ArcPy M-values – How to Set M-values to Cumulative Length of Line Using ArcPy

arcgis-10.7arcpygeometrylinear-referencingm-values

ArcMap 10.7.1 — Oracle 18c EGDB — SDE.ST_GEOMETRY

I have existing SDE.ST_GEOMETRY polylines that have M-values (aka 'measure values' for linear referencing purposes).

I want to update the M-values for all the lines via an ArcPy script.

In my case, the M-values of lines should be the same as the cumulative length of the line. In other words, if we look at the image below, the M-values should be the same as the length of the line at a given vertex.

Question:

Using ArcPy, is there a way to set the M-values to the cumulative length of the line?

Note: Some of the lines have arcs. And some of the lines are multi-part.

Related:

Python: Working with Feature Data using ArcPy [YouTube Video from ESRI]

Best Answer

I came up with a script seems to work:

import arcpy
connection = "Database Connections\my_conn.sde"
feature_class = connection + "\my_owner.my_fc"
spatial_reference = arcpy.Describe(feature_class).spatialReference

with arcpy.da.Editor(connection) as edit_session:
    with arcpy.da.UpdateCursor(feature_class, "SHAPE@") as cursor:
        for row in cursor:
            geometry = row[0].densify("ANGLE", 10000, 0.174533)
            parts = arcpy.Array()
            for part in geometry:
                points = arcpy.Array()
                for point in part:
                    point.M = geometry.measureOnLine(point)
                    points.append(point)
                parts.append(points)
            row[0] = arcpy.Polyline(parts, spatial_reference)
            cursor.updateRow(row)

Notes:

The script works for both singlepart and multipart features.
The script densifies true curves -- since true curve functionality in ArcPy is limited.
Related: Code Review SE — Loop through polyline vertices and update coordinate

Related Solutions

[GIS] Segment road network line shapefile into new equal length segments line shapefile

As with everythin in GIS there's more than one way to do it:

ArcGIS has a feature to do just this, See Divide line function. Of course, remember that you rarely have an exact division unless you use the percentage option (i.e. you will get a bit left over). And, to be honest, I found it would not always work perfectly but it doesn't do too bad a job. Don't confuse the Divide function with Split.

Depending on your requirements, adding M values to the data can either provide a means for splitting it or may be the solution to your problem. I know you say you want to split the lines, but do you NEED to (you don't say what the end goal is). I'm not being funny, I'm just suggesting that lateral thinking in geoprocessing can often provide the answer you need more easily rather than the most obvious route.

If neither of the easy options will do and you want to do this as a script and have ArcGIS, you don't need OGR to grab a list of vertices. You can do it all in ArcPy. Have a look in the manual for examples on reading geometries and then read the section on writing geometries. These sections have ArcPy examples. I'd follow a recipe along the following lines:

Create an empty shapefile (destination)
For each feature, read the vertices, calculating the distance and bearing between each one and summing the distances until you need to split. As you go, add the vertices to a new line array object.
When you need to split use the bearing between your last two points and the remaining distance before 0.5mi to calculate the coordinates of your new point. Add this to your line array object.
Insert your line array object into your destination shapefile
Loop your code to make another line-array object using the last point as the first point of the new line and then continue reading as before.
Rinse and reapeat until done.
List item

Finally, Alternatively in any GEOS enabled software (PostGIS, SpatiaLite etc) you should frin an ST_Line_Interpolate_Point. This interpolates points along lines at set distances. You can then use the points as a means to split the lines.

I'm sure there are other ways to do it too :)

ArcPy – Editing Vertices Without Removing True Curves in ArcPy

I guess you can manipulate it using JSON.

For example, this field calculator expression, when I run it on a buffer of a point:

import json
def wJson(shp):
 pnts = json.loads(shp.JSON)
 #esriJSON = arcpy.AsShape(pnts, True)
 return str(pnts)
#----
wJson( !Shape!)

Returns:

{u'curveRings': [[[1727075.9359999998, 5946765.4923], {u'a': [[1727075.9359999998, 5946765.4923], [1717013.4664801424, 5946765.4923], 0, 1]}]], u'spatialReference': {u'wkid': 2193, u'latestWkid': 2193}}

Which is just a dictionary. You can use:

esriJSON = arcpy.AsShape(pnts, True)

after amending the dictionary.

Also see getPart() method returns incorrect geometry from buffer in ArcGIS

Best Answer

Related Solutions

[GIS] Segment road network line shapefile into new equal length segments line shapefile

ArcPy – Editing Vertices Without Removing True Curves in ArcPy

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