Home › Blog › GPS & Tangents

GPS Watch Pacing Mistakes: Bridges, Tall Buildings, Tangents (and How to Run Them)

Updated: · Reading time: ~12–16 min

Your marathon watch has one job: help you pace. But GPS pace is often wrong at the exact moments you care most: bridges, tunnels, tall buildings, and tight turns. This guide shows you (1) the most common GPS pacing mistakes, (2) the easiest setup to avoid them, and (3) how to run tangents so you don’t accidentally run “42.6 km” and wonder what happened.

Why GPS pace lies (and why it’s worst in races)

GPS watches estimate pace from position changes over time. That’s fine when the signal is clean and you run a straight line. But racing adds problems: you weave, you run close to buildings, you run under bridges, and GPS sampling/smoothing can lag behind your true speed.

The 10 GPS pacing mistakes that blow up marathons

1) Using instantaneous pace as your primary guide

Instant pace jumps when the signal jitters—then you surge to “fix it”—then it jumps again. That’s how runners accidentally turn a steady marathon into a series of micro-intervals.

Fix: Use lap pace (1 km) or average pace, plus a pace band.

2) Starting before GPS has a solid lock

If you start while your watch is still figuring out satellites, your early track can be messy, which impacts pace and distance.

Fix: Start GPS early. Wait for a solid lock. Don’t start recording in a tunnel or under a roof.

3) Urban canyon pacing (tall buildings)

Tall buildings can block and reflect signals, causing “urban canyon” problems and multipath errors. Your track may zig-zag on the map while you ran straight.

Fix: Don’t chase pace in cities. Use lap/average pace + RPE until you’re out of the canyon.

4) Bridges, tunnels, underpasses

Under/through obstructions, GPS may temporarily lose signal or mis-estimate position, producing random pace spikes.

Fix: Hold effort for 30–90 seconds, ignore spikes, and rely on the next km marker + lap pace.

5) Trees and heavy cover

Even outside cities, tree cover can degrade signal quality. Your “instant pace” becomes a mood swing.

Fix: Same solution: lap pace + RPE + pace band.

6) Auto-pause (or too-aggressive auto-stop)

Auto-pause can distort your pacing and splits if it triggers incorrectly in crowds or on tight turns.

Fix: Turn it off for races.

7) Chasing people (weave → surge → brake)

This is not just a pacing mistake—it’s a distance mistake. You run extra metres and spend extra energy.

Fix: Pick a clean line, accept small delays, and move through gaps smoothly—never with a surge.

8) Trusting watch distance over the course

On a certified course, the official distance is measured along the shortest possible route runners are allowed to take. Your GPS distance is your personal path, plus GPS error.

Fix: Pace off official km markers and your pace band. Use watch distance as “approximate.”

9) Running wide on every corner

Every wide turn adds distance. Do it 30–60 times and you’ve quietly changed the event.

Fix: Learn tangents (below). It’s free speed.

10) Trying to “make up” GPS error with effort

If GPS says you’re slow (but you feel fine), surging to “correct” it is the classic trap.

Fix: Trust effort and lap pace. GPS usually self-corrects after the obstruction.

The simplest watch setup that works

Suggested settings:

  • Auto-lap: 1 km (or manual lap at km markers if GPS is chaotic)
  • Turn off auto-pause
  • If your watch supports it: use the best GNSS mode you can afford battery-wise (especially for cities)
  • Display pace as lap pace / smoothed pace (avoid raw instant pace)

Backup plan: carry a pace band or memorize 5K/10K/HM checkpoints. When GPS lies, your band doesn’t.

Bridges, tunnels, tall buildings: what to do in the moment

When GPS breaks, your job is to avoid doing something stupid for 60 seconds.

Scenario What you’ll see What to do What NOT to do
Tall buildings Instant pace spikes, weird track Hold effort; use lap pace + RPE Surge repeatedly to “fix” pace
Bridge/underpass Sudden slow/fast pace, distance jump Ignore for 30–90s; reset at next marker Brake hard or sprint to correct
Tunnel Pace drops or freezes Run by feel; lap at marker after exit Panic-slow because the watch “says” slow
Tight turns/crowds Stop-start splits Run clean line; accept small delays Weave and surge (costs energy + distance)

Why your marathon “ran long” on GPS

If you ran a certified marathon and your watch says 42.5–43.0 km, you’re not alone. Here are the main reasons:

1) Certified courses are measured on the shortest possible route

Road race certification standards define measurement along the shortest possible route an athlete could take within the permitted road area. Your actual route (weaving, running wide, passing) is usually longer.

