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05/28/17, 01:45 AM

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Author Topic: "-0" degrees on the telemetry?  (Read 1786 times)
pipedream
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Posts: 595


"-0" degrees on the telemetry?
« on: 01/03/17, 12:36 PM »

While making my routine morning check of the NWAC telemetry, I noticed that the temperature at the telemetry site at the top of Tye Mill @ Stevens Pass had reported both a "0" and "-0" temperature reading. It since has flipped back and forth until finally warming above 0F at 0900 PST. Anybody have a good explanation for how this can be? I'm guessing a rounding issue, but is it possible the telemetry isn't using two's complement like it should be?

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mfonda
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Posts: 47


Re: "-0" degrees on the telemetry?
« Reply #1 on: 01/03/17, 01:14 PM »

I know nothing about how these systems actually work, but my guess would be the raw data is stored as floats, then rounded when displayed. This would then indicate that it's very near but slightly below zero. For example, it might look something like the following:

Code:
mfonda@dev-mfonda:~$ cat example.c
#include <stdio.h>

int main() {
    float n = -1 / 1e10;
    printf("%.0f", n);
    return 0;
}
mfonda@dev-mfonda:~$ gcc example.c && ./a.out
-0
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runningclouds
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Posts: 417


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Re: "-0" degrees on the telemetry?
« Reply #2 on: 01/03/17, 02:06 PM »

The problem might be in the raw data...

http://www.nwac.us/data-portal/location/stevens-pass/


* telemetry3211123.jpg (87.72 KB, 800x775 - viewed 192 times.)
« Last Edit: 01/03/17, 02:09 PM by runningclouds » Logged
pipedream
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Posts: 595


Re: "-0" degrees on the telemetry?
« Reply #3 on: 01/03/17, 04:14 PM »

I feel like I've asked this question before, but I can't find anything about it in my post history.

I do believe the issue is with rounding the float but retaining the '-' sign, like the example in mfonda's reply. Either way, it's entertaining to a programmer like me Grin
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