BN-880 GPS/Compass Module With Arduino Uno - The Compass 

I am running on an ESP32 using pins 21 and 22 for the I2C connected to my BN-880Q GPS receiver. Any idea why this is all I see with the unit sitting still? Rotating is does not change these numbers. I will order an HMC5883L. Heading: 357.00 Heading: 357.00 Heading: 87.00 Heading: 357.00 Heading: 87.00

don't know if you figured this out, but your phone reads mag heading, not true. the declination value you are entering is to convert from mag to true. So to directly compare the bn-880 to phone, you need to set declination to 0. next you have the phone right next to the bn-880. There are components in the phone that will affect the readings, you see that when you move the phone around. Need to keep the phone and bn-880 at least 6-10 inches apart. Being in a basement does not impact readings, and even if it did, it would affect both the same. I can't tell if you have calibrated the bn-880. every chip will have different amounts of offset to each of the x, y and z direction. I you have not calibrated those offsets, then the accuracy you are seeing is actually pretty good. I have calibrated these same types of magnetometer chips, and also taken tilt into account, and I can get within 2 degrees in all orientations, including with a big tilt.