Swift script to Convert GPX files to GeoJSON files for import into Compass

GPXToGeoJSON.swift

// A Swift script to Convert GPX files to GeoJSON files for import into Compass
// The script takes a single argument, the GPX file, and saves out a matching GeoJSON file
//
// The GeoJSON is meant to be imported into Compass (https://github.com/aaronpk/Compass)
// and thus is not "correct" GeoJSON

import Foundation

/// An object to handle converting a GPX file to GeoJSON
struct GPXtoGeoJSON {
    
    /// URL of the GPX file
    let gpxURL: URL
    
    
    /// URL of the GeoJSON file
    ///
    /// This URL is the same as the `gpxURL` with the extension replaced with `geojson`
    var jsonURL: URL {
        return gpxURL.deletingPathExtension().appendingPathExtension("geojson")
    }
    
    /// Return a GeoJSON dictionary by reading the GPX XML
    ///
    /// GPX trackpoints, `trkpt`, are extracted from the document using an XPath:
    /// `/gpx/trk/trkseg/trkpt`
    func geoJSON() throws -> [String: Any] {
        let doc = try XMLDocument(contentsOf: gpxURL, options: [])
        
        let points = try doc.nodes(forXPath: "/gpx/trk/trkseg/trkpt")
        
        let locations = points.compactMap {
            makeFeature(fromNode: $0 as! XMLElement)
        }
        
        return [
            "locations": locations
        ]
    }
    
    /// Write the specified GeoJSON disctionary to the `jsonURL`
    /// - Parameter geoJSON: The GeoJSON object to write
    func write(geoJSON: [String: Any]) throws {
        let data = try JSONSerialization.data(withJSONObject: geoJSON, options: [])
        
        try data.write(to: jsonURL)
    }
    
    
    /// Convert a GPX `trkpt` to a GeoJSON feature.
    ///
    /// The `XMLElement` needs to have the following attributes, along with a `time` element:
    /// - `lat`
    /// - `lon`
    ///
    /// Similar to the following:
    ///
    /// ```
    /// <trkpt lat="43.876595" lon="5.389074">
    ///     <ele>273.561401</ele>
    ///     <time>2012-10-06T07:17:20Z</time>
    /// </trkpt>
    /// ```
    ///
    /// - Parameter node: The XML object to convert
    /// - Returns: A GeoJSON feature dictionary
    func makeFeature(fromNode node: XMLElement) -> [String : Any]? {
        
        // Get coordinates from node
        guard
            let latString = node.attribute(forName: "lat")?.stringValue,
            let lonString = node.attribute(forName: "lon")?.stringValue
        else {
                return nil
        }

        // Get the timestamp from the node
        let timeNode = node.elements(forName: "time")[0]
        guard let time = timeNode.stringValue else {
            return nil
        }
        
        return [
            "type": "Feature",
            "geometry": [
                "type": "Point",
                "coordinates": [
                    Decimal(string: lonString),
                    Decimal(string: latString)
                ]
            ],
            "properties": [
                "timestamp": time
            ]
        ]
    }
    
}



do {
    let args = CommandLine.arguments
    
    guard args.count == 2 else {
        print("Please provide a GPX file")
        exit(1)
    }
    
    let gpxURL = URL(fileURLWithPath: args[1])
    
    let gpx = GPXtoGeoJSON(gpxURL: gpxURL)
    print("Converting \(gpxURL.lastPathComponent)")
    let geoJSON = try gpx.geoJSON()
    try gpx.write(geoJSON: geoJSON)
    print("GeoJSON written to \(gpx.jsonURL.path)")

} catch {
    print(error.localizedDescription)
}