# LGCPs - Spatial covariates

#### David Borchers and Finn Lindgren

#### Generated on 2024-07-10

Source:`vignettes/articles/2d_lgcp_covars.Rmd`

`2d_lgcp_covars.Rmd`

Set things up

```
library(INLA)
library(inlabru)
library(fmesher)
library(RColorBrewer)
library(ggplot2)
bru_safe_sp(force = TRUE)
bru_options_set(control.compute = list(dic = TRUE)) # Activate DIC output
```

## Introduction

We are going to fit spatial models to the gorilla data, using factor
and continuous explanatory variables in this practical. We will fit one
using the factor variable `vegetation`

, the other using the
continuous covariate `elevation`

(Jump to the bottom of the practical if you want to start gently with a 1D example!)

## Get the data

`data(gorillas_sf, package = "inlabru")`

This dataset is a list (see `help(gorillas_sf)`

for
details. Extract the objects you need from the list, for
convenience:

```
nests <- gorillas_sf$nests
mesh <- gorillas_sf$mesh
boundary <- gorillas_sf$boundary
gcov <- gorillas_sf_gcov()
```

## Factor covariates

Look at the vegetation type, nests and boundary:

Or, with the mesh:

```
ggplot() +
gg(gcov$vegetation) +
gg(mesh) +
gg(boundary, alpha = 0.2) +
gg(nests, color = "white", cex = 0.5)
```

#### A model with vegetation type only

It seems that vegetation type might be a good predictor because
nearly all the nests fall in vegetation type `Primary`

. So we
construct a model with vegetation type as a fixed effect. To do this, we
need to tell ‘lgcp’ how to find the vegetation type at any point in
space, and we do this by creating model components with a fixed effect
that we call `vegetation`

(we could call it anything), as
follows:

`comp1 <- geometry ~ vegetation(gcov$vegetation, model = "factor_full") - 1`

Notes: * We need to tell ‘lgcp’ that this is a factor fixed effect,
which we do with `model="factor_full"`

, giving one
coefficient for each factor level. * We need to be careful about
overparameterisation when using factors. Unlike regression models like
‘lm()’, ‘glm()’ or ‘gam()’, ‘lgcp()’, `inlabru`

does not
automatically remove the first level and absorb it into an intercept.
Instead, we can either use `model="factor_full"`

without an
intercept, or `model="factor_contrast"`

, which does remove
the first level.

`comp1alt <- geometry ~ vegetation(gcov$vegetation, model = "factor_contrast") + Intercept(1)`

Fit the model as usual:

Predict the intensity, and plot the median intensity surface. (In older versions, predicting takes some time because we did not have vegetation values outside the mesh so ‘inlabru’ needed to predict these first. Since v2.0.0, the vegetation has been pre-extended.)

The `predict`

function of `inlabru`

takes into
its `data`

argument an `sf`

object or other object
supported by the predictor evaluation code (for non-geographical data,
typically a `data.frame`

). We can use the
`inlabru`

function `pixels`

to generate an
`sf`

object with points only within the boundary, using its
`mask`

argument, as shown below.

```
pred.df <- fm_pixels(mesh, mask = boundary)
int1 <- predict(fit1, pred.df, ~ exp(vegetation))
ggplot() +
gg(int1, geom = "tile") + # gg() with sf points and geom = "tile" plots a raster
gg(boundary, alpha = 0, lwd = 2) +
gg(nests, color = "DarkGreen")
```

Not surprisingly, given that most nests are in `Primary`

vegetation, the high density is in this vegetation. But there are
substantial patches of predicted high density that have no nests, and
some areas of predicted low density that have nests. What about the
estimated abundance (there are really 647 nests there):

#### A model with vegetation type and a SPDE type smoother

Lets try to `explain`

the pattern in nest distribution
that is not captured by the vegetation covariate, using an SPDE:

```
pcmatern <- inla.spde2.pcmatern(mesh,
prior.sigma = c(0.1, 0.01),
prior.range = c(0.1, 0.01)
)
comp2 <- geometry ~
-1 +
vegetation(gcov$vegetation, model = "factor_full") +
mySmooth(geometry, model = pcmatern)
```

