The phone already carries the sensors. The Tools screen exposes them as three instruments, each feeding a specific input the calculator needs and cannot infer.
The clinometer
Reads the phone's tilt as an angle above or below horizontal. That angle is what the solver needs for an uphill or downhill shot, and eye estimates of slope are consistently poor — people routinely call 15 degrees as thirty.
- Rest the phone along the rifleOn the scope tube, the rail or the stock — anything parallel to the bore. You are measuring the rifle's angle, not the hillside's.
- Aim at the target and read the angleThe display shows the angle and whether it is up or down.
- Tare if your rest is not levelThe tool can zero itself against the current position, so a rail that is not quite flat does not bias every reading.
- Enter the angle and the true slope distanceNever an angle-corrected distance together with the angle — that applies the correction twice and you will shoot low.
The compass
Gives the azimuth you are shooting on, which the solver uses for the vertical component of the Coriolis correction. Shooting east makes the bullet land slightly high, west slightly low, and north or south neither.
The reading is magnetic, not true. For the purpose it serves — a correction measured in centimetres at long range — magnetic north is close enough everywhere the declination is not extreme, and precision to a few degrees is ample.
Because a fixed range has a fixed firing direction, this is normally a one-time reading rather than a per-shot input. The horizontal Coriolis component needs only your latitude and does not need the compass at all.
The map tool
Pick two points on a map and it returns the distance and the bearing between them. Two situations make this genuinely useful rather than a curiosity:
- Planning before you are there. Working out the distances across a valley from a known stand, the evening before, at a kitchen table.
- Targets you cannot laser. A gap in trees, a point beyond a rise, or a distance across water where the laser returns nothing.
The distance goes into the calculator the same way a lasered or reticle-ranged distance does. Map distance is horizontal by definition, so if there is significant elevation change between the two points you still need the angle from the clinometer.
Sensor limitations worth knowing
| Tool | Limitation | Practical effect |
|---|---|---|
| Clinometer | Needs the phone parallel to the bore | A phone held loosely in the hand reads the hand, not the rifle |
| Compass | Disturbed by metal and magnets | A rifle, a vehicle or a magnetic phone mount will shift it |
| Compass | Needs occasional figure-of-eight calibration | Android prompts for this; do it away from metal |
| Map | Gives horizontal distance only | Add the angle separately on sloped ground |
| All three | Not present on every device | The app says so rather than showing a fabricated reading |
The compass caveat is the one that bites. Taking an azimuth with the phone resting on a steel receiver, next to a vehicle, or in a magnetic case can be off by a large margin. Step away from metal, take the reading, then go back to the rifle.
How much these actually matter
- Shot angle — significant past 15 degrees, and the biggest of the three by a wide margin.
- Map distance — as important as any distance, because distance drives everything.
- Azimuth — a small correction that only earns attention past about 800 m. Worth setting once and forgetting.
Frequently asked
How accurate is a phone clinometer? +
Within a degree or so when the phone is resting solidly on something parallel to the bore. That is comfortably better than eye estimation and good enough for the correction, since the effect depends on the cosine of the angle.
Do I need true north or magnetic north for azimuth? +
Magnetic is fine. The vertical Coriolis component is small enough that a few degrees of declination does not change the answer meaningfully.
My phone says the compass is not supported. Is the app broken? +
No — some devices lack a magnetometer. The app reports that honestly rather than showing a reading it cannot take. Enter the azimuth manually if you need it.