2) The measurement line isn’t “the middle of the road”

Certification procedures measure close to the inside of turns (within a defined margin), and they include a small “short course prevention factor” so the course is not accidentally short.

3) Your GPS track isn’t a measuring wheel

GPS error increases in tall buildings, under bridges, and under cover. Even small errors can add up over 42 km.

How to run tangents (the shortest legal line)

A tangent is the straightest line connecting one turn to the next. If you run the outside of every corner, you run extra distance. If you weave through crowds, you add even more. Tangents are “free speed” because you run less distance for the same effort.

The basic technique: outside → apex → outside

  1. Look ahead (20–40 m) and identify the inside point of the corner (the apex).
  2. Approach from the outside of your permitted lane/road space.
  3. Clip the apex (closest legal line to the inside).
  4. Exit back to the outside so your next segment is as straight as possible.

How to handle “S turns” and gentle bends

  • On a gentle bend: avoid drifting wide; aim to “straighten” the curve within the legal space.
  • On S turns: focus on the second apex—don’t cut the first corner so hard that you’re forced wide on the second.
  • On crowded turns: prioritize safety and rhythm over the perfect apex. A smooth line beats a sharp stop-start.

Common tangent mistakes

  • Over-correcting: zig-zagging to chase the apex (you add distance).
  • Late apex: turning too late forces you wide on the exit.
  • Fighting crowds: surging to “hit” the tangent costs more energy than it saves.

How to practice tangents without overthinking

You don’t need a perfect course map. You need a habit: “smooth, straight, calm.”

3 simple drills

  • Easy run tangent reps: pick 5–10 corners on an easy run and practice outside–apex–outside with no surges.
  • Roundabout discipline: practice holding a consistent effort around a curve without drifting wide.
  • Group run line choice: in a group, practice staying smooth and avoiding weaving—this is where most extra distance comes from.

Race-day checklist (copy/paste)

  • ✅ GPS locked before the start
  • ✅ Auto-pause OFF
  • ✅ Primary pacing field = lap pace (1 km)
  • ✅ Backup pacing = pace band + course km markers
  • ✅ First 10K: no surges to “fix” GPS
  • ✅ Turns: outside–apex–outside (only where legal/safe)
  • ✅ Crowds: smooth line beats weaving

FAQ

Why does my GPS watch say the marathon was longer than 42.195 km?

Certified courses are measured on the shortest possible route runners are allowed to take; real runners often run wider, weave, and pass. GPS also has errors that increase under buildings/bridges/trees. The combination commonly produces “long” GPS distances.

Should I use instantaneous pace during a marathon?

Usually no. It’s noisy and encourages surging. Use lap pace (1 km), average pace, and a pace band.

How do I run tangents correctly?

Look ahead, run outside–apex–outside, and connect turns with the straightest line within the permitted course. Avoid weaving, and don’t cut where cones/barriers restrict you.

What should I do when GPS goes crazy under bridges or in cities?

Hold effort, ignore short spikes, and re-anchor at the next official km marker. Don’t chase the watch.

Do tangents matter if I’m not trying to win?

Yes—extra distance means either a slower time or a higher effort for the same time. Tangents are free speed if done smoothly and safely.

Related tools & guides

References

  1. Garmin Support. “Top FAQs About GPS Distance, Speed, and Pace Accuracy” (urban canyon / tall buildings). Garmin
  2. World Athletics / AIMS. “The Measurement of Road Race Courses” and Course Measurement Book (shortest possible route standard). PDF
  3. NYRR Help Center. “Course Measurement and Certification” (shortest distance vs runner path). NYRR
  4. DC Rainmaker. “Racing the line – understanding how courses are measured” (inside line + short-course prevention factor context). DC Rainmaker
  5. DC Rainmaker. GPS watch pace smoothing discussion (why pace is smoothed / displayed in increments). DC Rainmaker
  6. Running USA. Course measurement tips (measurer rides shortest possible route available to runners). Running USA

Educational content only. Race safely and follow course instructions at all times.