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I am trying to calculate a field for some points within a python script using arcpy functions. (This is calculating the percentage visibility from a cumulative viewshed, if that matters.)

# Process: Get Count
TotalFlightPoints = int(arcpy.GetCount_management(FlightPath_Pts).getOutput(0))

# Process: Add Field
Obsv_Vis = arcpy.AddField_management(Obsv_Vis, "PercentVis", "FLOAT")

# Process: Calculate Field (2)
Obsv_Vis = arcpy.CalculateField_management(Obsv_Vis, "PercentVis", "!RASTERVALU! / " + str(TotalFlightPoints), "PYTHON", "")

Even though I cast that field as a FLOAT when adding it, the result is consistently an integer (0 if less than 1, 1 if the point is 100% visible). I can get around this by "!RASTERVALU! * 100 / " + str(TotalFlightPoints), which will at least give me the percentage (without any trailing decimals). That level of accuracy may be sufficient for this application, at least for now.

But, if I use the Field Calculator within ArcMap on a "float" field (with, say, !RASTERVALU! / 121 as the equation), it gives me the result as a float. So I'm wondering why the field calculator seems to work differently in different situations, and/or what I can tweak to get what I really want.

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When calculating in the Python script, make sure you force float calculations, e.g. 12/2 = 6, but 12./2. = 6.0 –  Jason Mar 28 '13 at 15:22
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1 Answer

up vote 5 down vote accepted

@Jason's comment is correct: you need to be dividing by a float, but you define TotalFlightPoints as an integer. Try it with

TotalFlightPoints = float(arcpy.GetCount_management(FlightPath_Pts).getOutput(0))

and you should see different results.

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Ah yes, I glossed over the first line of code where TotalFlightPoints is cast as int. This should do it. –  Jason Mar 28 '13 at 16:57
THERE we go. Thank you. (I should have known better, sheesh!) –  Erica Mar 29 '13 at 1:02
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