title: Adding interpolated model data column to field dataset in r
body: <p>I'm trying to compare my model data with field data in r. Field data is at certain depths(1,20,50,75,100,150..). Model data output is not at the same depths and have to be interpolated(linear) and preferably added to a new column in the field data.</p>
  <p>I would like to run a for loop on each row of the model data and use the approx function to interpolate data to fit the depths of the field data and add it to the model_chla column. I have extracted the same dates(19 dates) from both data sets, so only interpolation on model depth is necesarry.</p>
  <pre><code>chla_FIELD         # some of the field data with the empty model column
            date depth  chla model_chla
  1   2008-02-21     1  0.06         NA
  2   2008-02-21    20  0.05         NA
  3   2008-02-21    50  0.03         NA
  4   2008-02-27     1  0.08         NA
  5   2008-02-27    20  0.04         NA
  6   2008-02-27    50  0.03         NA
  7   2008-02-27    75  0.03         NA
  8   2008-02-27   100  0.01         NA
  9   2008-03-07     1  0.07         NA
  10  2008-03-07    20  0.05         NA
  11  2008-03-07    50  0.03         NA
  12  2008-03-07    75  0.02         NA
  13  2008-03-07   100  0.02         NA
  14  2008-03-07   150  0.01         NA
  
  
  chla_MODEL          # Some of the model data to be interpolated to field data depths and added to column
               dt depth        chla
  766  2008-02-21   1.0  0.22385520
  767  2008-02-21   7.5  0.21676594
  768  2008-02-21  15.0  0.19189246
  769  2008-02-21  25.0  0.15524526
  770  2008-02-21  40.0  0.14638090
  771  2008-02-21  62.5  0.14301939
  772  2008-02-21  87.5  0.14094244
  773  2008-02-21 112.5  0.13897014
  774  2008-02-21 137.5  0.13680272
  775  2008-02-21 162.5  0.13430916
  776  2008-02-21 187.5  0.13133907
  777  2008-02-21 212.5  0.12757768
  778  2008-02-21 237.5  0.12237051
  779  2008-02-21 262.5  0.11396441
  780  2008-02-21 287.5  0.09206185
  856  2008-02-27   1.0  0.24240938
  857  2008-02-27   7.5  0.23447734
  858  2008-02-27  15.0  0.21238998
  859  2008-02-27  25.0  0.15545718
  860  2008-02-27  40.0  0.14592259
  861  2008-02-27  62.5  0.14171122
  862  2008-02-27  87.5  0.13900438
  863  2008-02-27 112.5  0.13662824
  </code></pre>
  <p>My start is something like this, but I'm new to the approx function and adding data to columns in loops.</p>
  <pre><code>chla_approx &lt;- function(FieldDATA, ModelDATA)
    
    for (row in FieldDATA) {
    
    }
  </code></pre>
  <p>Or is there an easier way to do this?   Thanks!</p>