Computer Compass Accuracy

[RonR]

The iPhone, anyway, has always had a compass chip, even before it included GPS capability, and the compass works with GPS off. Not sure about others, but GPS is very expensive for the battery and compass chips aren't, so I would guess they do. But the implementation in firmware can vary and greatly influences the displayed accuracy.

When you are looking at compass vectors in 3D the direction to magnetic north is not horizontal to the earth’s surface, but at an angle. In my location it’s about 20° downward. In addition to offsetting the influence of hardware and batteries, that is the orientation that the random rotation in Shearwater’s video is intended to capture (the Cobalt’s compass calibrates exactly the same way). The compass needs to hold that data in calibration to be able to work at any tilt angle- and this provides one of the advantages of digital over conventional compasses. This angle will change a great deal as you move along a north-south axis on the earth’s surface.

For most of us, it isn’t important to know true north, just to have relative bearing accuracy. But if you are trying to match a chart, for instance, then knowing if you are reading magnetic or true north becomes important.

-Ron

 

[lucca brassi]

have read a few spec : 3 axe gyro+ 3 axe accelometer+3 axe compass

http://www.invensense.com/products/motion-tracking/9-axis/

but i was surprised 3 axe compass https://www.adafruit.com/datasheets/HMC5883L_3-Axis_Digital_Compass_IC.pdf

BENEFITS :
''Sensors Can Be Used in Strong Magnetic Field Environments with a 1° to 2° Degree Compass Heading Accuracy''

also ''hard iron'' , ''soft iron'' compensation

Compensating for Tilt, Hard-Iron, and Soft-Iron Effects | Sensors

 

[DiveNav]

Phones don't use magnetic north, AFAIK. They use GPS and make calculations from that....

That is not completely accurate. It depends on the implementation within an app.

The iOS SDK Core Motion Framework provides quite a lot of data - more than plenty to implement a good enough compass .... but not deterministic enough to implement an acceptable dead-reckoning navigation

Same is true for Android SDK

 

[RonR]

BENEFITS :
''Sensors Can Be Used in Strong Magnetic Field Environments with a 1° to 2° Degree Compass Heading Accuracy''

These sensors will work in a magnetic field environment just fine, so long as it is stable. When coupled with an accelerometer the background field can be essentially mapped and canceled out, leaving the sensor to detect the changes due to the direction of the earth’s magnetic field. That’s how we can use magnetic buttons and batteries in the Cobalt and still have an accurate compass. A few years back this had to be done from scratch for each device, a complex bit of coding, but now there are libraries for the chips that most manufacturers supply. As I indicated above, the technology available for all kinds of sensors is just exploding.

"Soft iron" effects are more difficult to deal with, but if you prevent ferrous metal from being right next to your compass the changes should be minimal. Proximity has more influence than mass.

-Ron

 

[lucca brassi]

thanks for info