karti

A pedestrian navigator for Android that tracks your position without GPS, by dead reckoning: every step moves the marker a fixed stride length along the current compass heading. The network is only needed for map tiles and address search — tracking itself works in airplane mode.

Quick start

Build and install:

git clone ssh://forgejo@git.desu.church:61488/nero/karti.git && cd karti
nix develop
gradle assembleDebug
adb install app/build/outputs/apk/debug/app-debug.apk

Use:

1. Where am I — tap ✎ then tap the map, or tap the GPS button: a one-shot fix, progress in the status line, the map jumps straight to the point once it lands. Gives up after 45 s if nothing is caught.
2. Where to — type an address into search, or tap ⌖ then tap the map.
3. Check your stride length (the "Шаг" button, 0.7 m by default): enter your height and the stride is estimated as 0.414 × height, or use GPS-калибровка — walk ≥50 m steadily with GPS on, press finish, and the stride is measured for real (steps counted by the sensor, distance summed from GPS with jitter filtering; needs ≥30 steps and ≥30 m to apply).
4. Hit start walking and go: the marker advances step by step, drawing a trail and a line to the destination.
5. Position drifted? Re-anchor it (✎ or GPS) — accumulated drift resets.

Zoom is pinch, or one-handed: double-tap and keep the finger down on the second tap, then drag up to zoom in, down to zoom out (120 dp per level, anchored at the tapped point). A double tap without dragging zooms in a level as usual.

Map layer is switched with the ▦ button: OSM Mapnik, OpenTopoMap, Яндекс Карты/Спутник, Google Карты/Спутник/Гибрид, USGS Topo/Satellite, or your own tile server — an XYZ URL with {z}/{x}/{y} placeholders.

Paths can be recorded or drawn, then edited and exported:

• Record: press ⏺ Записать, walk, then 💾 Сохранить — the recording becomes an editable path and a compact ASCII string of it lands in the clipboard, safe to paste into any chat (a 500 m city walk is ~38 characters; ⏺ again discards the recording).
• Draw: the pencil-route button toggles draw mode — sketch the road with a finger; the stroke is recognized into lines and arcs, so you see clean geometry, not hand wobble. While a path exists, a stroke that starts and ends on it redraws that section; otherwise the stroke replaces the path.
• Edit: path vertices are draggable (long-press a vertex marker, then move). The ✂ button keeps only a section: tap its first and last vertex. Long-press 💾 Сохранить to delete the path.

Offline maps — the layers dialog (▦) has two entries that remove the tile-server dependency entirely:

• Скачать область в офлайн-архив — downloads the visible area (current zoom +4 levels, capped at z16) from the active online source into a standalone sqlite archive in app storage, then switches the map to it.
• Скачать город заранее — type a city or district name; its bounding box comes from Nominatim, and a dialog offers z14/z15/z16 depth with a tile-count and size estimate before downloading the whole city into an archive.
• Офлайн-архивы — pick a saved archive or import your own MBTiles/sqlite/zip/gemf file. In archive mode the app makes no network requests at all; tiles come only from the local file (zooming past the archive's levels shows blank tiles).

Requires Android 8.0+ (minSdk 26). A hardware step detector and compass are recommended; without a step detector, steps are counted from the accelerometer.

How it works

• Steps: TYPE_STEP_DETECTOR, falling back to an accelerometer peak detector (EMA baseline, hysteresis, ≥300 ms between steps).

• Heading: TYPE_ROTATION_VECTOR, EMA-smoothed on unit-vector components (no jump across 0°/360°). Holding the phone upright remaps the axes. Magnetic declination comes from GeomagneticField at the current position.

• Position: DeadReckoning.kt — equirectangular approximation; its projection error is negligible at walking distances. The real error sources are the compass and stride length, hence re-anchoring.

• Map/search: osmdroid and Nominatim. The tile provider is selectable (built-ins plus an arbitrary XYZ server), with a separate tile cache per source.

• Yandex tiles: Yandex renders in EPSG:3395 (elliptical Mercator) while the XYZ grid is EPSG:3857 (spherical) — the same tile index is ~20 km off in latitude at 55°N. TileSources.kt therefore fetches the one or two Yandex tiles a requested tile overlaps and resamples them row by row (longitude matches, only latitude shifts); the result is cached and archived like any other tile.

• Path format: PathCodec.kt, three layers. Geometry: points quantized to 1e-5° (~1.1 m), simplified with Douglas-Peucker, consecutive segments greedily merged into circular arcs (per-segment bulge = tan θ/4, 0 = straight). Binary: the first byte picks whichever representation is smaller — cartesian varint deltas (raw or deflated) or a polar bitstream (absolute start in 51 bits, then per segment: turn as signed Elias gamma on a 9-bit heading grid, length in meters as gamma, bulge; quantization is closed-loop, so error never accumulates). Text: Z85 base85 — printable ASCII with no space/quotes/backslash, so it pastes anywhere. Lossy within ~2 m, below dead-reckoning noise; 800 steps encode to 15 chars (straight), 38 (city walk), 90 (constant wobble); the floor is ~13 chars, almost all of it the absolute start point.

Last position, stride length, and the chosen map layer persist in SharedPreferences.

Code

File	Contents
app/src/main/java/dev/nerosama/karti/DeadReckoning.kt	pure dead-reckoning math
app/src/main/java/dev/nerosama/karti/PathCodec.kt	path fitting (lines/arcs) + encoding
app/src/main/java/dev/nerosama/karti/TileSources.kt	Google/Yandex tile sources + Yandex reprojection
app/src/main/java/dev/nerosama/karti/MainActivity.kt	UI, sensors, geocoding, GPS fix, path editor
flake.nix	dev environment: JDK 17, gradle, Android SDK 34