Distance calculation Flutter background Location - flutter

Question

I am trying to develop a Location service-based Distance calculation app. the problem is I need this to be run when the app is in the background as well. so I used the following plugin to do so.
https://pub.dev/packages/background_location
when this runs on AVD nothing happened but worked fine. but when run on some devices I got the wrong calculations. I have noticed that this happens when the app is in the background this is. following is a partial recreation of mine. I just want to know if my code get any error before posting this as an issue
Future<void> startLocationUpdate() async {
var permission = await Permission.locationAlways.isGranted;
if (!permission) {
var t = await Permission.locationAlways.request();
}
oldPosition2 = Location(
longitude: currentPosition.longitude, latitude: currentPosition.latitude);
isWaited = false;
waitButtonText = "WAIT";
BackgroundLocation.stopLocationService();
BackgroundLocation.setAndroidConfiguration(2000);
await BackgroundLocation.setAndroidNotification(
title: "XXXXXXXXXXXXXXXXX",
message: "XXXXXXXXXXXXXXXXXX",
icon: "XXXXXXXXXXXXXXXXXX",
);
//print('Inside startLocationUpdate');
await BackgroundLocation.startLocationService(distanceFilter: 20);
print('Inside startLocationUpdate 1');
BackgroundLocation.getLocationUpdates((location) {
try {
currentPosition = location;
LatLng pos = LatLng(location.latitude, location.longitude);
var rotation = MapKitHelper.getMarkerRotation(oldPosition2.latitude,
oldPosition2.longitude, pos.latitude, pos.longitude);
Marker movingMaker = Marker(
markerId: MarkerId('moving'),
position: pos,
icon: movingMarkerIcon,
rotation: rotation,
infoWindow: InfoWindow(title: 'Current Location'),
);
setState(() {
CameraPosition cp = new CameraPosition(target: pos, zoom: 17);
rideMapController.animateCamera(CameraUpdate.newCameraPosition(cp));
_markers.removeWhere((marker) => marker.markerId.value == 'moving');
_markers.add(movingMaker);
this.accuracy = location.accuracy.toStringAsFixed(0);
totalSpeed = location.speed;
totalDistance = totalDistance +
CalDistance(oldPosition2 != null ? oldPosition2 : location,
location, DistanceType.Kilometers);
var totalDistanceAdj = totalDistance != 0 ? totalDistance : 1;
//totalFare = 50 + (kmPrice * totalDistanceAdj);
print("distance $totalDistance");
});
oldPosition2 = location;
updateTripDetails(location);
Map locationMap = {
'latitude': location.latitude.toString(),
'longitude': location.longitude.toString(),
};
rideRef = FirebaseDatabase.instance
.reference()
.child("rideRequest/${widget.tripDetails.rideID}");
rideRef.child('driver_location').set(locationMap);
} catch (e) {
FirebaseService.logtoGPSData('Error in updates ${e}');
}
});
}
and the CalDistance method as follows
double CalDistance(Location pos1, Location pos2, DistanceType type) {
print("pos1 : ${pos1.latitude} pos2: ${pos2.latitude}");
double R = (type == DistanceType.Miles) ? 3960 : 6371;
double dLat = this.toRadian(pos2.latitude - pos1.latitude);
double dLon = this.toRadian(pos2.longitude - pos1.longitude);
double a = sin(dLat / 2) * sin(dLat / 2) +
cos(this.toRadian(pos1.latitude)) *
cos(this.toRadian(pos2.latitude)) *
sin(dLon / 2) *
sin(dLon / 2);
double c = 2 * asin(min(1, sqrt(a)));
double d = R * c;
//d = (d*80)/100;
return d;
}
double toRadian(double val) {
return (pi / 180) * val;
}
Any help or hint would be much appreciated