The local magnetic field is not constant. It’s affected by lumps of iron, the relative position of the Sun, and the activity of the Sun. There is a field measuring device positioned in a garden in southern Bath. It is a magnetometer built by UKRAA. It detects changes in two perpendicular axes A and B with the board aligned to the third axis. The measurements are recorded by a Raspberry Pi computer and regularly posted to this web site.
The view below show measurements over 1 day, 7 days, and 14 days. Each data point is an average of the prior 5 minutes. The daily cycle due to the position of the Sun can be seen during the daylight period. The data shown is raw counts from the PIC processor recovered over I2C to the Pi with the Gain set to 2. Further work is required to calibrate the sensor to nano Teslas.