GeoSnag

Automatically enrich GPS-less camera photos using coordinates from phone photos taken on the same day.

You shoot with a DSLR or mirrorless camera that has no GPS module. Your phone is always in your pocket, geotagging every shot. After a trip, your camera photos have no location data while your phone photos do.

GeoSnag fixes this. Point it at your photo directories, and it matches each GPS-less photo to the nearest geotagged photo by timestamp — then copies the coordinates over. No manual sorting, no separate camera/phone folder setup. It just figures it out.

Built for Synology NAS, but works anywhere Python runs.

How It Works

GeoSnag runs a four-stage pipeline:

1. Scan — reads EXIF metadata from every photo in your configured directories, using multithreaded I/O and an on-disk index cache so repeat runs are fast.
2. Classify — photos that already carry GPS coordinates become sources. Photos without GPS become targets. This happens automatically based on metadata — no folder-based configuration needed.
3. Match — for each target, GeoSnag finds the closest source by timestamp within the same calendar day. Each match gets a confidence score based on how close in time the two shots were. You can set a maximum time window (default ±2 hours) and a minimum confidence threshold.
4. Write — copies GPS coordinates from the matched source into the target. Supports writing directly to EXIF, creating XMP sidecar files, or both. Every modification is preceded by a backup.

Features

Broad Format Support

GeoSnag reads and writes EXIF metadata for all major camera formats: JPEG, ARW (Sony), NEF (Nikon), CR2 and CR3 (Canon), DNG, ORF (Olympus), RAF (Fuji), RW2 (Panasonic), PNG, HEIC, and HEIF. HEIC/HEIF support is provided through pillow-heif, so Apple ProRAW and modern iPhone photos work out of the box.

Intelligent Matching

Matching is timestamp-based with per-match confidence scoring. A phone photo taken 5 minutes before your camera shot scores higher than one taken 90 minutes later. The configurable time window (default ±2 hours, adjustable via --max-delta) and optional minimum confidence threshold let you tune the tradeoff between coverage and accuracy. All matching is day-scoped — GeoSnag won't accidentally pair a Monday photo with a Tuesday GPS source.

Three Write Modes

Choose how GPS data is written to your files:

• exif (default) — writes GPS coordinates directly into the photo's EXIF metadata using pyexiv2. This is the most compatible option: photo apps, map views, and cloud services will see the location immediately.
• xmp_sidecar — creates a .xmp companion file next to the original. Non-destructive — the original file is never modified. Useful for RAW workflows where you want to preserve the original file bit-for-bit.
• both — writes EXIF and creates an XMP sidecar. Belt and suspenders.

Performance at Scale

Photo libraries on a NAS can contain hundreds of thousands of files. GeoSnag is designed to handle this efficiently:

• Scan index — after the first run, EXIF metadata is cached to .geosnag_index.json. Subsequent runs only read metadata from new or modified files, detected by file size and modification time. A full rescan of 100k+ photos that takes minutes on first run completes in seconds on the next.
• Match cache — targets that had no match are remembered along with a fingerprint of the available GPS sources. If sources haven't changed, there's no point re-evaluating those targets. This is especially valuable for libraries where new photos are added incrementally.
• Multithreaded scanning — EXIF reads are parallelized across configurable worker threads (default: 4). On NAS hardware with slow disks but multiple cores, this makes a significant difference.

Safe by Default

GeoSnag defaults to dry-run mode. Running geosnag without --apply scans everything, finds all matches, prints a summary — and touches nothing. You review the output, then run geosnag --apply when you're satisfied.

Additional safety measures:

• No backup files needed — pyexiv2 is built on libexiv2, a mature C++ library that has been the EXIF standard for 20+ years (used by Lightroom, digiKam, darktable, ExifTool). libexiv2 performs writes atomically: it parses and modifies the full EXIF structure in memory, then writes it back in a single operation. If anything fails — disk full, permission denied, corrupt header — it raises an exception before the file is touched. The original is never partially overwritten.
• Processed-file tagging — after writing GPS data, GeoSnag stamps the file's Exif.Image.Software tag with a GeoSnag: marker. On future runs, tagged files are automatically skipped. This prevents double-processing and makes re-runs safe.
• GPS validation — latitude and longitude values are range-checked before writing. Invalid coordinates from corrupted EXIF data won't propagate.
• CSV reports — use --report matches.csv to export the full match table for review before committing to --apply.

NAS-Friendly

GeoSnag was designed for Synology NAS environments:

• Automatically excludes Synology thumbnail directories (@eaDir) and recycle bins (#recycle).
• Configurable glob-based exclusion patterns for anything else you want to skip.
• Runs on Python 3.9 (available via Synology Package Center) with minimal dependencies.
• Low memory footprint — metadata is processed in streaming fashion, not loaded entirely into RAM.

