In the 2020 TRB Annual Conference, Patrick Whiteford of ADOT and Shawn Blaesing of IaDOT led the discussion on the proverbial LRS question – “Measure or XY” – by presenting opposing arguments in the ABJ60 business meeting. The debate led to a coin theory – Measure and XY were the two sides of the LRS coin.
The LRS (Location Referencing System) coin, however, is unusual in that it used to have only one side! Long before the geodetic concept was conceived, our ancestors communicated locations in the form of “along a path near a known feature (tree or rock)”, the quintessential linear location referencing format. Human brains still haven’t evolved to relate a pair of X and Y values to a real-world location unaided.
We may also imagine an LRM (Linear Referencing Method) coin with two sides: Route Measure vs Route Reference w/ Offset. The former tracks the measure from the beginning of a route, while the latter references a nearby, visible, and stable landmark with a short offset distance.
Figure 1. Route Measure and Route Reference Illustration
On the surface, both sides of the coin describe distances from respective known points (Figure 1), however, they offer very different human experiences in location awareness. Route Reference allows users to visualize events without a map or diagram. Route Measure, on the other hand, requires our brains to make math computations before pictures of locations can emerge. Modern LRS implementations have exacerbated these challenges, because the main purpose of route measures is to maintain LRS’ structural integrity and to dyn-seg geometry in lieu of representing meaningful distance metrics.
Let us once again look back into history, this time at no-so-distant past, before GIS/LRS came into existence. Back then, transportation agencies in the USA managed their highway assets or events in tabular format using Route Reference LRMs especially on off-state systems without mileposts surveyed and planted. Arizona DOT’s ALISS was a great success story using a Route Reference LRM to manage statewide crash data developed in the 1980’s.
Which side of the LRM coin would you choose, “On SR001 at measure 13,200” or “On SR001 0.3 miles E of D St”?
The choice is profoundly consequential from a technical standpoint as well. Stay tuned for my next blog post: Stabilizing the LRS Castle – The Shifting Sands Ignored.