And plot the posterior median intensity surface

```
int2 <- predict(fit2, pred.df, ~ exp(mySmooth + vegetation), n.samples = 1000)
ggplot() +
gg(int2, aes(fill = q0.5), geom = "tile") +
gg(boundary, alpha = 0, lwd = 2) +
gg(nests)
```

… and the expected integrated intensity (mean of abundance)

```
Lambda2 <- predict(
fit2,
fm_int(mesh, boundary),
~ sum(weight * exp(mySmooth + vegetation))
)
Lambda2
#> mean sd q0.025 q0.5 q0.975 median sd.mc_std_err
#> 1 675.7446 28.47324 628.4281 673.7549 731.9011 673.7549 1.920159
#> mean.mc_std_err
#> 1 3.231356
```

Look at the contributions to the linear predictor from the SPDE and from vegetation:

```
lp2 <- predict(fit2, pred.df, ~ list(
smooth_veg = mySmooth + vegetation,
smooth = mySmooth,
veg = vegetation
))
```

The function `scale_fill_gradientn`

sets the scale for the
plot legend. Here we set it to span the range of the three linear
predictor components being plotted (medians are plotted by default).

```
lprange <- range(lp2$smooth_veg$median, lp2$smooth$median, lp2$veg$median)
csc <- scale_fill_gradientn(colours = brewer.pal(9, "YlOrRd"), limits = lprange)
plot.lp2 <- ggplot() +
gg(lp2$smooth_veg, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("mySmooth + vegetation")
plot.lp2.spde <- ggplot() +
gg(lp2$smooth, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("mySmooth")
plot.lp2.veg <- ggplot() +
gg(lp2$veg, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("vegetation")
multiplot(plot.lp2, plot.lp2.spde, plot.lp2.veg, cols = 3)
```

#### A model with SPDE only

Do we need vegetation at all? Fit a model with only an SPDE + Intercept, and choose between models on the basis of DIC, using ‘deltaIC()’.

```
comp3 <- geometry ~ mySmooth(geometry, model = pcmatern) + Intercept(1)
fit3 <- lgcp(comp3,
data = nests,
samplers = boundary,
domain = list(geometry = mesh)
)
```

```
int3 <- predict(fit3, pred.df, ~ exp(mySmooth + Intercept))
ggplot() +
gg(int3, geom = "tile") +
gg(boundary, alpha = 0) +
gg(nests)
```

```
Lambda3 <- predict(
fit3,
fm_int(mesh, boundary),
~ sum(weight * exp(mySmooth + Intercept))
)
Lambda3
#> mean sd q0.025 q0.5 q0.975 median sd.mc_std_err
#> 1 675.3594 31.74151 625.7299 672.8656 733.8027 672.8656 2.17885
#> mean.mc_std_err
#> 1 3.609921
```

Model | DIC | Delta.DIC |
---|---|---|

fit1 | -562.5418 | 0.000 |

fit3 | 525.1865 | 1087.728 |

fit2 | 619.0161 | 1181.558 |

NOTE: the behaviour of DIC is currently a bit unclear, and is being investigated. WAIC is related to leave-one-out cross-validation, and is not appropriate to use with the current current LGCP likelihood implementation.

Classic mode:

Model | DIC | Delta.DIC |
---|---|---|

fit2 | 2224.131 | 0.00000 |

fit3 | 2274.306 | 50.17504 |

fit1 | 3124.784 | 900.65339 |

Experimental mode:

Model | DIC | Delta.DIC |
---|---|---|

fit1 | -563.3583 | 0.000 |

fit3 | 509.4010 | 1072.759 |

fit2 | 597.6459 | 1161.004 |

#### CV and SPDE parameters for Model 2

We are going with Model `fit2`

. Lets look at the spatial
distribution of the coefficient of variation

Plot the vegetation “fixed effect” posteriors. First get their names
- from `$marginals.random$vegetation`

of the fitted object,
which contains the fixed effect marginal distribution data

```
flist <- vector("list", NROW(fit2$summary.random$vegetation))
for (i in seq_along(flist)) flist[[i]] <- plot(fit2, "vegetation", index = i)
multiplot(plotlist = flist, cols = 3)
```