Quick Start

pip install geosnag[all]    # full install (includes pyexiv2 write backend)

curl -O https://raw.githubusercontent.com/zheqd/geosnag/main/config.example.yaml
cp config.example.yaml config.yaml
nano config.yaml    # set your photo directories

geosnag             # dry run — preview matches
geosnag --apply     # write GPS data

On Synology NAS where pyexiv2/libexiv2 is unavailable, install ExifTool as the write backend:

pip install geosnag            # minimal install (no pyexiv2)
opkg install perl-image-exiftool

If PyPI is not available (e.g. air-gapped Synology NAS), install the latest wheel directly from GitHub:

pip install "$(curl -s https://api.github.com/repos/zheqd/geosnag/releases/latest \
  | grep browser_download_url | grep '\.whl' | cut -d'"' -f4)"

See INSTALL.md for detailed Synology NAS setup instructions.

Configuration

Copy config.example.yaml to config.yaml and edit it:

Always single-quote paths in scan_dirs. Unquoted paths with ! cause a YAML parse error, and paths with # are silently truncated at the # (treated as a comment). Single quotes are safe for all characters.

scan_dirs:
  - '/volume1/photo/personal'
  - '/volume1/photo/shared'
  - '/volume1/homes/user/Photos/!PhotoLibrary/2016/08'  # ← quotes required
  - '/volume1/homes/user/photo library'                  # ← quotes required

recursive: true

extensions:
  - .jpg
  - .jpeg
  - .arw
  - .nef
  - .cr2
  - .cr3
  - .dng
  - .heic
  - .heif
  - .png

matching:
  max_time_delta_minutes: 120   # ±2 hours
  min_confidence: 0             # 0–100, reject matches below this

write_mode: exif          # exif | xmp_sidecar | both
skip_processed: true
dry_run: true             # always start with dry run
workers: 4                # parallel scan threads
log_level: INFO

exclude_patterns:
  - "*/@eaDir/*"
  - "*/#recycle/*"
  - "*.geosnag.bak"

All directories in scan_dirs are scanned the same way. Photos with GPS become sources, photos without GPS become targets. There is no separate camera/mobile directory configuration.

CLI Reference

geosnag [OPTIONS]

--apply                  Write GPS data (default is dry run)
--dry-run, -n            Force dry run even if config says otherwise
--config, -c PATH        Path to config file (default: config.yaml)
--report, -r PATH        Save match report to CSV
--write-mode, -w MODE    exif | xmp_sidecar | both
--max-delta, -d MINUTES  Max time difference for matching
--workers N              Number of parallel scan threads
--reindex                Force full rescan, ignore cached index
--rematch                Force re-evaluation of all targets, ignore match cache
--no-index               Disable scan index entirely
--no-skip-processed      Re-process files already tagged by GeoSnag
--preview-count N        Number of matches to preview (default: 20)
--verbose, -v            Enable debug logging

You can also run GeoSnag as a Python module: python -m geosnag [OPTIONS].

Project Structure

geosnag/
  __init__.py        Version and project-wide constants
  __main__.py        python -m geosnag entry point
  cli.py             CLI argument parsing and pipeline orchestration
  scanner.py         EXIF metadata reader (supports all listed formats)
  matcher.py         Timestamp-based GPS matching with confidence scoring
  writer.py          GPS coordinate writer (EXIF and XMP sidecar)
  index.py           Scan index cache for incremental runs
  parallel.py        Multithreaded scanning
tests/
  test_e2e.py        End-to-end pipeline tests (61 tests)
  test_index.py      Index and cache tests (107 tests)
pyproject.toml       Package metadata, dependencies, CLI entry point
config.example.yaml  Configuration template

Requirements

• Python 3.9 or later
• exifread — EXIF metadata reading
• pillow-heif — HEIC/HEIF format support (pulls in Pillow automatically)
• PyYAML — configuration file parsing
• pyexiv2 (optional) — EXIF/XMP metadata writing (requires libexiv2 system library)

All Python dependencies are installed automatically via pip install .. pyexiv2 is optional — install with pip install geosnag[all] or use ExifTool as the write backend. On Synology NAS, you may need to install libexiv2 separately — see INSTALL.md.

Testing

python tests/test_e2e.py      # 61 end-to-end tests
python tests/test_index.py    # 107 index/cache tests

Or with pytest:

pip install pytest
pytest tests/ -v

License

Apache 2.0 — see LICENSE.