title: Interpolation of 15 minute values
body: <p>I have a dataframe that looks like this:</p>
  
  <pre><code>dat &lt;- data.frame(time = seq(as.POSIXct("2010-01-01"),
                               as.POSIXct("2016-12-31") + 60*99, 
                               by = 60*15),
                    radiation = sample(1:500, 245383, replace = TRUE))
  </code></pre>
  
  <p>So I have every 15 minutes a measurement value. The structure is:</p>
  
  <pre><code>&gt; str(dat)
  'data.frame':   245383 obs. of  2 variables:
   $ time     : POSIXct, format: "2010-01-01 00:00:00" "2010-01-01 00:15:00" "2010-01-01 00:30:00" "2010-01-01 00:45:00" ...
   $ radiation: num  230 443 282 314 286 225 77 89 97 330 ...
  </code></pre>
  
  <p>Now I want to interpolate, so my aim is a dataframe with values for every minute.
  I searched a few times and tried some methods with the zoo package. But I have some problems with the dataframe. I have to convert it to a text file i guess? I have no idea how to do that. </p>