topic Re: Creating small polylines from point coordinates in Python Questions

Creating small polylines from point coordinates

SachidanandaMishra — Fri, 25 Jul 2014 20:19:09 GMT

I am trying to create a polyline geometry from point coordinates using Arcpy (ArcGIS 10.1). I can do that if the length of lines are not short (>100 ft). However, my script creates an empt geometry when I try to create a line of length ~10s of feet. E.g, creating a line for the start points (32.297159, -90.211997) and end point (32.297179, -90.211997) does not work. I wonder if its a decimal problem or some kind of geometry creation issue. I would appreciate any help!

Thanks,

sachi

Re: Creating small polylines from point coordinates

DanPatterson_Retired — Fri, 25 Jul 2014 20:23:23 GMT

Sounds like you are working with decimal degree data and trying to produce a polyline in meters (or feet). I would suggest working with projected coordinates. I would also suggest including your script here so it can be examined for potential script errors

Re: Creating small polylines from point coordinates

SachidanandaMishra — Fri, 25 Jul 2014 20:57:32 GMT

Hi Dan,

Thanks for your reply. Please see part of my script below that intends to create the line layer.

#-----------------------------

import arcpy

arcpy.env.overwriteOutput = True

output = "C:\\temp\\line.shp"

createLine = arcpy.CreateFeatureclass_management("in_memory", "createLine", "POLYLINE", "", "DISABLED", "DISABLED", polygon)

cursor = arcpy.da.InsertCursor(createLine, ["SHAPE@"])

#Read the coordinates

x1 = -90.211997

y1 = 32.297159

x2 = -90.211997

y2 = 32.297179

array = arcpy.Array([arcpy.Point(x1, y1), arcpy.Point(x2, y2)])

array_line = arcpy.Polyline(array)

cursor.insertRow((array_line,))

del cursor

arcpy.CopyFeatures_management(createLine, output)

arcpy.Delete_management(createLine)

#-------------------------

Re: Creating small polylines from point coordinates

DanPatterson_Retired — Fri, 25 Jul 2014 21:11:38 GMT

I have some vague recollection that the spatial reference had to be set otherwise you got errors in geometry creation. I have included a small demo script...you can add to the list of values to calculate for...values are calculated from the origin...just copy and paste the code into a script window...tab spacing is 2 spaces

'''
import math
import arcpy
pnt = arcpy.Point()

polylines = []
vals = [0.001, 0.1, 10.0, math.sqrt(2.0), 2.0]
for a_val in vals:
polylines.append( [[0.0,0.0],[a_val,0.0]] ) #positive out
polylines.append( [[0.0,0.0],[-a_val,0.0]] ) #negative out
polylines.append( [[a_val,0.0],[0.0,0.0]] ) #positive in
polylines.append( [[-a_val,0.0],[0.0,0.0]] ) #negative in
array = arcpy.Array()
polys = []
for i in range(len(polylines)):
a_poly = polylines
print "\n", "Coordinates: ", a_poly
for pair in a_poly:
    pnt.X = pair[0]
    pnt.Y = pair[1]
    array.add(pnt)
    #print " X: %20.16f Y: %20.16f" % (pnt.X, pnt.Y)
'''
add an empty spatial reference when creating the
geometry to go from low to high precision
'''
poly = arcpy.Polyline(array,arcpy.SpatialReference())
cal_leng = poly.length
centroidX = poly.centroid.X
centroidY = poly.centroid.Y

print "Polyline properties:"
print " length: %20.16f" % (cal_leng)
print " centroid (x,y: %20.16f, %10.16f" % (centroidX, centroidY)

array.removeAll()
polys.append(poly) #for further use