patter: particle algorithms for animal movement

Particle filters, smoothers and sampling algorithms for animal movement modelling in R

patter provides particle filtering, smoothing and sampling algorithms for animal movement modelling, with a focus on passive acoustic telemetry systems. This wraps and enhances a fast Julia backend (Patter.jl). The methodology enables the reconstruction of movement paths and patterns of space use. patter unifies a suite of methods formerly known as the flapper algorithms and supersedes the experimental flapper package (Lavender et al., 2023).

Note: patter is a new R package. Like all new packages, you should use it with a degree of caution. Please share feedback and issues.

Highlights

patter is designed to reconstruct movement paths and emergent patterns of space use from animal tracking data. A powerful, flexible, process-orientated, particle-based framework is used for this purpose. This framework unifies the flapper algorithms and provides important opportunities for development, which we exploit here.

The essential functions are pf_filter() and pf_smoother_*():

• pf_filter() is the particle filter. This simulates the possible locations of an individual moving forwards in time, accounting for all of the data (for example, acoustic observations, depth observations and any other observations) up to each time point and the animal’s movement (a partial marginal distribution).
• pf_smoother_*() is a particle smoothing algorithm. At each time step, the smoother accounts for all of the data from both the past and the future (the full marginal distribution) and substantially refines maps of space use.

We hope to add backward sampling algorithms to the package in due course.

Evolution

patter evolved from the experimental flapper package, but is:

• More powerful, with a substantially revised methodology;
• Faster, with overhauled internal routines;
• Simpler to use and maintain;
• Stable, with fewer dependencies and an upgraded spatial ecosystem;
• Better tested, with comprehensive unit tests (in progress!);

See NEWS for a summary of the evolution of flapper to patter.

At the time of writing (May 2024), patter is more streamlined than flapper and focuses on the implementation of fast particle-based algorithms for the reconstruction of movements and patterns of space use. Please get in touch if you would like to see additional functionality brought into patter.

Installation

Note: patter currently works on Windows and MacOS. On Windows, everything should work if you follow the instructions below. On MacOS, some additional set up (such as compiler configuration) may be required, depending on your set up. In our (limited) experience, patter installs but crashes on Debian/Ubuntu. This is due to a conflict between the GDAL/GEOS/PROJ libraries used by R and Julia (which we hope to solve in due course). Please let us know your experiences if you are using other Linux distributions. In case of issues, you should be able to use Patter.jl directly, which on some systems may be simpler than getting R and Julia to play together!

1. Install R. This package requires R version ≥ 4.1 (but the most recent version is recommended). You can check your version from the R console using R.version.string.

2. Install build packages. Package installation and configuration (may) require the devtools, pkgbuild and here packages. Install them with:

install.packages(c("devtools", "pkgbuild", "here"))

3. Install system libraries.

• On Windows, package building requires RTools. You can check whether RTools is installed with pkgbuild::has_rtools(). If RTools is not installed, it is necessary to download and install the appropriate version of RTools before proceeding by following the instructions here.
• On MacOS, some system-specific step up (e.g., compiler configuration) may be required. Follow the steps below and address any issues as required for your system.
• On Linux, our experience is currently limited—please share yours.

4. Install Julia. Julia is high-performance programming language that patter uses as a backend. If you do not have Julia installed on your system, you can install Julia via R using JuliaCall:

# Install the {JuliaCall} package:
install.packages("JuliaCall")

# Install `Julia` via {JuliaCall}:
library(JuliaCall)
julia <- julia_setup(installJulia = TRUE)

# Validate the Julia installation:
# * TRUE: `Julia` is working! 
# * FALSE: `Julia` is not working (see below)! 
isTRUE(try(julia_eval('true'), silent = TRUE))

If this doesn’t work, download and install Julia from JuliaLang. Then retry julia_setup():

julia <- julia_setup()
isTRUE(try(julia_eval('true'), silent = TRUE))

If you run into issues at this stage, it is likely that Julia has not been installed properly or R can’t find it. You may need to tell R the location of the Julia binary via JULIA_HOME (see ?JuliaCall::julia_setup() and the JuliaCall README for troubleshooting and ways to get help).

