A line in 2D space is defined by a start and an and point, each associated with 2D coordinates.
This function takes a /codedata.frame as input and assumes that each row defines a line in space.
In order to do so, the data frame must have at least four columns and the `start.cols`

and
`end.cols`

parameters must be used to point out the names of the columns that define
the start and end coordinates of the line. The data is then converted to a
`SpatialLinesDataFrame`

`DF`

. If a coordinate reference system `crs`

is provided
it is attached to `DF`

. If also `to.crs`

is provided, the coordinate system of `DF`

is transfromed accordingly. Additional columns of the input data, e.g. covariates,
are retained and attached to `DF`

.

## Usage

`sline(data, start.cols, end.cols, crs = CRS(as.character(NA)), to.crs = NULL)`

## Arguments

- data
A data.frame

- start.cols
Character array poitning out the columns of

`data`

that hold the start points of the lines- end.cols
Character array poitning out the columns of

`data`

that hold the end points of the lines- crs
Coordinate reference system of the original

`data`

- to.crs
Coordinate reference system for the SpatialLines ouput.

## Examples

```
# \donttest{
# Create a data frame defining three lines
lns <- data.frame(
xs = c(1, 2, 3), ys = c(1, 1, 1), # start points
xe = c(2, 3, 4), ye = c(2, 2, 2)
) # end points
# Conversion to SpatialLinesDataFrame without CRS
spl <- sline(lns,
start.cols = c("xs", "ys"),
end.cols = c("xe", "ye")
)
if (require(ggplot2, quietly = TRUE)) {
# Plot the lines
ggplot() +
gg(spl)
}
# }
```