title: How to create an estUDm object from utilization distributions created with GIS?
body: <p>I created kernel densities/utilization distributions for multiple animals in ArcGIS Pro instead of using the adehabitatHR package in R because I needed a different smoothing parameter estimator and I had a difficult boundary to work with.</p>
  <p>Now I have several individual raster files of these UDs and I want to calculate overlap using the UDOI method. This would be much more efficient using the adehabitatHR package, but I'm having trouble converting these rasters into a single estUDm object to use with the kerneloverlaphr function. I apologize, I am not sure how to provide the rasters to create reproducible data, but my code is below.</p>
  <p>I appreciate any help with this error message that I'm receiving when running kerneloverlaphr: &quot;Error in if (slot(x[[1]], &quot;vol&quot;)) stop(&quot;x should not be a volume under UD&quot;) :
  argument is of length zero&quot;</p>
  <pre><code>library(raster)
  library(adehabitatHR)
  
  #Import kernel density rasters created in ArcGIS Pro
  UD1 &lt;- raster(&quot;UD1.tif&quot;)
  UD2 &lt;- raster(&quot;UD2.tif&quot;)
  
  #Convert to spatial pixels data frame
  UD1.px &lt;- as(UD1, &quot;SpatialPixelsDataFrame&quot;)
  UD2.px &lt;- as(UD2, &quot;SpatialPixelsDataFrame&quot;)
  
  #Convert to estUD
  UD1.estUD &lt;- new(&quot;estUD&quot;, UD1.px)
  UD2.estUD &lt;- new(&quot;estUD&quot;, UD2.px)
  
  #Combine both estUD's and convert to estUDm for overlap analysis
  UD.combined &lt;- list(animal1=UD1.estUD, animal2=UD2.estUD)
  class(UD.combined) &lt;- &quot;estUDm&quot;
  
  #Check individual UD's
  plot(UD.combined$animal1) #shows correct UD
  plot(UD.combined$animal2) #shows correct UD
  
  kerneloverlaphr(UD.combined, method=c(&quot;UDOI&quot;)) #Received error message: Error in if (slot(x[[1]], &quot;vol&quot;)) stop(&quot;x should not be a volume under UD&quot;) :   argument is of length zero
  </code></pre>