So to be clear, please separately confirm that you had no LRS routes based on postmiles already created prior to posting this issue. If that is the case, then I am assuming that the postmile values in your fields were measured/estimated by field staff relative to actual mile posts or by some other means that did not involved using LRS techniques and route calibration. Do you have any point layer for the actual mile posts positions that can be used for calibration in addition to the polyline end points? If such points exist between the end points of your lines they could alter the accuracy of the postmile distances interpolated between the line end points.
Please let me know if the following example accurately describes what you want to accomplish.
1. Say I have a highway made up of 5 polyline segments with the following from and to postmiles.
0 to 2
2 to 5
5 to 8
8 to 12
12 to 19
2. If the urban boundaries intersected these segments at postmile positions 1 to 6 inclusive and 10 to 17 inclusive that would produce the following segments designated either Rural or Urban as follows:
0 to 1 Rural
1 to 2 Urban
2 to 5 Urban
5 to 6 Urban
6 to 8 Rural
8 to 10 Rural
10 to 12 Urban
12 to 17 Urban
17 to 19 Rural
3. If I ignore the Rural segments and just extract the 5 Urban segments I would create two new fields to hold the reapportionment of the Urban postmile measures. I would continue to use the first postmile measures of 1 to 6 for the first sets of 3 urban segments, but I would adjust the postmile measures of the second set of 2 urban segments to eliminate the postmile offset created by the gap separating it from the first set of urban segments. So taking just the 5 urban segments I would have a polyline with two fields showing the original postmile measures and two fields showing the reapportioned postmile measures as follows:
1 to 2 original urban and 1 to 2 reapportioned urban
2 to 5 original urban and 2 to 5 reapportioned urban
5 to 6 original urban and 5 to 6 reapportioned urban
10 to 12 original urban and 6 to 8 reapportioned urban
12 to 17 original urban and 8 to 13 reapportioned urban
Total Urban postmiles = Beginning at original highway postmile 1 there are 12 Urban postmiles covered by these polylines (To postmile minus From postmile = postmile 13 - postmile 1 = 12 postmiles).
If the above example is a correct presentation of what you want to do, this can be done using LRS techniques to obtain the postmile segments of section 2, but you would need a custom Python script to perform the steps needed to create the output shown in section 3.
