[Numpy-discussion] Comparing variable time-shifted two measurements

Gökhan Sever gokhansever@gmail....
Fri Nov 6 17:20:15 CST 2009

On Thu, Nov 5, 2009 at 10:46 PM, Charles R Harris <charlesr.harris@gmail.com
> wrote:

> On Thu, Nov 5, 2009 at 7:21 PM, Gökhan Sever <gokhansever@gmail.com>wrote:
>> Hello,
>> I have two aircraft based aerosol measurements. The first one is
>> dccnConSTP (blue), and the latter is CPCConc (red) as shown in this screen
>> capture. (http://img513.imageshack.us/img513/7498/ccncpclag.png). My goal
>> is to compare these two measurements. It is expected to see that they must
>> have a positive correlation throughout the flight. However, the instrument
>> that gives CPCConc was experiencing a sampling issue and therefore making a
>> varying time-shifted measurements with respect to the first instrument.
>> (From the first box it is about 20 seconds, 24 from the seconds before the
>> dccnConSTP measurements shows up.) In other words in different altitude
>> levels, I have varying time differences in between these two measurements in
>> terms of their shapes. So, my goal turns to addressing this variable
>> shifting issue before I start doing the comparisons.
>> Is there a known automated approach to correct this mentioned varying-lag
>> issue? If so, how?

Sorry for the late response. I was working on a lab report and took more
than I thought to finish.

> Can you be more explicit about the varying lags? Do you know what the lags
> are?

I can identify the lag in between the two instrument by going over the
time-series data. I know exactly what causes the lag, that is the CPC device
cannot maintain 1L/min sample flow rate at higher altitudes. This goes my
varying lag complain. The flow rate of the sample air changes depending on
the ambient pressure. Unfortunately we couldn't record the sample flow rate
from the instrument during the experimentation period. However we have
pressure recordings throughout the flights, where I can somehow relate this
to correct the variable lag issue. However this requires me to do manual
observations on the CPCConc data. (i.e, get some sample points to determine
the lag manualy by panning and zooming in the data and looking the pressure
levels at these points and coming up with a fit and use that for the

> If not, how much information about them, such as range, probability, etc.,
> can you supply?

I think, I clarified these points by mentioning the pressure dependence.

> Are there dropouts, or do the lags varying sort of continuously? Can you
> parameterise the lags, so on and so forth.

There are no drop-outs. I don't know how would I parameterize the lags
besides using the external pressure information.

> Chuck

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