Open your family tree
Start MacFamilyTree (version 10 or 11, macOS) and open the tree you want to print. MacFamilyTree saves in its own format, so the poster needs a GEDCOM export of the tree.
Go to Publish › Export GEDCOM file
In the main menu on the left choose "Publish", then "Export GEDCOM file" in the function bar or overview pane. The export screen with all parameters opens.
Choose the persons and the GEDCOM version
In the "Persons to Export" list keep "Export all persons". As the version choose "GEDCOM 5.5.1". MacFamilyTree also offers GEDCOM 7.0.3, which RootsOnPrint reads as well; the manual recommends version 5 for compatibility with other programs. Leave the file format at "UTF-8" and the line feed at "macOS Default".
Set privacy, dates and media, then export
Leave "hide private persons" and "hide data of living persons" off if the living family should appear on the poster, keep the date format as the GEDCOM standard (not year only) so birth and death years print correctly, and skip media files. Click "Export GEDCOM File…" (not "Export as GedZip") and save the .ged file on the Desktop or in Downloads.
Upload to RootsOnPrint
Your GEDCOM file is ready. Upload it at RootsOnPrint and design your poster there.
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Troubleshooting
The upload rejects the file?
You probably exported a GedZip archive. Run the export again with the plain "Export GEDCOM File…" button; the file must end in .ged, not .gdz.
Living relatives are missing or anonymous?
The option to hide the data of still-living persons was switched on in the export screen. Switch it off and export again; you decide on the poster who appears.
Only one branch should be on the poster?
You can leave that to RootsOnPrint: export all persons and pick the root person and generations in the design step. If you prefer a smaller file, choose "Only export subset of all persons" › "Select persons to export" › "Add Ancestors of Person" in MacFamilyTree.
Ready to create your family tree poster?
Upload your GEDCOM file and design your printed family tree in under 5 minutes.
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