plot_hotspot: Hotspot mapping and visualisation

In realTimeSurv: Real Time Disease Surveillance Using Geospatial Statistical Modelling

Description

A function for mapping hotspots according to user defined criteria.

Usage

plot_hotspot(
  lg,
  covariates,
  threshold.var = NULL,
  threshold.value = NULL,
  labels,
  threshold.prob = 0.8,
  relative = TRUE,
  osm = FALSE,
  per.days = 10000,
  msq = 10000
)

Arguments

lg	Output from a call to lgcp
covariates	A spatialPolygonsDataFrame covering the area of interest and containing the covariate and population density data. Typically the same object as specified in the covariates argument in the call to lgcp.
threshold.var	A vector of one or two strings specifying the variables to define the hotspots, see Details for how to specify.
threshold.value	A vector or one or two values indicating the threshold(s) for determining a hotspot. Given in the same order as threshold.var.
labels	A vector of two or four labels for the hotspots, see Details.
threshold.prob	A vector of one or two values specifying the exceedence probabilities.
relative	A logical value. If one or both of the variable is with respect to a previous time period, whether the comparison should be relative (TRUE) or absolute (FALSE)
osm	A logical value indicating Whether to include a Open Street Map map under the plot.
per.days	If one or both of the variables is incidence, the denominator number of person-days.
msq	The denominator for the population density in m^2. Default is hectares (per 10,000m^2)

Details

A “hotspot” is defined as an area that exceeds a user-defined criterion with probability of at least p. The criterion can be a function of one or two variables derived from the model; where two variables are used then there are four possible hotspot classifications, where only one is used then there are two classifications (above or below the threshold).

The log-linear model can be divided into a set of multiplicative components:

(A) population density x (B) size of the area x (C) average disease rate x (D) RR observed covariates x (E) RR latent process

A threshold can be any combination of these factors, or their difference over time. The user can specify the combination using the labels (A)x(C) poppp (A)x(B)x(C) pop (D) obs (E) latent in the argument to threshold.var as an additive sum. For example, to specify the incidence (in person-days) as the variable 'poppp+obs+latent', or to specify the overall relative risk of an area 'obs+latent'. To difference the variable with respect to t time periods prior, add '+lag(t)'. So to use the incidence rate ratio relative to 7 days prior, we can specify 'poppp+obs+latent+lag(7)'. The 'hotspot' is an area where Pr(variable > threshold) > p.

Hotspots are labelled in the following way. For a single variable definition, the labels are given as c(a,b) where

a = Pr(variable > threshold) <= p

b = Pr(variable > threshold) > p

For a two variable definition the labels are c(a,b,c,d) where

a = Pr(variable 1 > threshold 1) <= p1 & Pr(variable 2 > threshold 2) <= p2

b = Pr(variable 1 > threshold 1) > p1 & Pr(variable 2 > threshold 2) <= p2

c = Pr(variable 1 > threshold 1) <= p1 & Pr(variable 2 > threshold 2) > p2

d = Pr(variable 1 > threshold 1) > p1 & Pr(variable 2 > threshold 2) > p2

The labels do not need to be unique.

Value

An lgcpRealPlot object comprising a list of two ggplot objects. The first is the hotspot classifications, the second the exceedence probabilities. An object outl is exported to the global environment to reduce needing to reload sampling data on further calls to the same lgcpReal object. This can be removed if needed as it can be large.

Examples

data(dat,square,square_pop)
lg1 <- lgcp(data=dat,
            pop.var = c("popdens"),
            boundary=square,
            covariates=square_pop,
            cellwidth=0.1,
            laglength = 7,
            mala.pars=c(200,100,1),
            nchains=2)
plot_hotspot(lg1,
             covariates = square_pop,
             threshold.var = c("poppp+obs+latent",
                               "poppp+obs+latent+lag(3)"),
             threshold.value = c(0.1,1),
             threshold.prob=0.8,
             labels=c('low','high incidence',
                      'rising incidence','both'))

realTimeSurv documentation built on May 18, 2021, 9:07 a.m.