Analyzing risk index using habitat connectivity

Krishna Keshav(

08 April, 2024


The network and connectivity of cropland can be used to analyse the potential spread of plant pathogen. While network plays a crucial role, there are several other factors that affects the spread and thus the connectivity. Although croplands may be geographically connected, the risk cannot be generalized as pathogen may not spread if it’s exclusive to specific crop. (Keshav et al. 2023) supports up to 10 parameters that has potential to impact risk and connectivity among croplands. The implementation is expanded upon (Xing et al. 2020), which discusses global cropland connectivity. This framework uses default values from the paper at the same time making them as parameters and eventually turning it into framework for the analysis of crops.

Although this article is focused on usage, it is useful to know for interested developers that package design is inspired from widely used Configuration-based design in software development (Majors 2022), (Nash and DeMore 2009), and (Allaire 2023) provides a text based interface to control the parameters values for risk analysis in this context.

Primary objective of this vignette is to help user in getting started, list capabilities and intuition behind them. It also describes underlying implementation at high level to support the intuition behind functions. Throughout the article, we will citing external sites and resources which is relevant to usage of this package.



External materials

Data sets

We use publicly available sources to obtain crop information -

Geodata provides set of APIs to access these data-sets. For visualization and plots, we use rnaturalearth.

Quick Start

Meta information

Installation and loading

Installing geohabnet will also install its dependencies. Please see the list of dependencies using desc::desc(package = "geohabnet")

if (!require("devtools")) {

if (!require("geohabnet")) {

Alternatively, install from github source.

#> Loading required package: usethis

if (!require("geohabnet")) {
  install_github("GarrettLab/HabitatConnectivity", subdir = "geohabnet")

At any point, access the help page using following -

# and
?geohabnet::msean # any function

Run analysis on default configuration

This is to run the analysis on default set of values for the supported parameters. Initially, the values are set based on the Xing et al(2021) (Xing et al. 2020) and crop is Avocado. This would run the workflow on global geographical extent quickly since the crop presence is relatively low.

res <- sensitivity_analysis()




The results are the side effects and the similar information is captured in the object returned from it. Its based on the values set for each of the parameters which we will see in details in the further sections.

Running new analysis

We provide 2 methods or entry points to run analysis by setting new values for the supported parameters.

Setting values in a function

The goal of the this function is to provide simple invasive factors as parameters. The internal implementation and program deals with object of terra (Hijmans 2023) and igraph(Csardi and Nepusz 2006) . The primary object is of type SpatRaster from terra . To get started, we will use quick way to obtain raster and later we will understand the details.

An effective way to look-up is search_crop(). It will return sources which contains the data-set for this crop.

avocado <- cropharvest_rast("avocado", "monfreda")

# verify the raster object
#> class       : SpatRaster 
#> dimensions  : 2160, 4320, 1  (nrow, ncol, nlyr)
#> resolution  : 0.08333333, 0.08333333  (x, y)
#> extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#> coord. ref. : lon/lat WGS 84 (EPSG:4326) 
#> source      : avocado_HarvestedAreaFraction.tif 
#> name        : avocado_HarvestedAreaFraction

Now that we have a raster object, it can be fed directly to the workflow. We will only set the required parameter - rast. The value represents Avocado plantation in geographical area of North America.

geo_net <- msean(avocado)




The functions msean is same as sean except msean has side effects and they return different S4 objects, GeoNetwork and GeoRasters respectively. We will use them interchangeably in this article. The results should be interpreted in accordance to the values of other parameters that have factored as arguments to sean. Run ?sean to see all the supported parameters. We will later see the usage of other parameters as well.

  1. rast - spatRaster. Represents map of crop presence in a geographical area.

  2. geoscale - Vector. Geographical coordinates in the form of c(Xmin, Xmax, Ymin, Ymax)

  3. global - Logical. When set to TRUE, geoscale is ignored.

    • Get geographical scales used in global analysis -

      #> $east
      #> [1] -24 180 -58  60
      #> $west
      #> [1] -140  -34  -58   60
    • Although recommended not to change the global scales since it has been finalized after several tries. Still, for advance use, set the global geographic scales using -

      #set_global_scales(list(east = c(-24, 180, -58, 60), west = c(-140, -34, -58, 60)))
  4. thresholds - Numeric. 2 types of thresholds: host density and link weights represented by hd_threshold and link_threshold respectively. The former threshold filters from the aggregation of input data-set/raster and latter will filter the cells from the adjacency matrix which is used to calculate the network connectivity.

