title: scikit-image LineModel() estimate incorrect
body: <p>I have some pretty straight forward code to segment out a blob and then do a total least squares line fit to it, but the estimate of the line (blue) is clearly not correct.  Anybody know what I did wrong?</p>
  
  <p><img src="https://i.stack.imgur.com/Essgg.png" alt="segment of dash image under analysis"></p>
  
  <pre><code>import skimage as s
  import skimage.feature as feat
  from skimage import measure
  #dash is the original image of the dash as a numpy array
  g= dash[2662:2800,3050:3263,0]
  edges = feat.canny(g/255., 3.2, .01, .45)
  fill = ndimage.binary_fill_holes(edges)
  t = s.img_as_ubyte(flt.sobel_h(fill))/2+s.img_as_ubyte(flt.sobel_v(fill))/2
  ptpairs = np.int0(np.where(t&gt;160))
  l = measure.LineModel()
  l.estimate(ptpairs.T)
  xdata = np.arange(60,80)
  ydata = l.predict_y(xdata)
  imshow(g+edges*100)
  plot(xdata,ydata)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/TN8ck.png" alt="Line Fit Overlay on Gauge Image"></p>