geodatabases is to create relatively short segments, such as may be defined by address blocks. In order to assure atomicity in segment attributes, you may want to consider breaking such block centerline features wherever there is an obvious change in an attribute at a midblock location. For example, if you will store segment width as an attribute for such applications as determining lane miles, then you will need to break segments wherever width changes. If there is a transition section, then make it part of the narrower segment and start the wider segment where the new full width begins. Cul-de-sac width should not affect lane mile calculations, so do not create a special cul-de-sac feature class unless you need to include culde-sac diameter as a network parameter to establish the cost of U-turns. (See chapter 13.)
You may also want to consider going with separate centerlines when faced with a divided road with different primary characteristics on each side of the road. An alternative would be to store attributes by side of the road.
At most locations, the logical centerline will closely approximate the location of the facility. The most common exception, save for the issue of roadway splits discussed earlier, is at limited-access highway interchanges. Most state DOTs use a form of linear location referencing, such as route-milelog. They also commonly place the point of intersection between a ramp and the main highway at the physical gore, as this is the location where the two pieces of pavement become one. For the ramp and mainline centerlines to intersect at this location, the ramp centerline needs to turn toward the main centerline and intersect at the appropriate milelog location at a right angle. The resulting appearance may look strange, but it ensures that the intersection will occur at the proper location from the perspective of the limited-access highway’s linear measures. If you are also providing carriageways, they can continue along the path of the ramp as it merges with the highway pavement.
Figure 4.6 Logical centerlines Logical centerlines need to intersect at the location that makes sense from the perspective of connectivity and the linear datum. In this example, the agency rule says that the point of intersection, for which the route measure is defined, is the end of the physical gore; i.e., where the pavement of the ramp merges with that of the mainline. A ramp carriageway need not be constrained by that definition, since the purpose of a carriageway is to provide a good map appearance at a large scale.
Intersections
An intersection is normally visualized as the location where two roads cross at grade. You may also want to look at using a number of intersection subtypes as a way of managing behaviors and symbology. For example, an interchange could be either a way to manage a collection of roads and at-grade intersections or just a big intersection with internal turns for pathfinding.
A railroad grade crossing is another intersection subtype, one between a railroad and a road. You may want to restrict the classification of this kind of intersection to those places where a track crosses a road through topology rules. Such a function can be easier to implement if you create a railroad grade crossing subtype for intersections.
Figure 4.7 Intersection subtypes Three of the four basic intersection subtypes are shown here. An at-grade intersection is the typical place where two or more routes meet. A railroad grade crossing is where a route crosses a railroad. If there are multiple tracks, you may want to use a multipart point feature class. The third subtype shown here is the access point, which represents a location on a route where a nonmapped facility connects to a mapped facility. Access points include shopping center driveways, recreational trail crossings, and anywhere you have intersection-like elements with characteristics to store, such as traffic signals and crashes.
There are also places that look like intersections where only one road seems to exist. These are access point intersections, and they occur where trails cross a road or a major driveway provides access to the road system. Like at-grade intersections, you will likely want to retain information about these locations, such as traffic signals and traffic crashes.
Using intersection subtypes will provide you with the ability to check cardinality to ensure that it meets your expectations. For example, you will need at least two intersecting roads for an at-grade intersection, one road and at least one railroad track for a railroad grade crossing, and only one road for an access point. An interchange will need to include at least one road classified as a limited-access highway.
Realignment
The way you segment centerlines will affect future editing workloads. One of the more common issues you will deal with is facility realignments. A typical approach to building a geodatabase for road features is to identify the entities to be included by name and route number. You may additionally subdivide these entities at county lines or district boundaries to reduce the extent of changes or to coincide with your data maintenance organizational structure. This is all fine and good, but once the features are created, you need to break the symmetry of entities and features and treat name or route number as just another attribute. If you try to preserve the symmetry, you will have a lot of extra work when a road is realigned, particularly if you are using a linear location referencing method. The problem is that realignment causes the total length of the facility to change, resulting in a ripple effect through all the downstream locations. You either have to revise all their measure values or create some sort of reconciliation structure in the measure values to deal with the missing or duplicative values within the area of realignment. You also may lose the ability to properly map event data applicable in datasets tied to old measure values.
A better choice is to treat the new section as a new facility. Add a facility status field to the database and give it one of three values: active, retired, and replaced. (See chapter 6.) You would leave the old alignment in the dataset; just list its status as retired. If you need to display historical data on the former alignment, the feature and its linear measures are still in place to do the job. You also do not need to change any downstream data. All the data about the new section is also new, so there is no lineage issue for the facility identifier and linear measures used by it. If you want to see the present state of the system, just select all the active facilities. You can select by date and status to produce various time-stamped views of the geodatabase.
Figure 4.8 A strategy for realignments One of the traditionally difficult aspects of a transportation dataset based on a route-milelog linear referencing method (LRM) is facility realignments that change the length of the facility at a midpoint. Such a change, if accomplished through traditional methods, will reshape the affected geometry and recalculate the measure for downstream events. A better way that avoids modifying all the event records is to make a new feature for the realigned portion and abandon the replaced segment. A status attribute identifies active and retired route segments. This approach has the benefit of retaining the original geometry and measures for time-based analyses.
Segmentation methods
Several references have been made to the traditional transportation dataset structure of short segments with repeating attributes. This is a perfectly valid way to go if you have a manageable number of segments and relatively few changes to the system other than added facilities, which is the case for most local governments.