5. Install patter. To install patter from the main branch, use:

devtools::install_github("edwardlavender/patter", 
                         dependencies = TRUE, 
                         build_vignettes = TRUE)

The dependencies = TRUE argument ensures that suggested packages are also installed, which are required for some functions and to build vignettes. This process may take several minutes. Set build_vignettes = FALSE for a faster installation.

To install patter from the development (dev) branch, if available, use:

devtools::install_github("edwardlavender/patter@dev", 
                         dependencies = TRUE, 
                         build_vignettes = TRUE)

This branch may include bug fixes and new features but should be used with caution.

We recommend using renv (or similar) and RStudio Projects to track the version of patter that you use in your projects. This will ensure that your code continues to work, even if we have to make breaking changes to patter as the package evolves in response to user feedback.

6. Connect to Julia. At the start of every R session, you need to connect R to Julia (and patter to Patter.jl):

# Load & attach {patter}:
library(patter)

# Option (A): Connect to `Julia` using default options: 
julia <- julia_connect()

#  Option (B): Connect to `Julia` within an RStudio Project (recommended):
julia <- julia_connect(JULIA_PROJ = here::here("Julia"))

The first time you run julia_connect(), it will connect to Julia and install (and pre-compile) Patter.jl and the additional Julia dependencies. This may take a few minutes. Subsequent julia_connect() calls will be faster.

7. Validate the R—Julia connection. To validate that patter works on your system, run:

julia_validate()

This should return NULL, invisibly, in which case you are good to go. Otherwise, the function will return an error (or R may crash). Please report any issues you experience during this process.

Functionality

Vignettes

For an introduction to patter, use:

• vignette("a-methodology", package = "patter") for a conceptual introduction to the methodology;
• vignette("b-workflow-outline", package = "patter") for an overview of the workflow;

For a full list of all functions, see help(package = 'patter').

For a glossary of key arguments, see glossary.

Datasets

For example datasets from the Movement Ecology of Flapper Skate project (datasets-mefs), which inspired patter, see:

• dat_moorings for acoustic receiver deployments;
• dat_acoustics for acoustic time series;
• dat_archival for archival (depth) time series;
• dat_gebco() for a bathymetry grid;

To validate new datasets for use with patter, see pat_setup_data() and/or the assemble_*() function documentation.

For example algorithm outputs (datasets-algorithms), see:

• dat_path() for an example output from sim_path_walk();
• dat_coa() for an example output from coa();
• dat_pff() and dat_pfb() for an example output from pf_filter();
• dat_tff() for an example output from pf_smoother_two_filter();

Set up Julia

To link patter and the Patter.jl Julia backend, use:

• julia_connect() to connect to R to Julia;
• julia_validate() to validate the R—Julia connection;
• set_seed() to set the seed in R and Julia;
• set_map() to make a SpatRaster of the study area available in Julia;

These functions should be run at the start of every R session.

Abstract Types

patter is based on three Abstract Types, defined in Julia:

• State structures hold the state (location) of an animal at a given time step;
• ModelMove structures hold movement model, used to simulate new states;
• ModelObs structures hold observation model parameters, used to evaluate the correspondence between simulated states and observations;

Simulation

To simulate animal movement time series, see:

• sim_path_walk() to simulate a movement path from a walk model (via ModelMove);
• sim_array() to simulate an acoustic array;
• sim_observations() to simulate observational time series (via ModelObs);

To evaluate model skill in reconstructing simulated patterns, see skill_*() functions:

• skill_mb() to calculate mean bias;
• skill_me() to calculate mean error;
• skill_rmse() to calculate root mean squared error;
• skill_R() to calculate Spearman’s rank correlation coefficient;
• skill_d() to calculate the index of agreement;

Data exploration

For help with data acquisition, processing, checking and preliminary analyses, see the flapper package. This facilitates:

• Data preparation;
• Spatial operations;
• Distance calculations;
• Movement analyses;

Please submit a feature request if you would like functions from flapper in patter.

