Building an offline-first travel app in .NET MAUI (on-device OCR, currency & maps, no backend)

A build note from Horizon Software, a one-person Android studio. , which sits on top of the platform's built-in recognizer (ML Kit on Android). That hands you raw text and bounding boxes, which is only half the problem. Turning a messy wall of receipt text into amount / date / merchant is a custom parser: pick out money-shaped tokens, prefer the ones sitting near words like "total", validate against date patterns, and fail gracefully when a crumpled receipt returns nonsense. That heuristics layer is where the real work went — and it never has to phone home.

3. An offline map with no map SDK

"Show my trip on a map" almost always pulls in a mapping SDK streaming tiles over the network — the one thing I don't have. So the map is drawn by hand, and it's simpler than it sounds. There's a single bundled equirectangular world image (a Creative-Commons one from Wikimedia), a tiny plate carrée projection that turns any latitude/longitude into a 0..1 coordinate on that image, and SkiaSharp to drop a pin for each leg and connect the route:

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// The entire projection. Lat/lon in, 0..1 canvas coordinates out.

public static (double X, double Y) Project(double lat, double lon)

{

    var x = (lon + 180.0) / 360.0;

    var y = (90.0 -  lat) / 180.0;

    return (Math.Clamp(x, 0, 1), Math.Clamp(y, 0, 1));

}

One image, a few lines of maths, a canvas. No SDK, no tiles, no API key, no network — and because it's all SkiaSharp it renders straight to a PNG you can share. The pins come from a small bundled table of country centroids, so even the "where is this place" lookup is offline.

Auto-budgets from a bundled dataset

The same offline-first thinking runs through the budgeting. A bundled JSON dataset of ~139 destinations — each with typical daily costs at three travel styles — lets the app suggest a per-leg budget the moment you add a stop, no lookup required. (It also quietly powers a little if you want to see it in action, and there's a — a small studio making simple, offline-first Android apps.

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