# sa.trans

##### Trigonometric transformation of a slope and aspect interaction

The Trigonometric Stage (1978) [slope * cos(aspect)] or [slope * sin(aspect)]

An a priori assumption of a maximum in the NW quadrant (45 azimuth) and a minimum in the SW quadrant can be replaced by an empirically determined location of the optimum without repeated calculations of the regression fit. In addition it is argued that expressions for the effects of aspect should always be considered as terms involving an interaction with slope (Stage, 1976)

For slopes from 0 bounded from -1 to 1. Greater than 100 out of the -1 to 1 range.

An alternative for slopes with values approaching infinity is to take the square root of slope/100 to reduce the range of values.By default this model test all values greater than 100 to 101

##### Usage

```
sa.trans(
slope,
aspect,
type = "cos",
slp.units = "degrees",
asp.units = "degrees"
)
```

##### Arguments

- slope
slope values in degrees, radians or percent

- aspect
aspect values in degrees or radians

- type
Type of transformation, options are: "cos", "sin"

- slp.units
Units of slope values, options are: "degrees", "radians" or "percent"

- asp.units
Units of aspect values, options are: "degrees" or "radians"

##### Value

A vector of the modeled value

##### References

Stage, A. R. 1976. An Expression of the Effects of Aspect, Slope, and Habitat Type on Tree Growth. Forest Science 22(3):457-460.

##### Examples

```
# NOT RUN {
sa.trans(slope = 48.146, aspect = 360.000)
library(raster)
data(elev)
sa <- raster::terrain(elev, opt=c("slope", "aspect"), unit="degrees")
scosa <- raster::overlay(sa[[1]], sa[[2]], fun = sa.trans)
# }
```

*Documentation reproduced from package spatialEco, version 1.3-2, License: GPL-3*