Use `spde.posterior( )`

to obtain and then plot the SPDE
parameter posteriors and the Matern correlation and covariance functions
for this model.

```
spde.range <- spde.posterior(fit2, "mySmooth", what = "range")
spde.logvar <- spde.posterior(fit2, "mySmooth", what = "log.variance")
range.plot <- plot(spde.range)
var.plot <- plot(spde.logvar)
multiplot(range.plot, var.plot)
```

```
corplot <- plot(spde.posterior(fit2, "mySmooth", what = "matern.correlation"))
covplot <- plot(spde.posterior(fit2, "mySmooth", what = "matern.covariance"))
multiplot(covplot, corplot)
```

## Continuous covariates

Now lets try a model with elevation as a (continuous) explanatory variable. (First centre elevations for more stable fitting.)

```
elev <- gcov$elevation
elev <- elev - mean(terra::values(elev), na.rm = TRUE)
ggplot() +
gg(elev, geom = "tile") +
gg(boundary, alpha = 0)
```

The elevation variable here is of class ‘SpatRaster’, that can be
handled in the same way as the vegetation covariate, with automatic
evaluation via an `eval_spatial()`

method. However, since in
some cases data may be stored differently, other methods are needed to
access the stored values, or there’s some post-processing to be done. In
such cases, we can define a function that knows how to evaluate the
covariate at arbitrary points in the survey region, and call that
function in the component definition. The method
`eval_spatial()`

is the method that handles this
automatically, and supports `terra`

`SpatRaster`

and `sf`

geometry points objects, and mismatching coordinate
systems as well. In the following evaluator example function, we only
add infilling of missing values as a post-processing step.

```
# Note: this method is usually not needed; the automatic invocation of
# `eval_spatial()` method by the component input evaluator is usually sufficient.
f.elev <- function(where) {
# Extract the values
v <- eval_spatial(elev, where, layer = "elevation")
# Fill in missing values; this example would work for SpatialPixelsDataFrame data
# if (any(is.na(v))) {
# v <- bru_fill_missing(elev, where, v)
# }
return(v)
}
```

For brevity we are not going to consider models with elevation only, with elevation and a SPDE, and with SPDE only. We will just fit one with elevation and SPDE. We create our model to pass to lgcp thus:

```
matern <- inla.spde2.pcmatern(mesh,
prior.sigma = c(0.1, 0.01),
prior.range = c(0.1, 0.01)
)
ecomp <- geometry ~ elev(f.elev(.data.), model = "linear") +
mySmooth(geometry, model = matern) + Intercept(1)
```

Note how the elevation effect is defined. We could alternatively use
the `terra`

grid object directly (causing
`inlabru`

to automatically call `eval_spatial()`

),
like in the vegetation case: we specified it like

`elev(elev, model = "factor_full")`

whereas with the special function method we specify the covariate like this:

`elev(f.elev(.data.), model = "linear")`

Most applications can use the automatic method, and the special function method is included only as an example of how to handle more complex cases.

We also now include an intercept term in the model.

The model is fitted in the usual way:

Summary and model selection

```
summary(efit)
#> inlabru version: 2.11.1.9001
#> INLA version: 24.06.27
#> Components:
#> elev: main = linear(f.elev(.data.)), group = exchangeable(1L), replicate = iid(1L)
#> mySmooth: main = spde(geometry), group = exchangeable(1L), replicate = iid(1L)
#> Intercept: main = linear(1), group = exchangeable(1L), replicate = iid(1L)
#> Likelihoods:
#> Family: 'cp'
#> Data class: 'sf', 'data.frame'
#> Response class: 'numeric'
#> Predictor: geometry ~ .
#> Used components: effects[elev, mySmooth, Intercept], latent[]
#> Time used:
#> Pre = 0.351, Running = 4.72, Post = 0.387, Total = 5.46
#> Fixed effects:
#> mean sd 0.025quant 0.5quant 0.975quant mode kld
#> elev 0.004 0.001 0.002 0.004 0.006 0.004 0
#> Intercept 1.125 0.478 0.151 1.136 2.037 1.136 0
#>
#> Random effects:
#> Name Model
#> mySmooth SPDE2 model
#>
#> Model hyperparameters:
#> mean sd 0.025quant 0.5quant 0.975quant mode
#> Range for mySmooth 1.76 0.217 1.376 1.75 2.23 1.714
#> Stdev for mySmooth 1.00 0.085 0.848 1.00 1.18 0.995
#>
#> Deviance Information Criterion (DIC) ...............: 521.26
#> Deviance Information Criterion (DIC, saturated) ....: 520.43
#> Effective number of parameters .....................: -825.43
#>
#> Watanabe-Akaike information criterion (WAIC) ...: 1604.04
#> Effective number of parameters .................: 153.51
#>
#> Marginal log-Likelihood: -1254.95
#> is computed
#> Posterior summaries for the linear predictor and the fitted values are computed
#> (Posterior marginals needs also 'control.compute=list(return.marginals.predictor=TRUE)')
deltaIC(fit1, fit2, fit3, efit)
#> Model DIC Delta.DIC
#> 1 fit1 -562.5418 0.000
#> 2 efit 521.2575 1083.799
#> 3 fit3 525.1865 1087.728
#> 4 fit2 619.0161 1181.558
```

