Phone compass and Ladybug Tools disagree on north by about 15° after calibration,what should I check first?

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I’m running into something that has been confusing me, and I’m wondering if anyone else has seen the same thing.

I was outside checking the orientation of an existing building before looking at the same location in Ladybug Tools. To get a quick idea of the direction, I opened an online compass on my phone. An online compass is a compass that works through your phone by using the device’s GPS location together with its built-in sensors to estimate which direction you’re facing. I know it’s meant for quick navigation rather than precise surveying, but I expected it to be reasonably close.

The strange part is that my phone kept showing north about 15 degrees away from what I expected. I recalibrated the compass a couple of times, turned airplane mode on and off, walked away from parked cars, and even stood in an open area where there weren’t any obvious sources of interference. My GPS location looked accurate, but the heading never really changed.

When I opened the same location in Ladybug Tools and checked the orientation against satellite imagery, it seemed to line up much better than what my phone had been showing. That’s what made me wonder if I was misunderstanding something.

Does Ladybug Tools always assume true north, or is there something else that could explain why my phone’s compass seems consistently rotated? If you’ve ever compared a phone compass with the orientation you’re using in Ladybug Tools, how close were the results?

Also, is there a simple way to figure out whether the problem is my phone’s compass sensor, local magnetic interference, or the difference between magnetic north and true north? I don’t use surveying equipment, so I’m just trying to understand which reference I should trust when I’m checking the orientation of a real building before looking at solar or daylight analysis. Has anyone else had a phone that kept showing the same offset even after calibration?

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I never use my phone’s GPS compass because it points towards the Magnetic North, and LBT needs True North instead: depending where you are located they could very well shifted by 15°.

I usually take building orientation angles directly from satellite imagery, and I use that for LBT simulations. However I’m reading up that if you are using 2D map service it’s not perfect either because satellite imagery services like Google Maps or Bing might have a slight distortion (~1-2°) due to the type of projection used called Web Mercator. If you use a 3D map (like Google Earth) with top down view you have virtually no distortion.

I once read that a 10° change in azimuth angle has an impact of less than 1% variation in annual energy demands, so I wouldn’t worry too much about measuring from satellite imagery… just avoid using the phone’s compass.