Linear Referencing Lab

Transportation agencies locate things on roads by route and milepoint. Every crash in the lecture's crash table already carries a GDOT route ID and a milepoint. Pick a real District 7 route, watch its curves and crashes become events along its measure, and compare the milepoint answer to “which crashes happened on a curve?” with the 150 ft buffer from Lecture 4.

1 · Locate Features Along Routes: curves become line events

The curve inventory is a set of lines with no milepoints. To put a curve on the diagram, project its first and last vertex onto the measured route, take the closest point on the route and read the measure there. Each curve gets a from-milepoint and a to-milepoint, the same RID, FMEAS and TMEAS fields that ArcGIS Pro's Locate Features Along Routes writes. The snap distance is how far the curve line sits from the route line.

2 · Make Route Event Layer: crashes become point events

A crash row with an Inventory Route ID and a MilePoint is a point event. Make Route Event Layer finds the route with that ID and walks along it to the milepoint, interpolating between the measures stored at the vertices. Nothing about the crash's latitude and longitude is used. Comparing the event location with the reported coordinates is a quick data-quality check.

3 · Overlay Route Events: curve crashes by milepoint, compared with the buffer

By milepoint, a crash is a curve crash when it carries this route ID and its milepoint falls inside a curve's from–to range, give or take a tolerance. In ArcGIS Pro that is Overlay Route Events with the INTERSECT type, after lengthening each curve event by the tolerance. The buffer method from Lecture 4 asks something else: is the crash's reported point within 150 ft of the curve line? Both run here on the same route's curves.

4 · Dynamic segmentation: cut the route into homogeneous pieces

Overlay the line events with Overlay Route Events (UNION), so the route is cut wherever any of them starts or ends, then merge neighbours that still match with Dissolve Route Events. Every piece that comes out has one value for each attribute you split on, which is what a crash-rate or safety-performance study needs.

Try this in class

Hover the straight-line diagram and watch the marker move along SR 154 on the map, then hover the map. Both views read the same measure. Find a place where the AADT row changes shade and read the marker on each side of the change.

Drag the milepoint tolerance to 0 ft. The two Reynolds Rd crashes at MP 19.66 leave the milepoint method, because they sit just under 150 ft past the end of a curve, yet the 150 ft buffer still counts them. Then set the tolerance back to 150 ft.

Read the two buffer-only crashes on SR 154. Their route IDs are city streets, so they happened on cross streets that pass within 150 ft of a curve. Should a study of curve crashes on SR 154 count them?

Pick SR 401 (I-75), Fulton, D side. All four crash events land more than 50 ft from their reported coordinates, and the one at W Paces Ferry Rd lands 573 ft away. Discuss the possible causes: a wrong milepoint in the crash report, or crash milepoints and route measures taken from different versions of the road network.

Pick SR 403 (I-85), Fulton, inventory direction. One crash cannot be placed because the route has no measure at its milepoint. Its report says I-75 at Langford Pkwy, on the Downtown Connector, where I-85 shares the road with I-75. Then find the buffer-only crashes that are coded on the D side.

In step 4, untick “Curve on or off” and watch the number of segments drop, then tick “Functional class”. Which attribute cuts the route into the most pieces? Keep in mind that the crash sample covers only one day.

The vocabulary

Term	What it means here
RoutePolylineM	A line with a route ID whose vertices also store a measure (m-value). Here each route is built from GDOT road segments sorted by their from-milepoint.
Measuremilepoint, MP	Distance along the route from its start, in miles. GDOT restarts the count at 0 at every county line.
Point eventroute ID + measure	A table row that says where something is by route and milepoint, such as a crash with Inventory_ and MilePoint_.
Line eventroute ID + from + to	A stretch of road with one value, such as a curve, an AADT count section or a speed zone.
Dynamic segmentationtwo meanings	Esri's term for computing where events sit on the map from their measures, at display time. Safety analysts also use it for cutting a route into homogeneous segments.
Calibrationmeasures vs ground	Adjusting route measures so they match known milepoints on the road. Events only land in the right place when the events and the routes use the same calibration.

The crash table already holds eventsLecture 3 · Table operations

The crash CSV you displayed with Display XY Data has the columns Inventory Route ID (Crash Level) and MilePoint (Crash Level), cut to Inventory_ and MilePoint_ in the shapefile. That pair is a point event. Placing it works like a Join: the route ID is the common key, many crashes match one route (many-to-one), and the milepoint says where along the route each row belongs.

Crashes whose route ID is blank, or names a road that is not in the route layer, are the rows a join would leave unmatched.

The buffer asks a different questionLecture 4 · Buffer and Select By Location

The exercise draws a 150 ft round buffer around the D7 curves and selects the crashes inside it with Select By Location, then joins curve attributes to them with Spatial Join. That test uses only coordinates, so it also catches crashes on cross streets and on the other side of a divided road. The milepoint test uses only the route ID and the measure, so it misses crashes whose milepoint is wrong.

Neither method is the truth on its own. A careful study runs both and checks the crashes they disagree on by hand.

The tools in ArcGIS ProLinear Referencing toolbox

Create Routes builds routes from GeorgiaRoadSegments_SR with the route ID field, using two fields (from and to milepoint) as the measure source.
Locate Features Along Routes writes RID, FMEAS and TMEAS for line features such as curves, or RID and MEAS for points, within a search radius.
Make Route Event Layer places a table of crashes on the routes by route ID and milepoint.
Overlay Route Events: INTERSECT finds crashes inside curve events; UNION cuts line events into pieces.
Dissolve Route Events merges neighbouring pieces that share the same values.

ArcMap has the same tools under the same names in its Linear Referencing toolbox.