proposal: clarify interpretation of stop_headsign

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Joe Hughes

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Nov 11, 2007, 11:05:00 PM11/11/07

to Google Transit Feed Spec Changes

Summary:
Specify that when stop_headsign is empty for a stop_times entry in a
given trip, the most recent previous stop_headsign value on that trip
is used.

Motivation:
The stop_headsign field in stop_times.txt is useful for specifying
headsigns that change over the course of the trip. However, the
specification is unclear about the meaning of specifying a
stop_headsign value for some but not all stop times in a particular
trip. Example:

trip_headsign: "North Branch, Downtown, South Side"

stop_headsign values:
@ stop_sequence 1: ""
@ stop_sequence 2: "Downtown, South Side"
@ stop_sequence 3: ""
@ stop_sequence 4: ""
@ stop_sequence 5: "South Side"

What is the headsign value when the vehicle is at the location
indicated by stop_sequence 4? It's either "North Branch, Downtown,
South Side" (if we use the value of trip_headsign) or "Downtown, South
Side" (if we use the previous stop_headsign in the trip).

I propose that we adopt the latter interpretation. This allows us to
only specify the new headsign value at the point at which it changes
during the trip, rather than repeating it for every single stop_times
entry. This allows us to avoid a great deal of repeated content in a
feed, since headsign text is relatively long, and stop_times.txt is
generally the longest file in a feed by several orders of magnitude.

Proposal:
The following text will be added to the details of the stop_headsign
field in stop_times.txt:
--------------------------
When stop_headsign is empty for a given stop_times entry, the most
recent non-empty stop_headsign value in the same trip should be used
for the headsign at that point. When no previous stop_times entries
on that trip have a stop_headsign value, the trip_headsign value for
that trip should be used as the headsign.
--------------------------