Probe data may be collected by various data collection devices including global positioning systems (GPS) systems and other navigation systems. Regardless of the device that collects the probe data, the probe data may include a plurality of probe points. Each probe point may identify the location, such as in terms of latitude and longitude, at which the device that collected the probe data was located at the time at which the probe point was captured. A probe point may also include or otherwise be associated with a z-axis component indicative of the altitude or height of the device at the time at which the probe point was captured. However, information relating to the altitude associated with the probe point generally has a much greater tolerance and a much larger error associated therewith than the latitude and longitude associated with the probe point. For example, the altitude associated with a probe point may have an error of 10, 20, 30, 40, 50 or more meters. Additionally, not all probe points have an associated altitude with some probe points defining a location merely in terms of latitude and longitude.
During the creation of maps that identify road segments, the error associated with the altitude of the probe point has made it particularly difficult to determine if the probe points are associated with a road that passes over another road or a road that passes under another road. In this regard, roads sometimes cross one another or are otherwise stacked in such a manner that one road passes beneath another road. For example, an interchange of an interstate may include an underpass and an overpass with the road that forms the underpass passing beneath the road that forms the overpass. By way of another example, roads may be stacked with one road on top of another even in instances in which the roads do not cross. In these instances, the error associated with the altitude of the probe points may be sufficiently large that the probe points may not be reliably associated with either the upper or the lower road with a sufficient degree of confidence in the map creation or refinement process. In this regard, the error associated with the altitude of the probe points may be so large as to equal or exceed the anticipated vertical spacing between the roads such that the probe points may not be matched to the road that forms the overpass or the road that forms the underpass with the desired degree of confidence.