  5. resolution - Numeric. This is a resolution value. In the context of SpatRaster, it’s the number of pixels that are aggregated to produce a new finer/coarser version raster data. Default is

    #> [1] 12
  6. Side effect - Calculates, produces and plots the maps which is same as sensitivity_analysis() [sensitivity_analysis()]. An alternative is sean() which can be called to obtain the results from the function call and has no side effects.

Using configuration

More parameters are available under configuration and thus more control over the analysis. The configuration file name is parameters.yaml, currently supporting up to 10 parameters. The intuition behind this methodology is to provide a basic interface for setting new values. The snippet below describes the basic usage of configuration.

Get the initial configuration file. By default this function will save the file in temporary directory tempdir(), however we recommend saving to path where program will have write permissions. Using iwindow = TRUE will prompt a selection window to save config file.

config_file <- get_parameters(out_path = tempdir())
#> [1] "/var/folders/r5/zggvft9d3yn5kh51wqp78rd00000gn/T//RtmpseEzla/parameters.yaml"

The file should look something like this -

Initial parameters.yaml
Initial parameters.yaml

The values must be modified without modifying the structure. The order don’t matter for the program. The new values in the configuration must be fed to the workflow using -

set_parameters(new_params = config_file)
#> [1] TRUE
#using iwindow = true will prompt a selction window to choose config file.

get_parameters() was only to fetch the initial parameters. While you can, it is not required to re-fetch if the parameters has not been modified in the configuration. Modify the value and feed it to workflow using set_parameters().



We have some helper functions to make the analysis easier. Available sources can be seen using -

#> [1] "monfreda"          "mapspam2010"       "mapspam2017Africa"

More specifically, we support 2 data set sources - Monfreda and Mapspam using the APIs provided by Geodata. These sources have crop presence in the geographical areas in the form of raster(tiff). To start with, we can check if a certain crop is available in the source -

search_crop("avocado") #1
#> [1] "monfreda"
search_crop("bean") #2
#> [1] "monfreda" "spam"

The #1 result suggests that we have crop information available under monfreda . Now, #2 suggests that banana is present in 2 sources. We will use the results from here to obtain its raster data from the one of the specified sources.

# get avocado data
rast_avocado <- crops_rast(list(monfreda = "avocado"))

old_to <- options("timeout")
options(timeout = 6000)

#get data of banana crop
rast_ban <- crops_rast(list(mapspam2010 = "banana"))

The returned is actually if type SpatRaster. In above, we will have separate raster information for each of the crop. Crops can be combined and be returned as one raster.

# Back to back downloads causes connection to fail, this is a workaround
rast_avo_ban <- crops_rast(list(monfreda = c("avocado", "banana"), mapspam2010 = c("banana")))
old_to <- options(old_to)

In this case for banana, information from all the sources will be first combined using mean and then all the crop information i.e. banana and avocado both are again combined into one raster. This is done to produce a single map which contains information of all the requested crops. We have one more method to obtain raster, using tiff - tiff_torast() . Finally, we have get_rasters() method, which is an abstraction of . To see the difference in data that was generated in last 3 methods -

gplot(c(rast_avocado, rast_ban, rast_avo_ban))

The result obtained from one of these methods can be used as an argument to sean() . Value of results is list of risk indices.

results <- lapply(list(rast_avocado, rast_avo_ban), msean)

Internally, cropharvest_rast() is used to fetch the crop data-set in it’s supported form.

So far, we have now run the sensitivity analysis workflow for avocado and banana on global scale and default set of parameter values. Alternatively, we can just specify the crops under parameters.yaml and run the workflow -

results <- sensitivity_analysis()





Thresholds are used to select subset of values from the SpatRaster on which the operations are applied. It directly effects the connectivity and gives a sense of sensitivity in the network. The intermediate goal is to produce a adjacency graph which essentially determines the connectivity. Cells which doesn’t meet the threshold are removed from the consideration by dispersal models.

risk_indexes <- msean(avocado, global = FALSE, hd_threshold = 0.00001, link_threshold = 0.00001)



Density Thresholds

host density threshold. The host density threshold is the minimum cropland proportion in the grid cells (or locations) that will be included in the analysis. This parameter is called HostDensityThreshold and supports a list of values between 0 and 1. Before running the sensitivity_analysis() function, check that the values for the host density threshold are smaller than the maximum host density in the map to prevent errors. The values are rounded off to 5 decimal points.


