contourPlot2: Contour Plot of pollutant concentration field

In simulariatools: Simularia Tools for the Analysis of Air Pollution Data

Contour Plot of pollutant concentration field

Description

The function contourPlot2 generates a contour plot of a given quantity, such as the ground concentration of an airborne pollutant or odour, defined on a regular grid.

Usage

contourPlot2(
  data,
  x = "x",
  y = "y",
  z = "z",
  domain = NULL,
  background = NULL,
  underlayer = NULL,
  overlayer = NULL,
  legend = NULL,
  levels = NULL,
  size = 0,
  fill = TRUE,
  tile = FALSE,
  transparency = 0.75,
  colors = NULL,
  mask = NULL,
  inverse_mask = FALSE,
  bare = FALSE
)

Arguments

data	A dataframe in long format with three columns for Easting, Northing and values to be plotted.
x	charactrer. Name of the column containing Easting (longitude) coordinates (default "x").
y	character. Name of the column containing Northing (latitude) coordinates (default "y").
z	character. Name of the column containing concentration values (default "z").
domain	optional list of six numeric values defining the boundaries of the domain to be plotted (minimum X, maximum X, minimum Y, maximum Y) and the number of ticks on X & Y axis. Example: c(340000, 346000, 4989500, 4995500, 5, 5). If missing, all the full domain of the input data is considered, with 5 ticks.
background	filename. Optional path to a raster file to be plotted as the basemap.
underlayer	optional list of layers to be plotted between base map and contour plot. See Details
overlayer	optional list of layers to be plotted on top of the contour plot. See Details.
legend	character. Optional title of the legend.
levels	numeric vector of levels for contour plot. If not set, automatic pretty levels are computed. If -Inf and Inf are used as the lowest and highest limits of the array, the lowest and highest bands are unbounded and the legend shows < and >= symbols.
size	numeric. Width of the contour line.
fill	logical. If TRUE the contour plot is filled with colour (default = TRUE).
tile	logical. If TRUE rectangular tiles are plotted (default = FALSE).
transparency	transparency level of the contour plot between 0.0 (fully transparent) and 1.0 (fully opaque). Default = 0.75).
colors	colour palette for contour plot, as an array of colours.
mask	character. Path to shp file used as a mask. It must be a closed polygon.
inverse_mask	logical. If TRUE areas on mask are masked. Default is to mask areas outside the polygon defined in the shp file.
bare	boolean (default FALSE). If TRUE only the bare plot is shown: axis, legend, titles and any other graphical element of the plot are removed.

Details

This is a convenience function to plot contour levels of a scalar quantity such as pollutants computed by a dispersion model, with ggplot2 version >= 3.3.0.

Data are required to be on a regular grid, typically (but not necessarily) in UTM coordinates. The input dataframe has to be in long format, i.e. one line per value to be plotted. The names of the columns corresponding to x, y and z can be specified in the input parameters.

If tile = TRUE data are shown as they are, without any graphical interpolation required for contour plots. This is helpful when you want to visualise the raw data. Since version 2.4.0, when tile = TRUE the intervals include the lowest bound and exclude the highest bound: ⁠[min, max)⁠. Note: In previous version it was the opposite.

underlayer and overlayer layers are ggplot2 objects to be shown at different levels of the vertical stack of the plot. These are useful to show topographical information related to the plot, such as sources or receptors locations.

When a shp file is given to the mask argument the plot is drawn only inside the polygon. In order to avoid boundary artifacts due to reduced resolution, original data are resampled to higher resolution (currently set to 10 times the original one.) Ifinverse_mask is set to TRUE, the plot is drawn outside the polygon. The mask feature is based on the terra::mask() function. The CRS of the shp file is applied to the data in the data.frame. Please, keep in mind this feature is still experimental.

Value

A ggplot2 object.

Examples

# Load example data in long format
data(volcano)
volcano3d <- reshape2::melt(volcano)
names(volcano3d) <- c("x", "y", "z")
# Contour plot with default options
v <- contourPlot2(volcano3d)
v

# Set levels, and properly format the legend title:
contourPlot2(
    volcano3d,
    levels = c(-Inf, seq(100, 200, 20), Inf),
    legend = expression("PM"[10] ~ "[" * mu * "g m"^-3 * "]")
)

# Sometimes, instead of a contour plot it is better to plot the original
# raster data, without any interpolation:
contourPlot2(
    volcano3d,
    levels = c(-Inf, seq(100, 200, 20), Inf),
    tile = TRUE
)

# Since contourPlot2 returns a `ggplot2` object, you can add instructions as:
library(ggplot2)
v +
    ggtitle("Example volcano data") +
    labs(x = NULL, y = NULL)

simulariatools documentation built on Sept. 1, 2025, 5:09 p.m.