Predict and plot the density

```
e.int <- predict(efit, pred.df, ~ exp(mySmooth + elev + Intercept))
e.int.log <- predict(efit, pred.df, ~ (mySmooth + elev + Intercept))
p1 <- ggplot() +
gg(e.int, aes(fill = log(sd)), geom = "tile") +
gg(boundary, alpha = 0) +
gg(nests, shape = "+")
p2 <- ggplot() +
gg(e.int.log, aes(fill = exp(mean + sd^2 / 2)), geom = "tile") +
gg(boundary, alpha = 0) +
gg(nests, shape = "+")
library(patchwork)
p1 | p2
```

Now look at the elevation and SPDE effects in space. Leave out the Intercept because it swamps the spatial effects of elevation and the SPDE in the plots and we are interested in comparing the effects of elevation and the SPDE.

First we need to predict on the linear predictor scale.

```
e.lp <- predict(
efit,
pred.df,
~ list(
smooth_elev = mySmooth + elev,
elev = elev,
smooth = mySmooth
)
)
```

The code below, which is very similar to that used for the vegetation factor variable, produces the plots we want.

```
lprange <- range(e.lp$smooth_elev$mean, e.lp$elev$mean, e.lp$smooth$mean)
library(RColorBrewer)
csc <- scale_fill_gradientn(colours = brewer.pal(9, "YlOrRd"), limits = lprange)
plot.e.lp <- ggplot() +
gg(e.lp$smooth_elev, mask = boundary, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("SPDE + elevation")
plot.e.lp.spde <- ggplot() +
gg(e.lp$smooth, mask = boundary, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("SPDE")
plot.e.lp.elev <- ggplot() +
gg(e.lp$elev, mask = boundary, geom = "tile") +
csc +
theme(legend.position = "bottom") +
gg(boundary, alpha = 0) +
ggtitle("elevation")
multiplot(plot.e.lp,
plot.e.lp.spde,
plot.e.lp.elev,
cols = 3
)
```

You might also want to look at the posteriors of the fixed effects and of the SPDE. Adapt the code used for the vegetation factor to do this.

```
LambdaE <- predict(
efit,
fm_int(mesh, boundary),
~ sum(weight * exp(Intercept + elev + mySmooth))
)
LambdaE
#> mean sd q0.025 q0.5 q0.975 median sd.mc_std_err
#> 1 672.12 25.48524 628.8362 668.5368 728.1858 668.5368 1.872771
#> mean.mc_std_err
#> 1 2.923078
```

```
flist <- vector("list", NROW(efit$summary.fixed))
for (i in seq_along(flist)) {
flist[[i]] <- plot(efit, rownames(efit$summary.fixed)[i])
}
multiplot(plotlist = flist, cols = 2)
```

Plot the SPDE parameter posteriors and the Matern correlation and covariance functions for this model.

```
spde.range <- spde.posterior(efit, "mySmooth", what = "range")
spde.logvar <- spde.posterior(efit, "mySmooth", what = "log.variance")
range.plot <- plot(spde.range)
var.plot <- plot(spde.logvar)
multiplot(range.plot, var.plot)
```

```
corplot <- plot(spde.posterior(efit, "mySmooth", what = "matern.correlation"))
covplot <- plot(spde.posterior(efit, "mySmooth", what = "matern.covariance"))
multiplot(covplot, corplot)
```

