Position accuracy and fix age

Make state, control, location, and consent accessible and recoverable: compare position uncertainty with fix age.

UX Uma: Keep every control usable when permission or a device is unavailable., your practice guide

UX Uma: Keep every control usable when permission or a device is unavailable.
Predict the reading, then compare it with the measurement.

Phone browser (Web APIs)

Phone browser

Make state, control, location, and consent accessible and recoverable: compare position uncertainty with fix age.

Tier 2 · Phone browser · No account

Version tested: Playwright mobile Chromium 147.0.7727.15 on 2026-10-09; 390×844 at device scale 3; browser controlled location or test microphone inputs. Date: 2026-10-09.

Open the lab on your phone over HTTPS, then use its permission button.

Open the phone lab (new tab)

Steps

Screens captured against Phone browser (Web APIs) Playwright mobile Chromium 147.0.7727.15 on 2026-10-09; 390×844 at device scale 3; browser controlled location or test microphone inputs on 2026-10-09; the tool may have moved on — the text steps are the contract.

  1. Step 1

    Do
    In the fix controls panel, tap First fix and inspect accuracy and fix time.
    You will see
    Input: Request first fix. Emulated location in mobile Chromium. Observed: First returned fix 1. Readout: First: fix 1 Latitude 51.48160; longitude -3.17910 Accuracy radius 25 m Fix time 2026-10-09T19:12:49.823Z Fix age 0.1 s Interpretation: The first fix carries an uncertainty radius.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: The first fix carries an uncertainty radius. Next check: In the readout panel, wait two seconds and read the increasing fix age.

  2. Step 2

    Do
    In the readout panel, wait two seconds and read the increasing fix age.
    You will see
    Input: Wait for age to rise. Emulated location in mobile Chromium. Observed: First returned fix 1. Readout: First: fix 1 Latitude 51.48160; longitude -3.17910 Accuracy radius 25 m Fix time 2026-10-09T19:12:49.823Z Fix age 2.5 s Interpretation: Age rises without a new position.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: Age rises without a new position. Next check: In the fix controls panel, request a fresh second fix and compare displacement with accuracy.

  3. Step 3

    Do
    In the fix controls panel, request a fresh second fix and compare displacement with accuracy.
    You will see
    Input: Move emulated location and require fresh fix. Emulated location in mobile Chromium. Observed: Fresh required returned fix 2. Readout: Fresh required: fix 2 Latitude 51.48180; longitude -3.17900 Accuracy radius 30 m Fix time 2026-10-09T19:12:52.639Z Fix age 0.2 s Displacement from first 23.3 m Interpretation: Displacement is comparable to the reported radius.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: Displacement is comparable to the reported radius. Next check: In the fix controls panel, tap Allow cached fix and inspect its timestamp and age.

  4. Step 4

    Do
    In the fix controls panel, tap Allow cached fix and inspect its timestamp and age.
    You will see
    Input: Allow cached fix. Emulated location in mobile Chromium. Observed: Cached allowed returned fix 3. Readout: Cached allowed: fix 3 Latitude 51.48180; longitude -3.17900 Accuracy radius 30 m Fix time 2026-10-09T19:12:52.639Z Fix age 0.3 s Displacement from first 23.3 m Interpretation: A cached request may reuse the previous fix.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: A cached request may reuse the previous fix. Next check: In the fix controls panel, tap Require fresh fix after changing location and compare fix time.

  5. Step 5

    Do
    In the fix controls panel, tap Require fresh fix after changing location and compare fix time.
    You will see
    Input: Move emulated location and require fresh fix. Emulated location in mobile Chromium. Observed: Fresh required returned fix 4. Readout: Fresh required: fix 4 Latitude 51.48200; longitude -3.17900 Accuracy radius 18 m Fix time 2026-10-09T19:12:53.277Z Fix age 0.0 s Displacement from first 45.0 m Interpretation: A fresh request changed the fix time.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: A fresh request changed the fix time. Next check: In the readout panel, wait briefly and compare the last fix age with its unchanged timestamp.

  6. Step 6

    Do
    In the readout panel, wait briefly and compare the last fix age with its unchanged timestamp.
    You will see
    Input: Wait with final fix displayed. Emulated location in mobile Chromium. Observed: Fresh required returned fix 4. Readout: Fresh required: fix 4 Latitude 51.48200; longitude -3.17900 Accuracy radius 18 m Fix time 2026-10-09T19:12:53.277Z Fix age 1.0 s Displacement from first 45.0 m Interpretation: Freshness is time dependent even with static coordinates.
    Why it matters
    Evidence: The real browser API and page produced the quoted state. Boundary: These locations are emulated inputs, not physical phone readings. Reason: Freshness is time dependent even with static coordinates. Next check: This result does not establish a physical route or exact position.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

  1. An app uses a rough zone estimate to guide a visitor. The team wants the same estimate to trigger a door automatically. What should change in the review?

    Return to the chapter’s knowledge check

  2. A home geofence provides a hint to warm the house. How should a door-unlock feature use that same hint?

    Return to the chapter’s knowledge check

Return to 'Location Awareness: Positioning Basics' · Browse Labs