Available with Location Referencing license.
The ArcGIS Roads and Highways extension tools and capabilities support projected and unprojected data. Minimum schema items such as centerlines, calibration points, and redline, along with LRS networks, and LRS events are modeled as feature classes, which allows use of the data beyond ArcGIS Pro.
Roads and Highways is a linear referencing system (LRS) used across ArcGIS. An LRS involves calculations of length and distance at a high level of accuracy. To support this in a geographic coordinate system (GCS), the distance between vertices and the x,y and z-tolerances of the feature classes are considered.
Distance between vertices and x,y and z-tolerance
In ArcGIS, projected and unprojected data is rendered differently. Projected data is drawn as a straight line between the points (vertices) on the line, while unprojected data in a GCS is drawn as an arc between the points on the line as it attempts to incorporate the curvature of the earth. The image below shows the projected data polyline in blue and the unprojected data polyline in purple.
Many Roads and Highways operations are spatial in nature. These include route editing tools like Realign and operations such as measure interpolation to populate M values on polylines. For data in a GCS, the spacing of the vertices along with the tolerance of the feature class can impact the level of accuracy in the calculations. Once the distance between two vertices reaches a specific threshold, the level of accuracy in calculations decreases. In the image below, two polylines in a geographic coordinate system are shown. The polyline in blue has multiple vertices that make up the polyline, while the polyline in purple has only two vertices total. The blue polyline has smaller segments, which results in smaller arcs. Compared to the single large arc on the purple polyline, the calculations on the blue polyline would be more accurate.
Additionally, the x,y and z-tolerance of the feature class contributes to the level of accuracy in Roads and Highways spatial operations. The tolerance is used as a search radius. The larger the tolerance, the larger the search radius when looking for polyline segments or arcs. The image below shows two polylines with two different tolerance values overlaid. Notice the larger tolerance radius in orange not only intersects with the blue polyline but also the purple polyline. As linear referenced data is modeled and maintained, it’s important to keep the density of vertices and the x,y and z-tolerances in mind to ensure measures are accurate.
Suggestions for high-level accuracy
As linear referenced data is maintained in a geographic coordinate system with Roads and Highways, there are a few considerations to be aware of that can ensure a high level of accuracy is maintained in calculations.
Densification
The distance between vertices on the route polyline in a GCS impact the level of accuracy of the LRS calculations. To ensure the most accurate LRS calculations on your data, determine the maximum distance between two vertices that correspond to the tolerances configured for your data. You can use the Identify Sparse Vertices tool.
If there are routes returned by the Identify Sparse Vertices tool with vertices that are spaced larger than the maximum distance calculated, it is recommended that you use the Densify tool to densify those routes to the maximum distance to ensure accurate LRS calculations.
x,y and z-tolerance
When modeling the feature classes that participate in the LRS, whether in advance using the Create LRS From Existing Dataset tool or using the Create LRS tool, set the tolerance and resolution values for your data to values that align with the way your data is collected.
Caution:
Once the feature classes are created, the x,y and z-tolerances cannot be changed during the configuration process.
The default tolerances for spatial references tend to be small, and the targeted level of accuracy for calculations at organizations may be larger than those default values. For example, if 10 centimeters accuracy is targeted, a 10 centimeter x,y and z-tolerance would be an acceptable value to configure. The smaller the tolerance is set for LRS data, the smaller the acceptable distance is between vertices.
Projected coordinate system
Another option is to project your data from a geographic coordinate system to a projected coordinate system. Projected data doesn’t have the same considerations related to vertex spacing and tolerance due to all calculations being completed on straight line segments. If this is an option for your data as you configure your LRS, consider using another projected coordinate system by using the Project tool.