# 5. Area of polygons and sampling¶

## Area and perimeter of polygons¶

You can compute the area and perimeter of spherical polygons like this.

```
pol <- rbind(c(-120,-20), c(-80,5), c(0, -20), c(-40,-60), c(-120,-20))
areaPolygon(pol)
## [1] 4.903757e+13
perimeter(pol)
## [1] 27336557
```

## Sampling longitude/latitude data¶

Random or regular sampling of longitude/latitude values on the globe needs to consider that the globe is spherical. That is, if you would take random points for latitude between -90 and 90 and for longitude between -180 and 180, the density of points would be higher near the poles than near the equator. In contrast, functions ‘randomCoordinates’ and ‘randomCoordinates’ return samples that are spatially balanced.

```
plot(wrld, type='l', col='grey')
a = randomCoordinates(500)
points(a, col='blue', pch=20, cex=0.5)
b = regularCoordinates(3)
points(b, col='red', pch='x')
```