Also estimate abundance. The `data.frame`

in the second
call leads to inclusion of `N`

in the prediction object, for
easier plotting.

```
Lambda <- predict(
efit, fm_int(mesh, boundary),
~ sum(weight * exp(mySmooth + elev + Intercept))
)
Lambda
#> mean sd q0.025 q0.5 q0.975 median sd.mc_std_err
#> 1 670.425 25.70681 621.9903 672.066 723.358 672.066 1.811383
#> mean.mc_std_err
#> 1 2.932958
Nest.e <- predict(
efit,
fm_int(mesh, boundary),
~ data.frame(
N = 200:1000,
density = dpois(200:1000,
lambda = sum(weight * exp(mySmooth + elev + Intercept))
)
),
n.samples = 2000
)
```

Plot in the same way as in previous practicals

```
Nest.e$plugin_estimate <- dpois(Nest.e$N, lambda = Lambda$median)
ggplot(data = Nest.e) +
geom_line(aes(x = N, y = mean, colour = "Posterior")) +
geom_line(aes(x = N, y = plugin_estimate, colour = "Plugin"))
```

### Non-spatial evaluation of the covariate effect

The previous examples of posterior prediction focused on spatial
prediction. From `inlabru`

version 2.2.8, a feature is
available for overriding the component input value specification from
the component definition. Each model component can be evaluated
directly, for arbitrary values by functions named by adding the suffix
`_eval`

to the end of the component name in the predictor
expression, and disabling normal component evaluation for all components
with `include = character(0)`

(since we’re both bypassing the
normal input to the `elev`

component, and not supplying data
for the other components). From version `2.8.0`

,
`inlabru`

attempts to automatically detect which model
components are used in the expression, and the `include`

argument can be left out entirely.

Since the elevation effect in this model is linear, the resulting plot isn’t very interesting, but the same method can be applied to non-linear effects as well, and combined into general R expressions.

```
elev.pred <- predict(
efit,
data.frame(elevation = seq(0, 100, length.out = 1000)),
formula = ~ elev_eval(elevation)
# include = character(0) # Not needed from version 2.8.0
)
ggplot(elev.pred) +
geom_line(aes(elevation, mean)) +
geom_ribbon(
aes(elevation,
ymin = q0.025,
ymax = q0.975
),
alpha = 0.2
) +
geom_ribbon(
aes(elevation,
ymin = mean - 1 * sd,
ymax = mean + 1 * sd
),
alpha = 0.2
)
```

## A 1D Example

Try fitting a 1-dimensional model to the point data in the
`inlabru`

dataset `Poisson2_1D`

. This comes with a
covariate function called `cov2_1D`

. Try to reproduce the
plot below (used in lectures) showing the effects of the
`Intercept + z`

and the `SPDE`

. (You may find it
helpful to build on the model you fitted in the previous practical,
adding the covariate to the model specification.)

```
data(Poisson2_1D)
ss <- seq(0, 55, length = 200)
z <- cov2_1D(ss)
x <- seq(1, 55, length = 100)
mesh <- fm_mesh_1d(x, degree = 1)
comp <- x ~
beta_z(cov2_1D(x), model = "linear") +
spde1D(x, model = inla.spde2.matern(mesh)) +
Intercept(1)
fitcov1D <- lgcp(comp, pts2, domain = list(x = mesh))
pr.df <- data.frame(x = x)
prcov1D <- predict(
fitcov1D,
pr.df,
~ list(
total = exp(beta_z + spde1D + Intercept),
fx = exp(beta_z + Intercept),
spde = exp(spde1D)
)
)
ggplot() +
gg(prcov1D$total, color = "red") +
geom_line(aes(x = prcov1D$spde$x, y = prcov1D$spde$median), col = "blue", lwd = 1.25) +
geom_line(aes(x = prcov1D$fx$x, y = prcov1D$fx$median), col = "green", lwd = 1.25) +
geom_point(data = pts2, aes(x = x), y = 0.2, shape = "|", cex = 4) +
xlab(expression(bold(s))) +
ylab(expression(hat(lambda)(bold(s)) ~ ~"and its components")) +
annotate(geom = "text", x = 40, y = 6, label = "Intensity", color = "red") +
annotate(geom = "text", x = 40, y = 5.5, label = "z-effect", color = "green") +
annotate(geom = "text", x = 40, y = 5, label = "SPDE", color = "blue")
```