Dissolving line segments while maintaining original route sequence/vertex order using ArcGIS Pro

Question

I have many polylines that represent the GPS derived route of a vehicle. I have created a layer with all route segments that meet a certain wind speed threshold (not all of these line segments touch), and have made an additional field with segment grouping values that I want to dissolve segments on (segments share group values based on the time they were generated). Unfortunately, when I dissolve segments together based on group value, the new polylines don't maintain the vertex order I need, as seen below:

Note the end vertex is on one of the middle lines; however, the vehicle was travelling West, so I need it to be on the far left segment. It seems that dissolve randomly chooses start/end line segments when creating the new vertex order. (Note, all original segments have correct vertex orders. I.e. for this image, each segment vertex ends on the Western vertex.)

I need to figure out if there is any way to control the vertex order of a dissolved polyline?

I have a time associated with each line segment that I can use to order them properly, I just am not sure how to apply this in dissolve. Context: I need a proper vertex order so that I can generate segment angle changes along the route.

Here is a link to some sample data (pre-dissolve). If you dissolve these segments by the "segment_group" field you will see the issue. https://github.com/Austin-Willoughby/Projects/blob/master/example_segments.zip

EDIT:

I've updated the sample data to include more segment group types. I've also written some code that utilizes the Array method @Hornbydd suggested and generated the segments seen below.

(Original segments are pink, generated segments are blue)

The newly generated lines seem to have the desired vertex order; however, the vertex coordinates seem to be off slightly. Additionally, the standalone segments (i.e. segment_group = 0 which shouldn't be combined with other lines) don't appear in the new lines. Perhaps the issue is that I try to create polylines from these single segments in the same manor as the multipart lines?

See code below:

python version 3.7.10, arcpy version 2.8

import arcpy  
import os  
from arcpy import env  

arcpy.env.workspace= os.getcwd() + "\\Wind_Speed_Analysis.gdb"
# sort feature class by segment_group then startepoch (i.e. start time) for desired segment order
arcpy.Sort_management("sample_data", "sample_data_sort", [["segment_group", "ASCENDING"], ["startepoch", "ASCENDING"]])
fc = r"sample_data_sort"
fc_len = int(arcpy.GetCount_management(fc).getOutput(0))

# Enter for loop for each feature
with arcpy.da.SearchCursor(fc, ['SHAPE@','segment_group']) as cursor:
    row_indx = 0
    segments_wind_exceedance_merged = []
    
    for row in cursor: #iterate through rows
        segment_group = row[1]
        
        # can't access prev row at line 0
        if row_indx == 0: 
            polyline = []
            prev_group = 0
            for part in row[0]:
                polyline.append(arcpy.Array(part))
            segments_wind_exceedance_merged.append(arcpy.Polyline(arcpy.Array(polyline)))
            
        # last row will always have the same segment_group directly above
        elif row_indx == fc_len-1:
            for part in row[0]:
                polyline.append(arcpy.Array(part))
            segments_wind_exceedance_merged.append(arcpy.Polyline(arcpy.Array(polyline)))
                
        # if not first row, previous row can be used
        elif row_indx > 0:
            if segment_group == 0:  # segment_group = 0 denotes standalone segments that will not be merged
                polyline = []
                prev_group = 0
                for part in row[0]:
                    polyline.append(arcpy.Array(part))
                segments_wind_exceedance_merged.append(arcpy.Polyline(arcpy.Array(polyline)))

            # segment_group = 0 are stand alone lines.  segment_group > 0 will occur in at least two rows for each group 
            elif segment_group > 0: 
                
                if segment_group != prev_group: # this will occur when loop accesses a new segment_group
                    if prev_group > 0:
                        segments_wind_exceedance_merged.append(arcpy.Polyline(arcpy.Array(polyline))) #append previous segments
                    polyline = [] # create empty polyline list for new segment
                    for part in row[0]:
                        polyline.append(arcpy.Array(part))
                    prev_group = segment_group
                    
                elif segment_group == prev_group:
                    for part in row[0]:
                        polyline.append(arcpy.Array(part))
                    prev_group = segment_group
                        
        row_indx+=1
        
        
arcpy.CopyFeatures_management(segments_wind_exceedance_merged, "Wind_Speed_Analysis.gdb\sample_polylines")

Answer 1

Shorter version of your script, with specified projection:

import arcpy
infc = "sample_data"
d = arcpy.Describe(infc)
SR = d.spatialReference
arcpy.AddMessage(SR.name)
GROUPS = set([int(row[0]) for row in arcpy.da.TableToNumPyArray(infc,"segment_gr")])
listOfLines = []
arr = arcpy.Array()
for item in GROUPS:
    arr.removeAll()
    q = '"segment_gr" = %i' %item
    with arcpy.da.SearchCursor(infc,"Shape@",q) as cursor:
        for row in cursor:
            part = row[0].getPart(0)
            arr.add(part)
    pline = arcpy.Polyline(arr, SR)
    listOfLines.append(pline)
outfc = "in_memory/mparts"
arcpy.management.CopyFeatures(listOfLines, outfc)

Note: tested on shapefile you provided.

Answer 2

I've looked at your sample and able to replicate the issue. I also tried the pairwise dissolve tool and weirdly it produce a different sequence. The problem can also be replicated in ArcMap.

I'm unsure why its happening so the only thing I can suggest is you use python and arcpy to build your multipart line. You can append each part to an Array object to construct the multipart.