[rrd-users] Some simple questions about Spikes, Average and Points.

Simon Hobson linux at thehobsons.co.uk
Sat Sep 29 10:26:34 CEST 2007

Holger Rose wrote:

>I work on a small alternative weather station 
>diagram with rrd. In this time i can display 
>some values (temperature, rain ...), but with 
>others i have mental problems.
>When i will display Wind direction the graph 
>should be see like the Wind dir (deg) Graph in 
><http://owfs.org/index.php?page=weather----owfsws>http://owfs.org/index.php?page=weather----owfsws (only 
>a point for each value). Is that possible?

No, RRD tools does not do scatter plots, just 
line or area graphs. Have you considered an 
alternative tool such as GNUPlot to generate the 
graphs ? You can still use rrdtool to store the 
data, just extract it and use it as the input to 
another tool.

As an aside, how are you storing your wind 
direction ? I ask just in case you've forgotten 
to take account of the fact that if you simply 
store the direction in degrees, then RRD will see 
a change from 359š to 0š as a large negative step 
- and if you average it, the average of 359š and 
1š is not due north (0š) but due south (180š).

I've even seen Military systems fail on that one 
- submarine autopilot that can't steer north ! 
One system I worked on 'fixed' that by using 
0-720š so that 10š right from 355 is 365, not 005 
- of course if you went full circle, at some 
point it had to apply a 360š adjustment to put 
itself back in that range.

If you can arrange that any consolidations have a 
common point where all the period ends align, 
then I would suggest doing any such correction at 
that point so that you can change from (say) 540š 
to 180š (due south to due south) without it 
getting averaged to 360š (due north).

>When i will draw Wind speed, the graph has a 
>dropout when Wind Gust is there. Is a mechanism 
>in rrd who will delete values which are a lot 
>greater or smaller then the last value (dequeues 
>spikes)? If so, (how) can i disable this setting?

When you create an RRD you can specify limit for 
the values - did you do that ? We can't tell 
because we can't see what you did from here !
But what does your data show ? What did you put 
in, what do you get out if you dump the database ?

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