# Conrex

The National Association of Independent Schools developed an Elixir implementation of the Paul Bourke’s Conrec algorithm to calculate the drive-time to a particular location. The Conrex hex package is now available to developers here on GitHub.

Most map-based apps calculate the distance from a central point outward. That’s helpful if you want to see how long it will take you to fly somewhere else, but not so helpful if you want to calculate how long it will take customers to drive through traffic to get to your location.

Conrex uses a convergent isochrone to calculate real traffic and topographic conditions. An implementation of Paul Bourke’s CONREC algorithm in Elixir, Conrex is now available to the open source community.

NAIS developed Conrex for its Market View app. Market View helps schools find children are within a reasonable driving distance of the school. It can also be used to map bus routes, commute times, or to determine a new location for a business.

## Installation

Conrex can be installed by adding `conrex`

to your list of dependencies in
`mix.exs`

:

```
def deps do
[
{:conrex, "~> 1.0.0"}
]
end
```

## Usage

The main algorithm outlined by Bourke can be invoked with `Conrex.conrec`

:

```
iex> Conrex.conrec(values, x_coords, y_coords, contour_levels)
```

where `values`

is a 2D list of samples (heights, travel times, etc), `x_coords`

and `y_coords`

are lists of X and Y coordinates for the sample grid, and
`contour_levels`

is a list of values at which a contour should be calculated.
`Conrex.conrec`

outputs a list of line segments to match the classic algorithm.

If the X and Y values are GPS coordinates, you can use `Conrex.contour_polygons`

to generate GeoJSON polygons for each contour level:

```
iex> Conrex.contour_polygons(values, x_coords, y_coords, contour_levels, reference_point)
```

The additional parameter, `reference_point`

, is a point known to be within the
contour polygon, but outside any polygon holes. When converting the line
segments to GeoJSON `%Geo.Polygon{}`

s, Conrex will discard exterior polygon
rings, and correct the coordinate winding for the main ring and the polygon
holes to conform to the GeoJSON spec.