First off, by thermometer measurements, 50 sty is ~ 1.0 deg C cooler than ground level @ Cantonment or Duxton plain park. I measured several times before. This i can share with you...
26 sty is also warmer by heat index, due to the concrete and less "airy".
50th sty in terms of distance got chance that it is quite accurate per lap for my GT runner, even with the overhead concrete blockage at 1C. My GT runner usually registers over 800m (810 - 815m usually to be more exact, but of coz nothing is absolutely accurate).....so 400m is half that lap, and 200m is like half that like 1G to 1D's turnstile.
The 26th sty confirm cannot rely on GPS, everything is screwed up. But the track they claim is 800m.
I tried google map measurement of distance and it shd be pretty close to 800m though a bit longer .
So yeah if really want to have more absolute accuracy for TT purposes, gotta go track.
However, even on a track, 3k to 5k distances, 10s to even 20s timing range it is also not gonna be very indicative of whether fitness dropped or improved, coz dependent on so many other variables which are probably even more affecting, things like rest/taper, heat index etc. Much less 10k - HM distances.
But of coz it is something that you can, on paper, say that you have set a PB for that timing.
Not sure what is Google map's accuracy and my manual plotting accuracy, and i dont have a wheel distance measurement device. Unless i go ultra kee siao and use my 7m tape measure go and measure, that means i'd be dragging the metal tape measure over the whole lap for 114 times (800/7).
Actually to be frank, with some elbow grease and 18-20 mins duration, it is actually doable if you really want an exact answer to the distance there.

But seriously speaking also, that four 90 deg turns at 1A and 1G also would impact the timing of paces obelow 4:30