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How can I create an isohyetal map in ArcGIS 10. I have the precipitation values from a number of weather stations in my area of interest. What tools should I use to create these contour lines?

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You first need to interpolate your precipitation values to a continuous surface. Then you can use the contour tool to create your isohyete.

For contour lines (where you want something visually attractive), the smoothness and the generalisation are important, so I recommend using a spline function for the interpolation. However, this is not necessarily the best method in other applications.

Remark : you need spatial analyst to use these tools in ArcGIS

  • I have created a continuous surface using IDW method. are you recommending me to do it again with spline before moving ahead. – Mehnaz Oct 2 '14 at 7:43
  • You have a better view than me on your data, so if your results are satisfying then its OK. Sometimes IDW creates some "buttons" around the observations, so you could end up with somehow irregular patterns in your isohyetes. With spline it should be smoother. I assumed that you wan it for cartography (for analysis, I would stick to the data), so smoother lines are usually better. – radouxju Oct 2 '14 at 8:20
  • While spline might be more visually appealing, it could also create some particular problems with this type of data since it can create regions outside of the actual data range. For example, I did an exercise with snowfall data using IDW, Spline, and Kriging. Spline certainly looked the best, but it also gave several areas of both negative snowfall (which makes no sense) and some areas above the maximum recorded. It might be better to use one of the more data oriented methods and then worry about smoothing out/cleaning up the resulting interpolation later via generalize lines or something. – Chris W Oct 2 '14 at 18:53
  • I agree, there are drawbacks, but with contourlines you can simply erase "impossible" isohyete IF they occur. I won't use it for analysis but I would give it a try for deriving nice contour lines. – radouxju Oct 2 '14 at 19:13

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