[rrd-users] using rrdtool with weather data

[Greg Satz]

I am attempting to use rrdtool to capture and display weather data. I have
run into a few issues where I could use some clarification and help in using
rrdtool better.

I want to keep some historical data. However it appears the graphing
function will consolidate data to fit on a particular graph. It doesn't make
sense to keep data that will never be displayed. How do I determine
reasonable disk space usage (number of samples) based on my graphing
requirements (last month, last year, etc.)?

Most of the data types are gauges. However a few data types have different
properties.

Rain is a counter in that it increases. However it is nice to see the sum
over a certain period of time. The consolidation functions all appear to
interfere with the summation over time. How can I get the total rainfall out
of rrdtool? Can I use a LAST RRA? How would I get total rainfall for the
last year, month, day (assuming I keep that much data)?

Wind speed could be a gauge or a counter. Total wind isn't very interesting
so it doesn't have the same issue as rain. However it is nice to see
maximums (gusting). It looks like the MAXIMUM consolidation function would
keep this information. If the wind is a counter (wind rate), can I still get
the absolute number (kph or mph)?

Wind direction seems to be the most challenging for rrdtool to accommodate.
Wind direction can be broken down into 16 compass points. It would be nice
to be able to show the maximum wind gust came from a certain direction. This
would require a way to extract the data for a given time. It looks like
rrdtool fetch will do this, wont it?

The difficult part for wind direction is the consolidation function.
Averaging or max/min won't work. Looks like I need a "most common value"
(mode) consolidation function? Any other ideas on how to handle this?

On the display side, I wrote a perl cgi script that fetched all the data and
attempts to work around some of the issues above. However I couldn't avoid
all of them.

Any feedback would be appreciated.

Thanks,
Greg


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