# sl.checkposition: Check Position of Point Relative to a Line on a Sphere In helgegoessling/spheRlab: Spherical Geometry, Analysis, and Plotting of Geoscientific Data on Arbitrary Grids

## Description

Check if a point is located to the left or to the right of a (directed) line, defined by two other points, on a sphere. This is equivalent to checking the direction of rotation of the resulting triangle.

## Usage

 `1` ```sl.checkposition(a, b, c, ccw.defined.as = 1) ```

## Arguments

 `a` Vector of length 2 with longitude and latitude of the start point of the line. `b` Vector of length 2 with longitude and latitude of the end point of the line. `c` Vector of length 2 with longitude and latitude of the point for which the position relative to the (great circle) line spanned from `a` to `b` is sought. `ccw.defined.as` By default `ccw.defined.as=1`, meaning that the return value is `1` if `c` is located to the left of the line (that is, if the resulting triangle rotates counterclockwise). If `ccw.defined.as=-1`, the return value is inverted.

## Value

Scalar return value: `1` if `c` is located to the left (if the triangle `a-b-c` rotates counterclockwise), `-1` if `c` is located to the right (if the triangle `a-b-c` rotates clockwise), or `0` if all points lie on one (great circle) line. (Values inverted if `ccw.defined.as=-1`.))

Helge Goessling

## Examples

 ```1 2 3``` ```sl.checkposition(a=c(0,0),b=c(90,0),c=c(45,1)) ## Should return: ## [1] 1 ```

helgegoessling/spheRlab documentation built on Sept. 29, 2018, 12:09 p.m.