# [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
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 ?

```