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I am using Spatial Analyst to interpolate surfaces for several groundwater parameters based on point observations at monitoring bores, including groundwater level and pH.

It suddenly occurred to me however, that pH itself is a logarithmic measure of concentration of hydrogen ions. I can't seem to clarify in my head if this requires special attention.

In the most basic conceptual example, if point A is pH=4.0 and point B is pH=6.0, it could be assumed that a point C directly in the middle of the two (equidistant to both) might be ph=5.0 (left-hand mark on graphic attached). If we were looking at actual hydrogen ion concentration as the parameter, the value at point C might be closer to 5.7-5.8 (right-hand mark on the graphic attached).

Could someone please help clarify the assumptions I might be missing here and the correct way to handle this when interpolating?

pH graph

  • Stick with pH, because main error is introduced by interpolation method itself. Interpolation between massively different points will produce ugly pattern – FelixIP Apr 15 '16 at 5:34
  • It seems more likely that if point B is close to neutral, but point A is really acidic, that a point between them is probably more acidic than half the value. However its probably dubious to be using interpolation if they are really that varying. – BradHards Apr 15 '16 at 5:35
  • If we were looking at actual hydrogen ion concentration as the parameter, the value at point C might be closer to 5.7-5.8. Actually, it would be between 4 and 5. – Andre Silva Sep 9 '16 at 21:14
  • @AndreSilva Right you are, had that backwards. Thanks. – malcoholic Sep 12 '16 at 6:29
  • Right, no problem. Would you mind editing the question and the picture? It might help getting an answer. Tks – Andre Silva Sep 12 '16 at 10:24

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