Aggregation strategy refers to the function used to create a new map of host density with a lower resolution (larger cells). Reducing the spatial resolution helps to reduce the computational power needed to run the analysis.

By default, analysis is run on both but can be opted out from one. If only one method is used, then the difference map is skipped from the outcome.

Distance methods

For each pair of locations in the host map with values greater than the host density threshold, the sensitivity_analysis() function will calculate the physical distances and use them to calculate the relative likelihood of pathogen movement between locations based on their pairwise geographical proximity.

There are two different options to calculate the distance between locations.

· Vincenty ellipsoid distance

This option is highly accurate but more computationally expensive.

· Geodesic distance

This option is less computationally expensive and less accurate than the option above.

You can set the distance option either as DistanceStrategy: “vincentyEllipsoid” or DistanceStrategy: “geodesic”. One of these options should be used as input to run the analysis. Check for supported methods in analysis by running dist_methods() in the console.

#> [1] "geodesic"          "vincentyellipsoid"


The aggregation factor or granularity is the number of small grid cells that are aggregated into larger grid cells in each direction (horizontally and vertically). The finest value is 1 which can require analysis to run up to hours because of large number of cells in SpatRaster . The resolution is also used in calculation of variance while dis-aggregating the risk indices into coarser resolution for producing maps.

If not provided, the defaulted value is selected from reso()


See available metrics using

#> [1] "betweeness"               "node_strength"           
#> [3] "sum_of_nearest_neighbors" "eigenvector_centrality"  
#> [5] "closeness"                "degree"                  
#> [7] "page_rank"

Metrics corresponding to dispersal models are applied to distance matrix with specified weights. The weights must be specified in % and sum of all the weights should be equal to 100. We use functions from (Csardi and Nepusz 2006) to calculate metrics for each dispersal model. The 2 dispersal models that are applied to parameters inverse power law and negative exponential. More formally, metrics are way to determine connectivity among nodes in a network.

In a graph functions of (Csardi and Nepusz 2006), the links are interpreted as distances. However, in the context of habitat connectivity, the network is adapted to interpret links as weights which means that the likelihood of pathogen spread is lower if the distance is larger.\[ L = \frac{1}{W}, \\ W' = \sum_{i=1}^{N} \max(W - W_i) \]

L is link weights and W is the original weights in an undirected graph. W’ is the transformed weight vector for calculating network centrality.

Geographical Extent

Geographical extent is a subset of world map defined by coordinate reference system. The corresponding parameter to set the area in sean() and sensitivity_analyis() is geoscale and GeoExtent respectively. Default setting is global = TRUE which will ignore the value of geoscale. This will consider taking the world map into account using values from global_scales() . For non-global analysis, either set global = FALSE with or without the setting geoscale. By default, geoscale will be extracted from the extents from the input SpatRaster. We recommend using EPSG:4326 as coordinate reference system because the functions have been tested on it.

  1. Using function

    results <- msean(avocado, global = FALSE, geoscale = c(-115, -75, 5, 32))
  2. Using config

    Set Global = FALSE and CustomExt = [-115, -75, 5, 32] . The initial parameters.yaml already contains this value which would run in combination with other parameters.

When provided with geoscale, program will take the subset of provided raster (data-set of a crop). The workflow will apply graph operations and network connectivity only to the subset.


By default, 3 maps are produced for each analysis. sean() also returns risk indices without maps which can, then be fed to connectivity() . This flexibility is supposed to allow users to use the risk indices for their purposes or use our function to produce maps with further different parameters.

In a code below, after obtaining results, the maps are produced. In order to calculate variance, cells of SpatRaster are extended to coarser value using res parameter. Setting maps = FALSE will suppress the calculation of outputs.

sean(avocado) + .connectivity(georast) is equivalent to sensitivity_analysis() or msean() which produces maps as side effect. The user function equivalent is connectivity() which accepts primitives types instead of S4 class.

final <- msean(avocado, link_threshold = 0.000001, hd_threshold = 0.000025)




# checkout the type of an object
#> [1] "GeoNetwork"
#> attr(,"package")
#> [1] "geohabnet"

Based on the result obtained from last cell, let’s navigate the object final object. You would have noticed the maps as a side effect and that’s really the whole point. See the result summary by simply calling the final object and navigate using the standard S4 classes approach.

