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

Alex van den Bogaerdt alex at ergens.op.het.net
Sat Sep 29 10:50:39 CEST 2007

On Sat, Sep 29, 2007 at 09:51:29AM +0200, Holger Rose wrote:
> Hi!
> 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.

Rates, not values.  As soon as you forget this, you will have a hard time
understanding what's happening.

It is OK if you "abuse" rrdtool to display other values, but you need to
understand that what you are doing is somewhat a hack.

Especially normalization is important to understand, if you are going
to work with wind directions.  If the wind direction is 350 at one moment
and is 010 at another moment, and if you were not careful, there's a good
possibility that RRDtool will eventually store 180 for one interval. That
is not a bug, it is documented behaviour and is easy to understand as long
as you keep in mind that RRDtool is about rates.

A similar problem (for you, not for RRDtool) exists when you are going to
consolidate data.

If the wind direction is 350 during 5 minutes, and 010 during the next 5
minutes, and when you consolidate these two directions into an averaged
value, you will get (350+10)/2=180.

You could use LAST, or MAX, instead of AVERAGE but that's not really what
you want.

I haven't thought about this much yet, but I guess you should be thinking
about splitting directions into north-south and east-west, let RRDtool do
its thing, and combine the two at graph time.

> 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 (only a point for each value). Is that possible?

You could try a transparent area with a stacked line of width 2 at rate 0.
Depending on the amount of pixel rows per interval, this may work.

> In wind dir i would like to display a normalized average of the values as additional graph (line as main wind direction). How can i do that?

Maybe man rrdgraph_rpn and look for TREND ?

> 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?

No. But as soon as consolidation kicks in, you may want to think hard
about how the maximum of a series of averages is quite different from
the maximum of a series of maximums.  Make sure you keep MAX RRAs as
well as AVERAGE RRAs. You also may want MIN RRAs.

Alex van den Bogaerdt

More information about the rrd-users mailing list