How to Stitch and Merge Two Scanned Halves of a Large Document Into One PDF

To turn a document that is too big for your scanner into one PDF page, scan it in overlapping pieces with identical settings, join the pieces into a single image with a stitching tool, then place that image on a PDF page set to the document's real paper size. Photoshop's Photomerge, the free Hugin, your scanner software's own stitch mode, or a careful manual join in GIMP will all do the joining.

One detail trips up most people: on an ordinary letter or A4 flatbed, two passes over an A3 or tabloid sheet leave almost no shared strip for the software to match. Plan for three passes, or use a scanner utility built to join exactly two halves.

Why two passes rarely give enough overlap

Most home flatbeds scan an area of about 216 x 297 mm (8.5 x 11.7 in). The large document has to lie with its short side along the glass's long side, so each pass captures only 216 mm of the document's length. Two passes therefore cover 432 mm, and whatever is left after the document's length is the overlap.

Those figures assume the scanner reaches the very edge of the glass. Many lose a few millimetres at the border, which wipes out the A3 overlap entirely. Stitching software also needs something to match in the shared strip, such as lines of text, a drawing or a border. A 12 mm band of blank margin gives it nothing to work with.

With three passes, the middle scan overlaps each end scan by several centimetres. For an A3 sheet, three 216 mm passes cover 648 mm, leaving more than 100 mm of overlap at each of the two seams. That is plenty for automatic matching.

Scan the pieces so they can be joined

1. 1.Fix the resolution. 300 dpi suits text, forms and line drawings. Use 400 or 600 dpi for maps or plans with fine lettering. Every pass must use the same value, or the pieces will not match in size.
2. 2.Turn off automatic adjustments. Auto exposure, auto color correction, auto crop, document detection and deskew all treat each pass on its own. One pass comes out a little brighter or rotated differently, and you get a visible step at the seam.
3. 3.Keep one edge against the glass guide. Slide the sheet along that edge between passes rather than lifting and re-placing it. For the last pass you can turn the sheet 180 degrees so the far end reaches the glass, then rotate that scan back afterwards.
4. 4.Press the paper flat. The lid cannot close properly over a large sheet. Rest the lid down and press gently, or lay a clean, heavy book over the part on the glass. Paper that lifts near the hinge goes soft and grey.
5. 5.Save lossless files. Use PNG or TIFF, named in order, such as plan_1.png, plan_2.png and plan_3.png. JPEG artifacts in the overlap make alignment harder.

Let the scanner software do the join

Some scanner utilities stitch two halves for you. Canon's IJ Scan Utility has a Stitch mode for items up to about twice the size of the glass. You pick an output size such as A3 (A4 x 2), 11 x 17 (Letter x 2), B4 (B5 x 2) or Full Platen x 2. You scan the left half, rotate the item 180 degrees, scan the right half, then nudge and crop the two images in a preview before saving. Because you place the halves yourself in that preview, it copes with little or no overlap. Look in your own scanner software for an option called stitch, combine or large document before installing anything else.

Stitch automatically in Photoshop

1. 1.Choose File > Automate > Photomerge.
2. 2.Under Layout, pick Reposition, which only moves the pieces, or Collage, which can also rotate and scale them. Collage is the better choice when a sheet sat at a slight angle in one pass. Avoid Auto, Perspective and Cylindrical: they are made for camera panoramas and will bend a flat page.
3. 3.Click Browse and add all your scans.
4. 4.Tick Blend Images Together so Photoshop builds masks that hide the seams. Leave Geometric Distortion Correction off, as it corrects camera lenses, not scanners.
5. 5.Click OK, then zoom to 100% and follow each seam. Text lines should run straight across without doubling.
6. 6.Choose Layer > Flatten Image, crop away the ragged outer edges, and save as PNG or TIFF.

Stitch for free with Hugin

Hugin is an open-source panorama stitcher that also handles flat scans. Its own tutorial for scanned images treats each scan as a photo from a perfect camera, so the steps differ from a normal panorama:

1. 1.Choose Interface > Expert, then on the Photos tab click Add images and load the scans.
2. 2.Set the field of view, HFOV (v), to 10 for each image. Any moderate value works, as the tutorial notes.
3. 3.On the Control Points tab, generate matching points with CPFind. Each overlapping pair needs at least two points.
4. 4.Back on the Photos tab, set Geometric optimization to Custom parameters. On the Optimizer tab, untick yaw and pitch and tick the translation values X (TrX), Y (TrY) and Z (TrZ), then click Optimize now!
5. 5.Open the Fast Panorama preview, set Projection to Rectilinear, and crop to the page.
6. 6.On the Stitcher tab, click Calculate Optimal Size, then Stitch Now.

Join them by hand in GIMP

If automatic tools cannot find a match, for example because the overlap is a plain margin, align the pieces yourself. GIMP's Difference layer mode makes this precise, because identical pixels turn black when they sit exactly on top of each other.

1. 1.Choose File > Open as Layers and select all the scans.
2. 2.Use Image > Canvas Size to make the canvas tall enough for the whole document.
3. 3.Select the upper layer and set its Mode to Difference in the Layers panel.
4. 4.With the Move tool, drag the layer close, then use the arrow keys to nudge it until the shared strip goes black. If lines line up at one end but not the other, use Layer > Transform > Arbitrary Rotation for a fraction of a degree.
5. 5.Set the mode back to Normal. Add a white layer mask with Layer > Mask > Add Layer Mask and paint along the seam with a soft black brush to hide the edge.
6. 6.Choose Image > Flatten Image, crop, and use File > Export As to save a PNG.

Put the stitched image on a correctly sized PDF page

A 300 dpi A3 image measures about 3508 x 4961 pixels, and a 300 dpi tabloid image about 3300 x 5100. Crop the stitched image to the document's proportions first, so it fills the page without white bands. Then:

1. 1.Open Image to PDF, add the stitched PNG or JPEG, choose the Fit page size so the page matches the image, and save the PDF.
2. 2.Fit treats each pixel as one point (1/72 inch), so a 300 dpi scan produces a page roughly four times larger than the paper. Open the PDF in Resize PDF, choose A3 or 11 x 17 in, keep Scale to fit, and save. The pixels are untouched; the page now simply reports its true size to printers and viewers.

Both tools run in your browser, so a confidential site plan or legal exhibit is never uploaded anywhere.