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GMapCatcher: Offline Maps for Travelers and Field Workers

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Building a Custom Map Archive with GMapCatcherGMapCatcher is an open-source desktop application that lets you download and store tiled map data from various online map providers for offline use. Building a custom map archive with GMapCatcher lets you create a localized repository of map tiles that you can browse without internet access, speed up repeated tasks, support fieldwork in remote locations, or preserve map snapshots for projects and privacy-sensitive workflows.

This guide walks through planning, preparing, downloading, organizing, and maintaining a custom map archive using GMapCatcher. It includes practical tips for tile selection, storage layout, performance tuning, legal considerations, and alternatives if GMapCatcher doesn’t meet your needs.

What you’ll get from this guide

  • A practical step-by-step workflow to design and build a tile archive with GMapCatcher
  • Advice on choosing map providers, zoom levels, and geographic bounds to optimize storage and usefulness
  • Instructions for using GMapCatcher’s download features, plus tips for batch and automated downloads
  • Storage, organization, backup, and performance optimization strategies
  • Notes on legality, attribution, and ethical use of map data
  • Alternatives and complementary tools where relevant

1. Plan your archive: scope, providers, and zoom levels

Before downloading tiles, define what you need—this minimizes wasted bandwidth and storage.

  • Purpose: navigation, field surveying, historical snapshot, teaching resources, etc.
  • Geographic coverage: single city, region, country, or the entire world. The larger the area, the exponentially more tiles.
  • Zoom levels: choose the zoom range carefully. Each increase in zoom multiplies tile count roughly by four. For example, if you need city-level detail, you might use zoom 12–16; for national coverage, 5–10 might be sufficient.
  • Map styles/providers: GMapCatcher supports several sources (OpenStreetMap, Google Maps variants, Bing, etc.) depending on version and installed plugins. Prefer OSM or providers with clear licensing for archival use.
  • Storage estimate: calculate approximate tile count. Rough rule: number of tiles ≈ area_in_tiles_at_min_zoom × 4^(zoom_range). There are calculators online but doing a small test download of a representative area helps.

2. Install and configure GMapCatcher

GMapCatcher is cross-platform (Linux, Windows, macOS) and typically packaged as source or binary depending on your OS.

  • Install dependencies (Python, PyQt or PySide, SQLite, etc.) as required by the version you use.
  • Launch GMapCatcher and set default map providers and cache directories in Preferences. Choose a location with plenty of free space — external drives or network storage are common for archives.
  • Configure cache size and tile storage behavior. GMapCatcher stores tiles locally in a cache; for a permanent archive you may prefer to move or mirror the cache to a structured archive directory.

3. Selecting areas and tiles to download

GMapCatcher provides tools to pan/zoom and save tiles for offline use.

  • Use the app’s tile downloader: zoom to the area and use “save” or “download” to specify a rectangular area and zoom range.
  • For precise boundaries, mark corners and preview tile count before starting. Cancel and adjust if estimated storage looks too large.
  • Consider creating several project-specific archives rather than one monolithic collection. This simplifies updates and sharing.

Practical tip: Start with a small area and a limited zoom range to measure actual tile sizes and download times, then scale up.

4. Automating and batching downloads

For large archives, manual clicking is inefficient. Use automation where possible.

  • Scripting: Some GMapCatcher builds expose command-line or Python APIs you can script. Check your version’s documentation or source for a downloader module.
  • External tools: If you need more powerful batch downloading, tools such as Mobile Atlas Creator (MOBAC) or dedicated tile downloaders (that support the same tile URL format as your chosen provider) can complement GMapCatcher. Exported tiles can be organized to match GMapCatcher’s cache layout.
  • Throttling and politeness: Respect provider rate limits and terms. Throttle requests, add delays, and avoid simultaneous high-volume downloads.

5. Organizing the archive on disk

A clear structure makes maintenance and usage easier.

  • Tile layout: Tiles are normally stored by provider/zoom/x/y. Keep this structure to allow easy browsing and serving. Example: /archive/OSM/12/2048/1365.png
  • Metadata: Maintain a small index (JSON or SQLite) describing each archive: provider, bounding box, zoom levels, date created, and attribution/licensing details. This is invaluable when revisiting archives later.
  • Versioning: If you plan to update areas over time (for example, periodic snapshots), keep dated folders or use a simple naming convention: city_2025-09-05/.

Example folder tree:

/map-archive/   OpenStreetMap/     london_2025-09-01/       zoom_12-16/         12/...         13/...   Bing/     ...   metadata.sqlite

6. Serving and using your archive

Once tiles are stored, you can use them in multiple ways.

  • Point GMapCatcher’s cache directory to your archive to view offline. Some versions let you add a “local” provider pointing to a directory of tiles.
  • Run a lightweight tile server (e.g., TileServer-GL for vector tiles or simple static HTTP server for raster tiles) to serve tiles to map apps or other devices on your local network.
  • Use mobile apps that support MBTiles or XYZ tiles—convert or package tiles as MBTiles (SQLite-based) for portability with tools like mbutil or tippecanoe (vector) where applicable.

7. Performance and storage optimization

  • Limit zoom range and trim unnecessary peripheral tiles.
  • Compress tiles: PNGs can often be optimized; use pngcrush/optipng. For large archives, consider converting to JPEG where acceptable (satellite imagery) to save space.
  • Use SSDs for faster random access; HDDs are cheaper for cold archives.
  • For very large archives, consider using MBTiles (single SQLite file per area) for easier transfer. MBTiles also works well with many mobile/offline map clients.
  • Licensing: Always check the map provider’s terms of service. OpenStreetMap data is ODbL-licensed and allows local copies with attribution; many commercial providers forbid bulk downloading.
  • Attribution: Keep provider attribution metadata alongside your archive. Many providers require visible credit when tiles are used.
  • Rate limits and fair use: Don’t overload servers. Use caching responsibly and prefer providers that allow bulk exports or provide offline products or APIs.

9. Backups, updates, and maintenance

  • Backups: Treat your archive like any important data—use versioned backups, RAID or cloud backup for critical archives.
  • Updates: Decide on update cadence (daily, monthly, yearly). For incremental updates, re-download only the changed zoom levels or tiles within bounding boxes.
  • Integrity checks: Periodically validate tile files and metadata; store checksums if needed.

10. Alternatives & complementary tools

  • Mobile Atlas Creator (MOBAC) — powerful tile selection and export to various formats (MBTiles, RMaps, etc.).
  • MBTiles — pack tiles into a single SQLite database for portability. Useful with many offline mobile clients.
  • Tile servers (TileServer GL, MapServer, TileStache) — for serving archives on local networks or intranets.
  • QGIS — for advanced GIS analysis and converting/exporting map snapshots.

Example workflow (concise)

  1. Plan area and zooms (e.g., a 50 km×50 km area, zoom 12–16).
  2. Install GMapCatcher and set cache to /map-archive/OpenStreetMap/london_2025-09-01.
  3. Use the downloader to save the bounding box; monitor estimated tile count.
  4. Post-process: optimize PNGs, generate metadata.json with provider and bbox.
  5. Serve via simple HTTP server or use MBTiles for mobile distribution.

Building a custom map archive with GMapCatcher is a practical, flexible way to ensure reliable offline access to maps. With careful planning around scope, storage, provider licensing, and maintenance, you can create a durable, shareable archive tailored to your needs.

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