title: What are the units of distance in gstat variogram?
body: <p>This is a follow-up to <a href="https://stackoverflow.com/questions/10736070/calculate-variogram-of-raster-data-with-nas-in-r">this question</a>. I have now managed to plot variograms of the data using the answers given to that question. The code I am using (where <code>im</code> is a variable containing the <code>dput</code> data given at <a href="https://gist.github.com/2780792" rel="noreferrer">https://gist.github.com/2780792</a> is</p>
  
  <pre><code>library(gstat)
  point_data &lt;- as(im, 'SpatialPointsDataFrame')
  v &lt;- variogram(band1 ~ 1, data = point_data)
  plot(v)
  </code></pre>
  
  <p>This gives me the following plot:
  <img src="https://i.stack.imgur.com/b1rvd.png" alt="enter image description here"></p>
  
  <p>As you can see, the numbers on the distance axis range from 0 to around 150. However, the units of the data are degrees:</p>
  
  <pre><code>&gt; head(coordinates(point_data))
               x        y
  [1,] -1.849353 52.06165
  [2,] -1.759728 52.06165
  [3,] -1.401227 52.06165
  [4,] -1.311602 52.06165
  [5,] -1.221977 52.06165
  [6,] -1.132352 52.06165
  </code></pre>
  
  <p>Given that, what are the units of the distance measures on the X axis? I would expect them to be in the same units as the co-ordinates. It doesn't make sense for them to be metres, I guess they could be km.</p>
  
  <p>I can't find anything in the gstat manual about this, so I'd have assumed they used the underlying data's units, but this doesn't seem to be the case.</p>
  
  <p>Any ideas?</p>