Building a Ghost Trail Engine with CoreLocation

Building a ghost trail feature for a GPS app sounds complex, but the core idea is simple: store your original path as waypoints, then replay the ghost position in real time as the user walks.

The Core Concept

When you record a trail in FootSteps, every GPS coordinate is saved as a Waypoint in SwiftData with a timestamp. The ghost trail replay uses these timestamps to calculate where the “ghost” should be at any given moment.

struct Waypoint {
    let coordinate: CLLocationCoordinate2D
    let timestamp: Date
    let altitude: Double
}

Interpolating Position

The ghost doesn’t jump between waypoints — it smoothly interpolates between them using linear interpolation on coordinates.

func ghostPosition(at elapsed: TimeInterval, waypoints: [Waypoint]) -> CLLocationCoordinate2D? {
    guard let first = waypoints.first else { return nil }
    let targetTime = first.timestamp.addingTimeInterval(elapsed)

    for i in 0..<waypoints.count - 1 {
        let a = waypoints[i]
        let b = waypoints[i + 1]
        if targetTime >= a.timestamp && targetTime < b.timestamp {
            let t = targetTime.timeIntervalSince(a.timestamp) /
                    b.timestamp.timeIntervalSince(a.timestamp)
            return CLLocationCoordinate2D(
                latitude: a.coordinate.latitude + t * (b.coordinate.latitude - a.coordinate.latitude),
                longitude: a.coordinate.longitude + t * (b.coordinate.longitude - a.coordinate.longitude)
            )
        }
    }
    return waypoints.last?.coordinate
}

The Animated Marker

The ghost marker on the map is a custom GhostMarkerView — it floats up and down using a CSS-style animation in UIKit, has a pulsing ring to indicate it’s “alive,” and blinks to give personality.

What I Learned

The trickiest part was keeping the ghost marker in sync with Lenis scroll (joke — that’s the web, not iOS). The real challenge was handling GPS accuracy spikes — if the user pauses, GPS drift can make the ghost jump 20m randomly. The fix: ignore waypoints where accuracy > 15m when storing the trail.

This feature alone drove most of our positive App Store reviews.