title: Return attributes within given radius of observation in R
body: <p>Novice R programmer struggling to add a spatial component to large data set (>1500 obs)</p>
  
  <p>Given a list of: observations, corresponding x/y points (not lat/long), and attributes; how can I write an algorithm to return the sum of variable "Z" for all observations within a given radius (10) of the observation in question. I need to do this for each observation. </p>
  
  <p>Simplified data headers read as follows:</p>
  
  <pre><code>OBS      X        Y          Z**
  A       56.55     -289.65  
  B       52.59     -287.82   
  C       58.34     -284.58     
  </code></pre>
  
  <p>Any assistance would be appreciated. Thanks.</p>