# Log-Gaussian Cox processes on metric graphs

#### David Bolin, Alexandre B. Simas

#### Created: 2023-01-30. Last modified: 2024-07-19.

Source:`vignettes/pointprocess.Rmd`

`pointprocess.Rmd`

### Introduction

In this vignette we will introduce how to work with log-Gaussian Cox
processes based on Whittle–Matérn fields on metric graphs. To simplify
the integration with `R-INLA`

and `inlabru`

hese
models are constructed using finite element approximations as
implemented in the `rSPDE`

package. The theoretical details
will be given in the forthcoming article (Bolin,
Simas, and Wallin 2023).

### Constructing the graph and the mesh

We begin by loading the `rSPDE`

, `MetricGraph`

and `INLA`

packages:

```
library(rSPDE)
library(MetricGraph)
library(INLA)
```

As an example, we consider the default graph in the package:

```
graph <- metric_graph$new(tolerance = list(vertex_vertex = 1e-1, vertex_edge = 1e-3, edge_edge = 1e-3),
remove_deg2 = TRUE)
graph$plot()
```

To construct a FEM approximation of a Whittle–Matérn field, we must first construct a mesh on the graph.

```
graph$build_mesh(h = 0.1)
graph$plot(mesh=TRUE)
```

The next step is to build the mass and stiffness matrices for the FEM basis.

` graph$compute_fem()`

We are now ready to specify the and sample from a log-Gaussian Cox
process model with intensity
$\lambda = \exp(\beta + u)$
where
$\beta$
is an intercept and
$u$
is a Gaussian Whittle–Matérn field specified by
$(\kappa^2 - \Delta)^{\alpha/2} \tau u = \mathcal{W}.$ For this we can use the function
`graph_lgcp`

as follows:

```
sigma <- 0.5
range <- 2
alpha <- 2
lgcp_sample <- graph_lgcp(intercept = 1, sigma = sigma,
range = range, alpha = alpha,
graph = graph)
```

The object returned by the function is a list with the simulated Gaussian process and the points on the graph. We can plot the simulated intensity function as

`graph$plot_function(X = exp(lgcp_sample$u), vertex_size = 0)`

To plot the simulated points, we can add them to the graph and then plot:

```
graph$add_observations(data = data.frame(y=rep(1,length(lgcp_sample$edge_loc)),
edge_number = lgcp_sample$edge_numbers,
distance_on_edge = lgcp_sample$edge_loc),
normalized = TRUE)
```

`## Adding observations...`

`graph$plot(vertex_size = 0, data = "y")`

### Fitting LGCP models in R-INLA

We are now in a position to fit the model with our
`R-INLA`

implementation. When working with log-Gaussian Cox
processes, the likelihood has a term
$\int_\Gamma \exp(u(s)) ds$
that needs to be handled separately. This is done by using the mid-point
rule as suggested for SPDE models by Simpson et al. where we approximate
$\int_\Gamma \exp(u(s)) ds \approx \sum_{i=1}^p \widetilde{a}_i \exp\left(u(\widetilde{s}_i)\right).$ Using the fact that
$u(s) = \sum_{j=1}^n \varphi(s) u_i$
from the FEM approximation, we can write the integral as
$\widetilde{\alpha}^T\exp(\widetilde{A}u)$
where
$\widetilde{A}_{ij} = \varphi_j(\widetilde{s}_i)$
and
$\widetilde{a}$
is a vector with integration weights. These quantities can be obtained
as

```
Atilde <- graph$fem_basis(graph$mesh$VtE)
atilde <- graph$mesh$weights
```

The weights are used as exposure terms in the Poisson likelihiood in R-INLA. Because of this, the easiest way to construct the model is to add the integration points as zero observations in the graph, with corresponding exposure weights. We also need to add the exposure terms (which are zero) for the actual observation locations:

```
#clear the previous data in the graph
graph$clear_observations()
#Add the data together with the exposure terms
graph$add_observations(data = data.frame(y = rep(1,length(lgcp_sample$edge_loc)),
e = rep(0,length(lgcp_sample$edge_loc)),
edge_number = lgcp_sample$edge_number,
distance_on_edge = lgcp_sample$edge_loc),
normalized = TRUE)
```

`## Adding observations...`

```
#Add integration points
graph$add_observations(data = data.frame(y = rep(0,length(atilde)),
e = atilde,
edge_number = graph$mesh$VtE[,1],
distance_on_edge = graph$mesh$VtE[,2]),
normalized = TRUE)
```

`## Adding observations...`

We now create the `inla`

model object with the
`graph_spde`

function. For simplicity, we assume that
$\alpha$
is known and fixed to the true value in the model.

