Use elevation data to compute the locations that can be seen, or how much higher they would have to be to be seen, from a certain position. The raster data coordinate reference system must planar (not lon/lat), with the elevation values in the same unit as the distance unit of the coordinate reference system.

```
# S4 method for SpatRaster
viewshed(x, loc, observer=1.80, target=0, curvcoef=6/7, output="yes/no", filename="", ...)
```

## Arguments

- x
SpatRaster, single layer with elevation values. Values should have the same unit as the map units

- loc
location (x and y coordinates) or a cell number

- observer
numeric. The height above the elevation data of the observer

- target
numeric. The height above the elevation data of the targets

- curvcoef
numeric. Coefficient to consider the effect of the curvature of the earth and refraction of the atmosphere. The elevation values are corrected with: `elevation = elevation - curvcoeff * (distance)^2 / (earth_diameter)`

. This means that with the default value of 0.85714, you lose sight of about 1 meter of elevation for each 385 m of planar distance

- output
character. Can be "yes/no" to get a binary (logical) output showing what areas are visible; "land" to get the height above the current elevation that would be visible; or "sea" the elevation above sea level that would be visible

- filename
character. Output filename

- ...
Options for writing files as in `writeRaster`

## References

The algorithm used is by Wang et al.: https://www.asprs.org/wp-content/uploads/pers/2000journal/january/2000_jan_87-90.pdf.

## Examples

```
if (gdal() >= 3.1) {
f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)
x <- project(r, "EPSG:2169")
p <- cbind(70300, 96982)
v <- viewshed(x, p, 0, 0, 0.85714)
}
```