Yellow tape arrow on platforms

Every Red Line platform of late has an "arrow" (really just two 3"-wide pieces of tape mounted at a right angle) pointed a a survey spike with a reference number written alongside. What are these for?

I've noticed them on the Orange and I think even some Green Line platforms in the last few weeks as well.

octr202 wrote:I've noticed them on the Orange and I think even some Green Line platforms in the last few weeks as well.

I noticed them at the rear ends of Downtown Crossing and Tufts Medical Center yesterday. Could be a stopping point when running wrong-rail.

So far they appear to be at both ends of the platform.

I have seen them on both the Orange & Green Lines. I noticed on some of them a small surveying marker at the arrow point, also a number written in yellow.

My guess measuring the property for the real estate agent !

JoeD

Gerry6309 wrote:Could be a stopping point when running wrong-rail.

Saw one at the north end of the southbound platform at State today. It lined up with the cab window at the rear of the train. I'm almost inclined to suspect they're installing a screen for running wrong-iron or for short turning. It would certainly be necessary in some cases with some of the ever-so-gently curved platforms downtown.

I saw one at Central on the red line today, Alewife-bound side.

At the station formerly known as Mechanics!

JoeD

They're systemwide, and not only on platforms.

I saw one at Packard's Corner today as well.

If they are also showing up on Blue Line platforms, that blows my theory that it has to do with measurements for the new cars (Red / Orange and Type 9 Green).

My next-best theory is it has to do with the video surveillance that's been installed at the ends of almost every platform. I have yet to get this confirmed, but as far as I can tell there are cameras at every tunnel entrance and platform end backed by motion-sensing video processing. I believe that's why there is a ID reader PIN-pad with the red LED at every platform end. I'm speculating that if someone enters the tunnel and/or goes beyond the end of the platform the video surveillance would pick them up and alert Transit Police, unless they had signed-in first on that PIN keypad.

If I"m right about that, then the yellow arrow tape markers could be used to calibrate / test the video surveillance.

StefanW wrote:...the yellow arrow tape markers could be used to calibrate / test the video surveillance.

However if the arrows are also on above-ground stations that blows that theory also... unless there are general-purpose security cameras also above ground...?

Is there any video surveillance at Packard's Corner?

A friend who is an Inspector on Rapid Transit confirms the marks are for a GPS survey that was recently done. Once in the tunnel, he says, they had
a limited time for accurate surveying due to loss of GPS signal.

Diverging Route wrote:A friend who is an Inspector on Rapid Transit confirms the marks are for a GPS survey that was recently done. Once in the tunnel, he says, they had
a limited time for accurate surveying due to loss of GPS signal.

The markers are reference points for old-fashioned surveying. GPS becomes unusable for surveying the instant you lose direct line-of-sight to GPS satellites.