Mapbox方向API返回bearing-after与实际罗盘方位不符问题咨询

Let's break down why you're seeing such a big discrepancy between the API's bearing-after field and your phone compass readings—this is usually down to a few key mismatches in how the values are calculated or interpreted:

• True North vs. Magnetic North Difference
  Mapbox's bearing-after uses True North (geographic north, based on the Earth's axis) as its 0-degree reference. Most phone compasses default to Magnetic North (based on the Earth's magnetic field), which can be off by 5-20 degrees depending on your location (this is called magnetic declination). If you haven't already, switch your compass to True North mode and recompare the values—this often fixes the bulk of the discrepancy.

• Misunderstanding What bearing-after Represents
  It's easy to assume bearing-after is the direct compass heading from the maneuver point to your final destination, but that's not how it works. The field actually describes the initial heading of the route segment immediately following the maneuver. For example, if you're turning right onto a straight road that runs east, bearing-after will be 90 degrees (True North)—even if your final destination is southeast of that point. To verify this, visualize the route in Mapbox Studio and check the direction of the road right after the maneuver point; your True North compass should match that direction.

• How Request Parameters Affect Results
  The bearing and radius parameters you mentioned are used to help the API match your current position to the correct route (critical in areas with parallel roads or complex intersections), not directly adjust the bearing-after calculation. However, if you don't include them, the API might calculate the route based on the nearest road centerline, while your phone's GPS picks up your position slightly off-road. Try adding bearing=<your-true-north-heading>&radius=10 to your request (replace <your-true-north-heading> with your compass reading in True North mode) to ensure the API aligns with your actual position and direction, then check if bearing-after matches your compass when you face the next route segment.

If you've checked all these and still see a mismatch, double-check that you're looking at the correct bearing-after value for the right maneuver step—sometimes it's easy to mix up steps in a multi-leg route!

内容的提问来源于stack exchange，提问作者Adarsh