Algorithms

The main thrust of patter is the provision of fast, integrated modelling workflow based on particle filtering for reconstructing animal movement paths and emergent patterns of space use from observational time series (with a focus on passive acoustic telemetry systems).

To assemble datasets for particle filtering, use assemble_*() functions:

• assemble_timeline() assembles a timeline;
• assemble_acoustics() assembles an acoustic time series;
• assemble_archival() assembles an archival time series;

Ancillary time series should be structured in the same way for inclusion in the particle filter.

To implement particle filtering (PF) routines, use:

• pf_filter() to implement the particle filter;
• pf_smoother_two_filter() to implement the two-filter smoother;

These functions return pf_particles-class objects.

For convenience plotting functions, see:

• pf_plot_xy() to plot particle locations;

For mapping utilisation distributions, use:

• map_pou() to map probability-of-use;
• map_dens() to create smooth maps using spatstat, plus the supporting functions:
  • as.im.SpatRaster(), to convert SpatRasters to pixel images;
  • as.owin.SpatRaster(), to convert SpatRasters to observation windows;
  • as.owin.sf(), to convert sf objects to observation windows;
• map_hr_*() to map home ranges, specifically:
  • map_hr_prop() for a custom range;
  • map_hr_core() for the ‘core’ range;
  • map_hr_home() for the ‘home’ range;
  • map_hr_full() for the full range;

Options

For additional options in patter, see:

• patter-progress to monitor function progress;

Usage

Set up

This is the basic patter workflow to reconstruct movement paths and patterns of space use from animal tracking data. First, we load some essential packages:

library(patter)
#> This is {patter} v.1.0.1. For an overview, see `?patter`. For support, contact edward.lavender@eawag.ch.
library(data.table)
library(dtplyr)
library(dplyr, warn.conflicts = FALSE)
options(patter.verbose = FALSE)

Second, we connect R to Julia and set the seed in R and Julia to ensure reproducibility of our simulations:

julia_connect()
julia_validate()
set_seed()

Third, we define the properties of our study area; namely, a SpatRaster of our study area that defines the area within which movements are possible and the timeline over which we will model movements:

# Define map 
map <- dat_gebco()
set_map(map)

# Define timeline 
timeline <- seq(as.POSIXct("2016-03-17 01:50:00", tz = "UTC"),
                as.POSIXct("2016-03-18 01:48:00", tz = "UTC"), 
                by = "2 mins")

Movement

We will reconstruct the movements of a tagged flapper skate (Dipturus intermedius) within a study area off the west coast of Scotland, based on electronic tagging and tracking data. To do so, we need a model for the individual’s movements and a series of observation models that connect movements to observations. In this example, we are interested in the two-dimensional (x, y) location of our animal through time (that is, the animal’s ‘state’ is an object of type StateXY). The animal can move up to 750 m in two minutes, which is the resolution at which we will model movement, and we formulate a random walk model accordingly based on step lengths and turning angles:

# Define the animal's state:
state      <- "StateXY"

# Formulate a corresponding movement model:
mobility   <- 750.0
model_move <- move_xy(dbn_length = glue::glue("truncated(Gamma(1, 250.0), upper = {mobility})"),
                      dbn_angle = "Uniform(-pi, pi)")

# Visualise realisations of the movement model:
map |> 
  sim_path_walk(.timeline = timeline,
                .state = state,
                .model_move = model_move, 
                .n_path = 4L, .one_page = TRUE) |> 
  invisible()

Observations

We have collected acoustic and archival (depth) observations from tagged flapper skate. Let’s pull out the time series for a selected individual:

# Define acoustic detections
acc <-
  dat_acoustics |>
  filter(individual_id == 25L) |>
  mutate(individual_id = NULL) |>
  as.data.table()

# Define archival (depth) observations
arc <-
  dat_archival |>
  filter(individual_id == 25L) |>
  mutate(individual_id = NULL,
         depth_sigma = 50,
         depth_deep_eps = 30) |>
  rename(obs = depth) |>
  as.data.table()

Individual movements are connected to the observations by models of the observation process for each dataset. Without going into details, here we bundle together the observations with the parameters of the observation models:

model_1   <- "ModelObsAcousticLogisTrunc"
acoustics <- assemble_acoustics(.timeline = timeline,
                                .acoustics = acc,
                                .moorings = dat_moorings)

model_2  <- "ModelObsDepthNormalTrunc"
archival <- assemble_archival(.timeline = timeline,
                              .archival = arc)

Of course, you do not need acoustic and archival data to implement the algorithms (these are just the data we have collected from flapper skate)—other datasets can be used just as easily. To simulate observations instead, see sim_observations().

