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Archive for January 18th, 2015

APRS Turn Slope Units

There’s a fundamental error in my writeup about setting the APRS Smart Beaconing parameters for the bike trackers: I blundered the units of Turn Slope.

Rich Painter recently explained how that works:

I ran across your blog on Smart Beaconing and saw something that needed correction.

You state the Turn Slope is in units Degrees / MPH

This is incorrect. Although the term Turn Slope is not a real slope (such as rise/run classically) that is what the originators used albeit incorrectly. They do however correctly attribute the units to MPH * Degrees (a product and hence not really a slope).

In their formula they calculate a turn threshold as:
turn_threshold = min_turn_angle + turn_slope / speed

Looking at the units we see:
= Degrees + (MPH * Degrees) / MPH

which yields
= Degrees + Degrees

Which makes sense. It is too bad that the originators used the wrong term of Turn Slope which confuses most people. A better term would have been Turn Product.

In looking back over that post, I have no idea where or how I got the wrong units, other than by the plain reading of the “variable name”.

As he explained in a followup note:

As for units… I was introduced to making unit balance way back in 1967-1968 science class in HS by a really fine science teacher. It has served me all my life and I’m thankful for that training.

I have ever since told that teacher so!

A while back, our Larval Engineer rammed an engineering physics class head-on and sent me a meme image, observing that I’d trained her well: if the units don’t work out, then you’re doing it wrong.

Yes, yes, I do care about the units:

Give a shit about the units

Give a shit about the units

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