`rspde_model <- rspde.metric_graph(graph, nu = alpha - 1/2)`

Next, we compute the auxiliary data:

`data_rspde <- graph_data_rspde(rspde_model, name="field")`

We now create the `inla.stack`

object with the
`inla.stack()`

function. At this stage, it is important that
the data has been added to the `graph`

since it is supplied
to the stack by using the `graph_spde_data()`

function.

```
stk <- inla.stack(data = data_rspde[["data"]],
A = data_rspde[["basis"]],
effects = c(data_rspde[["index"]], list(Intercept = 1)))
```

We can now fit the model using `R-INLA`

:

```
spde_fit <- inla(y ~ -1 + Intercept + f(field, model = rspde_model),
family = "poisson", data = inla.stack.data(stk),
control.predictor = list(A = inla.stack.A(stk), compute = TRUE),
E = inla.stack.data(stk)$e)
```

Let us extract the estimates in the original scale by using the
`spde_metric_graph_result()`

function, then taking a
`summary()`

:

```
spde_result <- rspde.result(spde_fit, "field", rspde_model)
summary(spde_result)
```

```
## mean sd 0.025quant 0.5quant 0.975quant mode
## std.dev 0.365505 0.0982022 0.197950 0.358244 0.579449 0.343971
## range 2.081110 0.9192950 0.845333 1.894760 4.394410 1.576900
```

We will now compare the means of the estimated values with the true values:

```
result_df <- data.frame(
parameter = c("std.dev", "range"),
true = c(sigma, range),
mean = c(
spde_result$summary.std.dev$mean,
spde_result$summary.range$mean
),
mode = c(
spde_result$summary.std.dev$mode,
spde_result$summary.range$mode
)
)
print(result_df)
```

```
## parameter true mean mode
## 1 std.dev 0.5 0.3655054 0.3439713
## 2 range 2.0 2.0811106 1.5769048
```

We can also plot the posterior marginal densities with the help of
the `gg_df()`

function:

```
posterior_df_fit <- gg_df(spde_result)
library(ggplot2)
ggplot(posterior_df_fit) + geom_line(aes(x = x, y = y)) +
facet_wrap(~parameter, scales = "free") + labs(y = "Density")
```

Finally, we can plot the estimated field $u$:

```
n.obs <- length(graph$get_data()$y)
n.field <- dim(graph$mesh$VtE)[1]
u_posterior <- spde_fit$summary.linear.predictor$mean[(n.obs+1):(n.obs+n.field)]
graph$plot_function(X = u_posterior, vertex_size = 0)
```

This can be compared with the field that was used to generate the data:

`graph$plot_function(X = lgcp_sample$u, vertex_size = 0)`

## An example with replicates

Let us now test show an example with replicates. Let us first simulate replicates of a latent field

```
n.rep <- 30
sigma <- 0.5
range <- 2
alpha <- 2
lgcp_sample <- graph_lgcp(n = n.rep, intercept = 1, sigma = sigma,
range = range, alpha = alpha,
graph = graph)
```

We now clear the previous data and add the new data together with the exposure terms

```
graph$clear_observations()
df_rep <- data.frame(y=rep(1,length(lgcp_sample[[1]]$edge_loc)),
e = rep(0,length(lgcp_sample[[1]]$edge_loc)),
edge_number = lgcp_sample[[1]]$edge_number,
distance_on_edge = lgcp_sample[[1]]$edge_loc,
rep = rep(1,length(lgcp_sample[[1]]$edge_loc)))
df_rep <- rbind(df_rep, data.frame(y = rep(0,length(atilde)),
e = atilde,
edge_number = graph$mesh$VtE[,1],
distance_on_edge = graph$mesh$VtE[,2],
rep = rep(1,length(atilde))))
for(i in 2:n.rep){
df_rep <- rbind(df_rep, data.frame(y=rep(1,length(lgcp_sample[[i]]$edge_loc)),
e = rep(0,length(lgcp_sample[[i]]$edge_loc)),
edge_number = lgcp_sample[[i]]$edge_number,
distance_on_edge = lgcp_sample[[i]]$edge_loc,
rep = rep(i,length(lgcp_sample[[i]]$edge_loc))))
df_rep <- rbind(df_rep, data.frame(y = rep(0,length(atilde)),
e = atilde,
edge_number = graph$mesh$VtE[,1],
distance_on_edge = graph$mesh$VtE[,2],
rep = rep(i,length(atilde))))
}
graph$add_observations(data = df_rep,
normalized = TRUE,
group = "rep")
```

`## Adding observations...`

We can now define and fit the model as previously

```
rspde_model <- rspde.metric_graph(graph, nu = alpha - 1/2)
data_rspde <- graph_data_rspde(rspde_model, name="field", repl = ".all")
stk <- inla.stack(data = data_rspde[["data"]],
A = data_rspde[["basis"]],
effects = c(data_rspde[["index"]], list(Intercept = 1)))
spde_fit <- inla(y ~ -1 + Intercept + f(field, model = rspde_model, replicate = field.repl),
family = "poisson", data = inla.stack.data(stk),
control.predictor = list(A = inla.stack.A(stk), compute = TRUE),
E = inla.stack.data(stk)$e, verbose=TRUE)
```

Let’s look at the summaries

```
spde_result <- rspde.result(spde_fit, "field", rspde_model)
summary(spde_result)
```

```
## mean sd 0.025quant 0.5quant 0.975quant mode
## std.dev 0.499984 0.0170322 0.466982 0.499855 0.53388 0.499813
## range 2.180410 0.1630560 1.878730 2.173900 2.51880 2.160720
```

```
result_df <- data.frame(
parameter = c("std.dev", "range"),
true = c(sigma, range),
mean = c(
spde_result$summary.std.dev$mean,
spde_result$summary.range$mean
),
mode = c(
spde_result$summary.std.dev$mode,
spde_result$summary.range$mode
)
)
print(result_df)
```

```
## parameter true mean mode
## 1 std.dev 0.5 0.4999837 0.4998126
## 2 range 2.0 2.1804071 2.1607161
```

### References

*In Preparation*.

*Biometrika*.