Groundspeed — how fast, which way, how high

01 — Where the number comes from

Anyone can
show you a
wrong number

Most speedometer apps hand you CLLocation.speed straight off the receiver and call it a day. On a train that is a number which twitches between 78 and 91 while nothing whatsoever changes. It is not wrong, exactly. It is just unusable.

Groundspeed takes the Doppler speed when the fix is good, falls back to differentiating position over time when it isn't, throws away anything that implies you moved four hundred metres a second, and then smooths what survives with an exponential average weighted by the accuracy of the fix that produced it. A sloppy fix moves the needle less than a sharp one.

Below a per-mode noise floor it reads a hard zero, because a parked car doing 0.3 mph is a lie. Pull away and it snaps to the real number immediately rather than fading up to it — smoothing should hide jitter, not hide the moment you started moving.

02 — Six modes, six temperaments

A train is not
a bicycle

The same receiver noise, the same ninety seconds.
Dashed is what the GPS said. Solid is what you see.
Pick a mode — these are the app's real constants.

Raw fix Groundspeed

TRAIN · α 0.12

Smoothing: 0.12
Reads zero below: 1.3mph
Auto-pause after: 60s
Climb counts above: 16ft
Default unit: mph

03 — Everything it does

No settings
you'll never find

The number, enormousReadable at arm's length on a moving train, in daylight, with one hand on a rail. mph, km/h, knots and m/s, switched with one tap.
Course, then compassAbove walking pace it shows your course over ground — where you are actually going. Stand still and it falls back to the magnetometer. True or magnetic north, your call.
Barometric altitudeGPS height is fused with the pressure sensor, so a lift, a climb or a pass reads smoothly instead of stepping forty feet at a time. No vertical accuracy, no number — it won't invent one.
Breadcrumb mapYour trail behind you, heading-up or north-up, on a map that keeps working when the signal doesn't.
Trips that pause themselvesStopped at a signal for a minute is not the end of the journey. Each mode waits a different length of time before deciding — ten seconds walking, two minutes flying.
Records with the screen offLocked in a pocket, in a bag, on a bike mount. Distance, moving time, max speed, elevation gain and loss, all of it still accruing.
Signal honestyA quality pill that tells you when the fix is poor, rather than quietly showing you a confident number built from nothing.
History that staysEvery trip, on a map, with its stats. Rename it, annotate it, or delete it and it's gone — no tombstone on a server somewhere.
GPX and CSV outFull-fidelity GPX 1.1 with speed and course extensions, or flat CSV. Your data leaves in a format other software already reads.
Built for the eyes you haveDynamic Type, VoiceOver labels on every readout, colourblind-safe, dark by default. Precision instrument, not a toy.

Nothing you record
leaves the phone. Nothing.

No account to make. No server to sync with. No analytics SDK, no crash reporter, no advertising identifier, no third-party code of any kind. The only network call Groundspeed can make is the one that draws the map.

On-device by default · Export or delete, any time

The Groundspeed dashboard on an iPhone: 76 mph in a compass ring reading 355 degrees, train mode, a signal pill showing plus or minus 5 metres, and a live map with the route trail behind. — Train mode · 76 mph · ±5 m · Westchester, NY

No ads.
No account.
No subscription
for the whole app.

That is the entire business model. Buy it once, or don't.

04 — When you can have it

Not yet

It is built and running — on a phone, on real track, at real speed, with the screen off and the trip still recording.

What is left is the part nobody photographs: review, the store listing, and the hundred small courtesies an app owes a stranger's phone. Groundspeed is not on the App Store yet.

When it is, this page changes and there will be a link here instead of a promise.

Tell me when it ships

Coming soonto the App Store

No mailing list, no tracking, no drip campaign — an email to a person, who will send you exactly one back on the day it goes live. Will need iOS 18 or later, on iPhone.
Barometric altitude needs a phone with a pressure sensor — iPhone 6 and later have one.
Background recording asks for Always location, once, and shows the indicator the whole time it uses it.

Anyone canshow you awrong number

A train is nota bicycle

No settingsyou'll never find

Not yet

Anyone can
show you a
wrong number

A train is not
a bicycle

No settings
you'll never find