#> class            :  GeoNetwork 
#> host density     :  118 , 320  (nrow, ncol) SpatRaster 
#> mean             :  /var/folders/r5/zggvft9d3yn5kh51wqp78rd00000gn/T//RtmpseEzla/plots/mean_20240408070127644237.tif 
#> mean raster      :  1416 , 3840  (nrow, ncol) 
#> variance          :  /var/folders/r5/zggvft9d3yn5kh51wqp78rd00000gn/T//RtmpseEzla/plots/variance_20240408070129166308.tif 
#> variance raster   :  1416 , 3840  (nrow, ncol) 
#> difference        :  /var/folders/r5/zggvft9d3yn5kh51wqp78rd00000gn/T//RtmpseEzla/plots/difference_20240408070129878418.tif 
#> difference raster :  1416 , 3840  (nrow, ncol)

The final operations are performed risk indices and on the 3 results that are produced -

  1. Mean

    A mean of all the SpatRasters resulting from combination of parameter values. The values in cells are added across all the indices and divided by number of indices. It represents the connectivity based on host density in the given area.

    Navigating the resulting object -


  2. Variance

    Uses stats::var on risk indices, subset is extracted for provided scale and finally pixels are dis-aggregated using factor = resolution value in original parameter to from previous step.


  3. Difference

    If both the aggregation methods (sum and mean) is selected, then difference is calculated between the rank of matrices which are essentially numeric cells of risk indices of type SpatRaster. The result is dis-aggregated in the same way as previous step.


The path to saved raster can be accessed using the ‘type_out’ slot. Additionally, access the risk indices and it’s corresponding adjacency are further accessible within slots in the GeoNetwork class.

# checkout the results
#> class   :  GeoRasters 
#> global  :  TRUE 
#> globals :  1

# global is TRUE because we original set the global analysis
# thus, we will have set of 2 risk indices, eastern and wetern hemisphere
#> [[1]]
#> class :  GlobalRast 
#> east  :  14 
#> west  :  14

# Number of elements from above determines the the number of parameter values provided

# To access the adjacency matrix,
final@rasters$global_rast[[1]]$east[[1]] # this is also s4 class 'GeoModel'
#> class            :  Model 
#> adjacency matrix :  839 ,  839  (nrow, ncol) 
#> risk index       :  ,   (nrow, ncol)

Checkout the adjacency matrices by running the code below -

# replace the indexing with any arbitary index,
# uncomment line below to see the results.


Set pmean, pvar, pdiff to FALSE to skip the any of the calculation in connectivity() to skip this calculation. In sean() or sensitivity_analysis(), set map = FALSE to skip the generation of maps as an outcome. In case of global analysis, result of eastern and western geographic extents are merged using terra::merge() . The outcome of each of the operation are saved in the new directory plots under the specified path in OutDir with name opt_datetime.tif, where opt is one of the above suffixed by datetime of the file created. If the outdir is empty, the value is defaulted to tempdir() . This applies to corresponding parameter outdir in all the functions.


To understand the motivation behind implementation, let’s analyze the complexity. Since link and host density threshold is a list, let the size be N. For the kernel models, let’s represent metrics and dispersal coefficients as x and 4 reprectively. The overall complexity turns out to be \(N \cdot N \cdot (7X + 7Y)\) . Here, we have assumed the availability of host density. Although, the crop data is fetched in parallel to minimize the download time. Considering \(X = max(X, Y)\) , the complexity will now be \(T = 7N^2 X^2 \approx 7N^4\). We discount the complexity of graph operations like those of centrality scores because we haven’t attempted to optimize these operations. We try to optimize the performance through scaling which is fixed problem size, but increasing parallelism.

The operations are compute intensive. The run_msean snippet under Hosts used up to 8.3GB of memory which was 81% of the memory allotted to RStudio. For the most part, the implementation has focused on performance over efficient memory usage.

This Package applies mechanisms such as vectorization and foreach to improve the performance and efficiency. The workflow has several parts running independently. There are independent functions which performs operations such adjacency matrix and or aggregation. This created an opportunity for task level parallelism in running functions within geohabnet. Each combination of parameters can be run independently and in parallel through most of the parts using future mechanism. The implementation supports workflow acceleration using (Bengtsson 2021a, 2021b) . It is also important to note that we use SpatRaster object throughout the computation. SpatRaster is an external pointer in C++ rather than an R object and thus adds overhead of conversion, neutralizing the performance gains.

avocado <- geohabnet::cropharvest_rast("avocado", "monfreda")

# see ?future::plan for details



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