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How does one calculate the median value of Z from a 3D line?

The Add Surface Information tool in 3D Analyst only does Min,Max and Mean (average), and requires a surface model. (In this case I have one, but all necessary info is in the line, so let's just use that.) Line Statistics can do Median as well as Minority, Majority and a few others, but it doesn't know about 3D lines and only operates on field attributes.

Primary environment is ArcGIS Desktop but I'm open to Gdal/Qgis methods that write results to the source feature class in place (file-gdb).

Desired outcome are lines with a populated Z_Median attribute.

2

Interpolate shapes using tool from 3d analyst. Run field calculator expression on numeric field type float, parser - Python

def getMedian(shp):
    aList=[]
    part=shp.getPart(0)
    n=len(part)
    for i in xrange(n):
        p=part.getObject(i)
        z=p.Z
        aList.append(z)
    aList.sort()
    return aList[n/2]

To call it use:

getMedian(!Shape!)

It will work on a single part 3D polylines

  • This is near perfect, thanks (though indentation screwed me up for awhile. I'd put aList.append in parent). I changed it to use numpy.median() so I didn't have to add logic to handle when the list has even number of items. Those with ArcGIS Pro can use statistics.median() instead ('cause python 3). Good python + median reading: stackoverflow.com/questions/24101524/… – matt wilkie Jan 19 '16 at 18:28
  • 1
    This will fail on a polygon interior ring ("AttributeError: 'NoneType' object has no attribute 'Z'"). This seems to work: p=part.getObject(i); if p: z=p.Z. – matt wilkie Feb 10 '16 at 21:55
-1

Try using the Add Z Information tool.

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  • It's not appreciably different from Add Surface Information tool, providing only min,max and mean statistics. – matt wilkie Jan 18 '16 at 23:02

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