Explanation sought concerning gps semicircles.

[Peter Hamilton-Scott]

Looking at the Garmin developers api it mentions that all positions returned by their gps devices are represented internally as a number of semicircles. For example latitude N51.7.0 is represented internally as 609843842 semicircles. To convert this into d.m.s you'd convert it with this, dms=semicircles*(180/2^31) and the math is correct.

Questions that puzzle me are...

Why semicircles? Why not circles, or rectangles, or triangles? What is so intrinsically flawed with a circle that only half of it will do or is necessary?

I just find this semicircle concept a bit weird but there is obviously a good reason for it. Can anyone explain?

Regards.

Peter.

 

[Snay]

Looking at the Garmin developers api it mentions that all positions returned by their gps devices are represented internally as a number of semicircles. For example latitude N51.7.0 is represented internally as 609843842 semicircles. To convert this into d.m.s you'd convert it with this, dms=semicircles*(180/2^31) and the math is correct.

Questions that puzzle me are...

Why semicircles? Why not circles, or rectangles, or triangles? What is so intrinsically flawed with a circle that only half of it will do or is necessary?

I haven't even seen the API you mentioned but it seems that the term semicircle means simply 180 degrees. In other words, the used unit is a fraction (1/2^31) of 180°. When you multiply the number of such units by the conversion factor of (180/2^31), you get the measurement in degrees.

 

[Hal Mueller]

The mapping Garmin uses (180 degrees to 2^31 semicircles) allows them to use a standard 32 bit unsigned integer to represent the full 360 degrees of longitude. Thus you get the maximum precision that 32 bits allows you (about double what you'd get from a floating point value), and they still get to use integer arithmetic instead of floating point on the internal processor. Pretty cool design decision, actually.

Hal

 

[Colin (archive)]

The mapping Garmin uses (180 degrees to 2^31 semicircles) allows them to use a standard 32 bit unsigned integer to represent the full 360 degrees of longitude. Thus you get the maximum precision that 32 bits allows you (about double what you'd get from a floating point value), and they still get to use integer arithmetic instead of floating point on the internal processor. Pretty cool design decision, actually.

Hal

You can also move seamlessly across the 180 meridian, since the 32 bit value overflows to produce the correct result. You do have to be careful with comparisons across the 180 meridian though, using "(a-b)<0" rather than "a<b".
- Colin.

 

[Peter Hamilton-Scott]

The mapping Garmin...<snip>...Pretty cool design decision, actually.

Hal

Nice answer. I can now appreciate why they use it, cool, like you say. I like your lil test program you wrote using the api. Works well!.
Thanks.

 

[Cherrie]

The mapping Garmin uses (180 degrees to 2^31 semicircles) allows them to use a standard 32 bit unsigned integer to represent the full 360 degrees of longitude. Thus you get the maximum precision that 32 bits allows you (about double what you'd get from a floating point value), and they still get to use integer arithmetic instead of floating point on the internal processor. Pretty cool design decision, actually.

Hal

You are one smart mofo.

Thanks,
Cherrie

 

[wagnerc]

Wow this was so helpful!