Particle filter

We are now in a position to run the particle filter. This runs a simulation forwards (or backwards) in time, sampling states (locations, termed ‘particles’) that are consistent with the movement model and the observations up to and including each time point. We end up with a time series (data.table) of particles that approximate the partial marginal distribution for the location of the animal, at each time step:

# List filter arguments
args <- list(.map = map,
             .timeline = timeline,
             .state = state,
             .xinit_pars = list(mobility = mobility),
             .yobs = list(acoustics, archival),
             .model_obs = c(model_1, model_2),
             .model_move = model_move,
             .n_record = 500L,
             .n_particle = 1e5L)

# Forward run
fwd <- do.call(pf_filter, args, quote = TRUE)
head(fwd$states)
#>    path_id timestep           timestamp map_value        x       y
#>      <int>    <int>              <POSc>     <num>    <num>   <num>
#> 1:       1        1 2016-03-17 01:50:00  59.76520 709142.1 6253007
#> 2:       1        2 2016-03-17 01:52:00  68.53316 709276.5 6253291
#> 3:       1        3 2016-03-17 01:54:00  45.86026 709476.1 6252964
#> 4:       1        4 2016-03-17 01:56:00  44.46762 709390.0 6252794
#> 5:       1        5 2016-03-17 01:58:00  60.64737 708976.4 6252849
#> 6:       1        6 2016-03-17 02:00:00  55.42853 709437.9 6253395

# Backward run
args$.direction <- "backward"
bwd <- do.call(pf_filter, args, quote = TRUE)

Particle smoother

Particle smoothers refine the outputs from the particle filter. Smoothed particles approximate the full marginal distribution for the location of the individual at each time step (accounting for all of the data before and after each step).

smo <- pf_smoother_two_filter(.n_particle = 100L, .n_sim = 100L)

Mapping

Particles can be used to reconstruct movement paths and patterns of space use. We can estimate a utilisation distribution from our particle samples as follows:

# Estimate UD
ud <- map_dens(.map = map,
               .coord = smo$states,
               sigma = spatstat.explore::bw.diggle)$ud
#> Observation window is gridded.

# Add home range
map_hr_home(ud, .add = TRUE)
mtext(side = 4, "Probability density", line = -3)

This basic workflow is highly customisable. You have the flexibility to define species-specific movement models, include any type of observational dataset and implement system-specific observation models. See the vignettes and function examples for further details and reach out with queries.

Resources

For full details on the methods, see the references below.

For further information of the patter package, see:

• ?patter::patter for an overview of package functions;
• ?patter::pf_filterfor information on specific functions (such as pf_filter());

For further code examples, see:

• patter-eval for an extensive simulation-based workflow and analysis;
• patter-flapper for a complete real-world analysis;

Disclaimer and troubleshooting

patter is a new R package. All routines are experimental. Researchers interested in using the package are encouraged to get in touch while the methods and package remain at an early stage of evolution (edward.lavender@eawag.ch).

Citation

To cite patter in publications, please use:

• Lavender, E. et al. (2023). An integrative modelling framework for passive acoustic telemetry. Methods in Ecology and Evolution. https://doi.org/10.1111/2041-210X.14193
• Lavender, E. et al. (in prep). Particle algorithms for animal movement modelling in autonomous receiver networks.
• Lavender, E. et al. (in prep). Particle algorithms for animal tracking in R and Julia. https://doi.org/10.1101/2024.07.30.605733
• Lavender, E. et al. (in prep). Particle algorithms reveal patterns of space use in a Critically Endangered elasmobranch.

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Please note that patter is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.