Finding mid-point of line using ArcPy?

Question

How can the mid-point of a polyline be calculated using the geoprocessing framework, e.g. in a Python script?

The polyline.centroid property returns The true centroid if it is within or on the feature; otherwise, the label point is returned. The centroid is rarely located on non-straight lines, which is useless for my purposes.

The Feature Vertices to Points tool has a midpoint option but this requires ArcInfo, which I don't currently have.

An option could be to add measures to the polylines and create a route event 50% along the line.

Another workaround is to use the Calculate Geometry option in ArcMap, but ideally I need to automate this process in a script.

Any better/faster suggestions?

I am limited to ArcGIS 10.0 (without access to an ArcInfo level license) for the moment.

Answer 1

The Polyline class has a new method called "positionAlongLine" in ArcGIS 10.1. This will return a PointGeometry object with exactly one point at a specified distance from the starting end of the line, or a fraction of the distance between the start and end. To find the midpoint, you would just need to do positionAlongLine(0.5,True). To find the midpoints for lines and add their coordinates to the attribute table, you could do Field Calculator on the following statement:

• !Shape!.positionAlongLine(0.5,True).firstPoint.X
• !Shape!.positionAlongLine(0.5,True).firstPoint.Y

Note that you need to be using the Python parser in field calculator for this to work.

If you wanted to access this point object in Python, you would just do the following:

Input_shp = "C:\Temp\Line.shp"
Cursor = arcpy.SearchCursor(Input_shp)
for Feature in Cursor:
    Midpoint = Feature.shape.positionAlongLine(0.50,True).firstPoint
    print Midpoint.X
    print Midpoint.Y

Answer 2

With ET Geo Wizards license you could try the function "Polyline to Point" with the option "Middle points" and "Remove Duplicate Points" (optional): http://www.ian-ko.com/ET_GeoWizards/UserGuide/convertPl2Pnt.htm

You can implement ET Geo Wizards like this:

toolbox_ETgeowizards = arcpy.GetParameterAsText(0) # Location ET Toolbox (Type: Toolbox)
arcpy.ImportToolbox(toolbox_ETgeowizards)

...

arcpy.ET_GPPolylineToPoints(pInFeatureClass, sOutFileName, sExportOption, bRemoveDuplicates)

For input variables of the ET function, please see: http://www.ian-ko.com/ET_GeoWizards/UserGuide/Scripting/scripting_PolylineToPoint.htm