topic Re: Why firstPoint and lastPoint does not match? in Python Questions

Why firstPoint and lastPoint does not match?

ValerioLuzzi1 — Wed, 19 Sep 2018 12:52:24 GMT

SR = arcpy.Describe(linepathname).spatialReference

polyline = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686221.91579999961,4929363.8212000001],[686224.60580000002,4929363.6297999993],[686227.87069999985,4929364.5913999993],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)

#segment = arcpy.Polyline(arcpy.Array([polyline.firstPoint,polyline.lastPoint]),SR)

segment = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)

I have tried to create a simple line that join the first point and the last point of the polyline but the render on the map is wrong (see the Image).

I have an offset and I cant figure the problem.

(The spatial reference it's the same)

Anyone can give me some suggestion?

Re: Why firstPoint and lastPoint does not match?

JoshuaBixby — Sat, 11 Dec 2021 19:03:59 GMT

Using this code:

>>> polyline = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686221.91579999961,4929363.8212000001],[686224.60580000002,4929363.6297999993],[686227.87069999985,4929364.5913999993],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)
>>> arcpy.CopyFeatures_management(polyline,"in_memory\polyline")
<Result 'in_memory\\polyline'>
>>> 
>>> segment = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)
>>> arcpy.CopyFeatures_management(segment,"in_memory\segment")
<Result 'in_memory\\segment'>
>>> ‍‍‍‍‍‍‍‍

I get:

What version of ArcGIS are you using? I am using ArcMap 10.6.1.

Re: Why firstPoint and lastPoint does not match?

ValerioLuzzi1 — Wed, 19 Sep 2018 20:44:44 GMT

My version is 10.4 but I think this cannot be the problem. I tried your code and all seems ok also for me,
so the problem is not here.
 
This code should duplicate a record and change the shape with the segment but here you should see the trouble.
 
outpathname = "in_memory\polyline"
polyline = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686221.91579999961,4929363.8212000001],[686224.60580000002,4929363.6297999993],[686227.87069999985,4929364.5913999993],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)
arcpy.CopyFeatures_management(polyline,outpathname)
SR = arcpy.Describe(outpathname).spatialReference
with arcpy.da.SearchCursor(outpathname, ['SHAPE@','*'] ) as cursor:
 with arcpy.da.InsertCursor(outpathname, ['SHAPE@','*'] ) as icur:
  for row in cursor:
   polyline = row[0]
   #print polyline
   segment = arcpy.Polyline(arcpy.Array([polyline.firstPoint,polyline.lastPoint]),SR)
   row = list(row)
   row[0] = segment
   icur.insertRow(row)
 
Please give me a feedbak. Thank you!

Re: Why firstPoint and lastPoint does not match?

DarrenWiens2 — Wed, 19 Sep 2018 23:38:54 GMT

When you print `polyline`, it shows the correct coordinates? And, CRS is defined in the output feature class (just verify that it actually gets set in the feature class)?

Re: Why firstPoint and lastPoint does not match?

JoshuaBixby — Sat, 11 Dec 2021 19:04:02 GMT

Nesting an insert cursor within a search cursor on the same feature class or table seems ripe for problems. That said, it isn't the cause of the issue you are seeing. Try the following code:

outpathname = "in_memory\polyline"
polyline = arcpy.AsShape("""{"paths":[[[686187.19259999972,4929374.2093000002],[686221.91579999961,4929363.8212000001],[686224.60580000002,4929363.6297999993],[686227.87069999985,4929364.5913999993],[686230.85759999976,4929365.9749999996]]],"spatialReference":{"wkid":25832,"latestWkid":25832}}""", True)
arcpy.CopyFeatures_management(polyline,outpathname)
SR = arcpy.Describe(outpathname).spatialReference
with arcpy.da.SearchCursor(outpathname, ['SHAPE@','*'] ) as cursor:
 with arcpy.da.InsertCursor(outpathname, ['SHAPE@'] ) as icur:
  for row in cursor:
   polyline = row[0]
   #print polyline
   segment = arcpy.Polyline(arcpy.Array([polyline.firstPoint,polyline.lastPoint]),SR)
   icur.insertRow([segment])‍‍‍‍‍‍‍‍‍‍‍

Re: Why firstPoint and lastPoint does not match?

ValerioLuzzi1 — Thu, 20 Sep 2018 13:46:26 GMT

Yes!, it works! Thank you.

The problem is probably hidden in nested cursors. I also solved with a workaround. First duplicating records and after with a (not nested ) UpdateCursor updating the geometry, but your solution is more compact and I think I will use it.

Thank you again.

Re: Why firstPoint and lastPoint does not match?

JoshuaBixby — Thu, 20 Sep 2018 13:52:37 GMT

I think the issue here is that you were passing conflicting geometry data to the insert cursor. Since you used ["SHAPE@", "*"] for the fields, you were returning both a geometry object as well as a tuple of the shape's centroid x,y coordinates. When you went to insert the record, you passed a new geometry object but with the same tuple containing the previous geometry centroid.

Re: Why firstPoint and lastPoint does not match?

ValerioLuzzi1 — Thu, 20 Sep 2018 14:10:44 GMT

Ok, I thinked something similar … but I always thinked that all spatial information was inside the SHAPE field and replacing it with a new geometry all should be fine.

Anyway Tahnk you again. I should be more carefull in using cursors.