sf-binary-predicates.qmd

---
title: "sf: Binary predicate functions"
format: html
---

```{r}
#| label: load-packages
library(sf)
```

## Resources

- [Spatial Data Science - Binary Predicates](https://r-spatial.org/book/03-Geometries.html#sec-de9im)
- [Geocomputation with R - Topological Relations](https://geocompr.robinlovelace.net/spatial-operations.html#topological-relations)
- `sf` [vignette on geometric operations](https://r-spatial.github.io/sf/articles/sf3.html#geometrical-operations)

## Geocomputation with R - Topological Relations

This follows the examples in *Geocomputation with R* Chapter 4.2.2

```{r}
#| label: build-features

# Polygon
polygon_matrix <- cbind(
  x = c(0, 0, 1, 1,   0),
  y = c(0, 1, 1, 0.5, 0)
)
polygon_sfc <- st_sfc(st_polygon(list(polygon_matrix)))

# line
line_sfc <- st_sfc(st_linestring(cbind(
  x = c(0.4, 1),
  y = c(0.2, 0.5)
)))

# points
point_df <- data.frame(
  x = c(0.2, 0.7, 0.4),
  y = c(0.1, 0.2, 0.8)
)
point_sf <- st_as_sf(point_df, coords = c("x", "y"))
```

```{r}
#| label: plot

par(pty = "s")
plot(polygon_sfc, border = "red", col = "gray", axes = TRUE)
plot(line_sfc, lwd = 5, add = TRUE)
plot(point_sf, add = TRUE, lab = 1:4, cex = 2)
text(point_df[, 1] + 0.03, point_df[, 2] + 0.05, 1:3, cex = 1.3)
```

### st_intersects

Where do the points intersect with the polygon? The result is a a `sgbp` object (a sparse geometry binary predicate). The result is a list of vectors that are either empty (`FALSE`) or have a length of the number of matching features (`TRUE`).

```{r}
#| label: st_intersects-sparse
st_intersects(point_sf, polygon_sfc)
```

Setting `sparse = FALSE` returns a dense matrix. Each row represents a feature in the target (argument `x`) object and each column represents a feature in the selecting object (`y`).

```{r}
#| label: st_intersects-dense
st_intersects(point_sf, polygon_sfc, sparse = FALSE)
```

### st_within and st_touches

More restrictive predicates.

```{r}
#| label: more-restrictive
st_within(point_sf, polygon_sfc)
st_touches(point_sf, polygon_sfc)
```

### st_disjoint
Opposite of `st_intersects()` is `st_disjoint()`:

```{r}
#| label: st_disjoint
st_disjoint(point_sf, polygon_sfc, sparse = FALSE)[, 1]
```

### st_within_distance

```{r}
#| label: st_within_distance
st_is_within_distance(point_sf, polygon_sfc, dist = 0.2)
```