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To digitise a map or plan that is larger than your scanner, capture it in overlapping sections with an overhead camera and stitch them into one image, or use a large-format flatbed if the sheet is flat and robust. Target 300–400 ppi at the full original size (400–600 ppi for fine detail), keep the sheet flat and supported, and use a colour and scale target so the result is measurable. Oversized maps are mostly an exercise in handling and stitching, not exotic equipment.
Why are large maps a special case?
A map can be a metre or two across, often rolled or folded, sometimes brittle along old creases. No ordinary flatbed fits it, pressing it flat risks the folds, and a single overhead frame at archival resolution may not have the pixels to cover the whole sheet at detail. So you break the job into manageable sections and reassemble them, which is the core skill here.
How do I capture a map in sections (tiling)?
The workflow, in plain steps:
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1. Lay the map flat on a clean, supported surface (or wall mount if approved).
2. Mount the camera on a stand/rail that can move in even steps.
3. Divide the sheet into a grid of tiles with ~30% overlap between neighbours.
4. Keep lighting, focus and exposure identical for every tile.
5. Capture row by row, naming files by grid position (r1c1, r1c2, ...).
6. Include a scale bar and colour target in at least one frame.The overlap gives the stitching software shared features to align on. Consistent lighting across tiles is what makes the seams invisible.
How do I stitch the sections together?
Feed the tiles into a panorama/stitching tool. Free and capable: Hugin (open source). Commercial: PTGui or Photoshop Photomerge.
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# Hugin batch flow (conceptual)
pto_gen -o project.pto tile_r*c*.tif # create project from tiles
cpfind --multirow -o project.pto project.pto # find control points in overlaps
autooptimiser -a -m -o project.pto project.pto # optimise alignment
hugin_executor --stitching project.pto # render the seamless mosaicBecause the map is flat (not a 360° panorama), choose a planar/rectilinear projection so the geometry stays true — vital if the map will later be georeferenced.
What resolution and file format should I use?
Measure resolution at the full sheet size, not the tile size. A 1 m wide map at 400 ppi is roughly 15,700 pixels wide — a large file, so plan storage. Save the stitched master as an uncompressed TIFF or lossless JPEG 2000 in a wide colour space with the profile embedded, and make smaller access derivatives (or IIIF tiles) for delivery.
| Use | Resolution (at full size) | Notes |
|---|---|---|
| General access | 300–400 ppi | Readable place-names |
| Fine cartographic detail | 400–600 ppi | Hachures, small text, contour lines |
| Map for analysis | 400+ ppi, then georeference | Coordinates added afterwards |
How do I flatten a rolled or folded map safely?
Never force it. Brittle paper cracks along creases. Instead:
- Relax the sheet gradually under controlled humidity (a conservator's humidification chamber) or under light, even weights over time.
- Support the whole sheet; do not let parts overhang an edge.
- Use an anti-reflective glass platen to hold it flat for capture only if a conservator approves, and only acid-free.
- For very fragile sheets, leave flattening to a conservator.
Do I need to georeference the map?
Only if it will be used for spatial analysis or overlaid on modern maps in GIS. Georeferencing assigns real-world coordinates by matching identifiable points (a church, a road junction) to known locations, letting QGIS or ArcGIS warp the scan onto the globe. For plain access imaging — putting a readable surrogate online — you can skip it. If in doubt, capture cleanly now and georeference later from the master.
A small worked example
Say you have an estate plan 80 × 60 cm, target 400 ppi:
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Full-size pixels needed: 80 cm = ~31.5 in -> 31.5 * 400 = ~12,600 px wide
Camera frame covers: ~30 cm wide at chosen distance/resolution
Tiles needed: 3 across x 3 down (with overlap) = 9 captures
Stitch -> single ~12,600 px master TIFF, plus a colour/scale target frame.That is the whole job: nine careful, identical captures and one stitch.
Key Takeaways
- For maps bigger than your scanner, capture overlapping tiles and stitch them.
- Use about 30% overlap, identical lighting, and a consistent file-naming grid.
- Stitch with Hugin (free), PTGui or Photomerge, using a planar projection for flat sheets.
- Measure resolution at the full sheet size; large maps mean very large files — plan storage.
- Flatten rolled or folded maps gradually and safely, with a conservator for fragile sheets.
- Georeference only if the map is for spatial analysis; access imaging does not require it.
- Save a lossless master and derive access copies or IIIF tiles for delivery.
Frequently Asked Questions
How do I digitise a map larger than my scanner?
Capture it in overlapping sections with an overhead camera and stitch them, or use a large-format flatbed or roller scanner if the map is robust and flat. Stitching from a copy stand is the common route for fragile or very large sheets.
What resolution should I scan a large map at?
Aim for 300–400 ppi at the original's full size for general use, and 400–600 ppi where fine cartographic detail, place-names or hachures matter. Remember resolution is measured at the real sheet size, so large maps produce very large files.
How do I stitch map sections together?
Photograph overlapping tiles (about 30% overlap), then stitch in software like Hugin, PTGui or Photoshop Photomerge. Stitching aligns control points across overlaps to build one seamless high-resolution image.
How do I flatten a rolled or folded map for capture?
Relax it gradually under controlled humidity or with gentle weights over time, never by force. For fragile sheets a conservator should humidify and flatten; capture under a sheet of anti-reflective glass only if approved.
What is georeferencing and do I need it?
Georeferencing assigns real-world coordinates to a scanned map so it overlays modern GIS layers. You only need it if the map will be used for spatial analysis; for plain access imaging it is optional.