zeroes as gray
plot(week26_proj, col = "#e6e6e6", 
     maxpixels = ncell(week26_proj),
     axes = FALSE, legend = FALSE, add = TRUE)

# define color bins
qcol <- abundance_palette(length(year_bins) - 1, "weekly")

# plot abundances
plot(week26_proj, breaks = year_bins, col = qcol, 
     maxpixels = ncell(week26_proj),
     axes = FALSE, legend = FALSE, add = TRUE)

# add zero to the bin labels
bin_labels <- signif(c(0, attr(year_bins, "labels")), 3)

# create colors that include gray for 0
lcol <- c("#e6e6e6", qcol)

# plot legend
plot(week14_proj, zlim = c(-0.05, 1), legend.only = TRUE, 
     col = lcol, 
     breaks = c(-0.05, seq(0, 1, length.out = length(lcol))), 
     legend.shrink = 0.97, legend.width = 2,  
     axis.args = list(at = c(-0.05, seq(0, 1, length.out = length(bin_labels) - 1)), 
                      labels = bin_labels,
                      col.axis = "black", fg = NA,
                      cex.axis = 0.9, lwd.ticks = 0,
                      line = -0.5))

# add state boundaries on top
plot(wh_states, border = "white", lwd = 1.5, add = TRUE)

Mapping Abundance Confidence Intervals

In addition to occurrence and abundance estimates, we also provide confidence intervals at an upper value of 90% and lower value of 10%. These can be used to calculate and map a confidence band width.

# load lower and upper stacks
lower <- load_raster(sp_path, "abundance_lower", resolution = "mr")
upper <- load_raster(sp_path, "abundance_upper", resolution = "mr")

# calculate band width
conf_band <- upper[[26]] - lower[[26]]

conf_week26 <- projectRaster(conf_band, crs = eck, method = "ngb")

par(mar = c(0, 0, 0, 2))
plot(trim(conf_week26), col = magma(20), 
     maxpixel = ncell(conf_week26),
     axes = FALSE, box = FALSE)

Extracting Trajectories with Uncertainty

With RasterStacks for relative abundance estimates, as well as upper and lower confidence intervals, we can extract an abundance trajectory with uncertainty intervals and plot them across the year for a single location.

# set a point
pt <- st_point(c(-88.1, 46.7)) %>% 
  st_sfc(crs = 4326) %>% 
  st_transform(crs = projection(abd)) %>% 
  st_coordinates()

# extract
abd_traj <- extract(abd, pt, fun = mean, na.rm = TRUE)[1, ]
upper_traj <- extract(upper, pt, fun = mean, na.rm = TRUE)[1, ]
lower_traj <- extract(lower, pt, fun = mean, na.rm = TRUE)[1, ]

# plot trajectories
plot_frame <- data.frame(x = 1:length(abd_traj),
                         y = unname(abd_traj),
                         upper = unname(upper_traj),
                         lower = unname(lower_traj))

ggplot(plot_frame, aes(x, y)) +
  geom_line(data = plot_frame) +
  geom_ribbon(data = plot_frame, 
              aes(ymin = lower, ymax = upper), 
              alpha = 0.3) +
  ylab("Expected Count") +
  xlab("Week") +
  theme_light()

It is also possible to extract trajectories for regions, but it takes a little more data work.

# set an extent based on polygon
mi <- ne_states(country = "United States of America", returnclass = "sf") %>% 
  st_transform(crs = projection(abd))
mi <- mi[mi$name == "Michigan"]

# extract
# because we're using a region, we get lots of values that we need to average
abd_traj <- extract(abd, mi, fun = mean, na.rm = TRUE)
abd_traj <- apply(abd_traj, 2, mean, na.rm = TRUE)

upper_traj <- extract(upper, mi, fun = mean, na.rm = TRUE)
upper_traj <- apply(upper_traj, 2, mean, na.rm = TRUE)

lower_traj <- extract(lower, mi, fun = mean, na.rm = TRUE)
lower_traj <- apply(lower_traj, 2, mean, na.rm = TRUE)

# plot trajectories
plot_frame <- data.frame(x = 1:length(abd_traj),
                         y = unname(abd_traj),
                         upper = unname(upper_traj),
                         lower = unname(lower_traj))

ggplot(plot_frame, aes(x, y)) +
  geom_line(data = plot_frame) +
  geom_ribbon(data = plot_frame, 
              aes(ymin = lower, ymax =upper), 
              alpha = 0.3) +
  ylab("Expected Count") +
  xlab("Week") +
  theme_light()