A GeoPDF is a PDF whose map frame carries real-world coordinates. It looks and prints like any other PDF, but hidden in each page are the coordinates of the map frame’s corners and the coordinate system the map was drawn in. Software that reads them can place the page on a map, show the coordinates under your cursor, and work out the scale. Software that doesn’t, which includes most PDF readers, shows you a picture and nothing more.
That is why GeoPDFs confuse people. A surveyor exports a georeferenced site plan, the client opens it in their usual PDF reader, and it is “not referenced”. The coordinates were there all along.
Check a file now: drop it on the free GeoPDF viewer. Pages with coordinates are marked in the page list, a small map shows where each page sits, and the status bar shows latitude, longitude and grid coordinates under your cursor. Nothing is uploaded.
How georeferencing in a PDF works
A PDF page is measured in points (1/72 of an inch) from its bottom-left corner. A GeoPDF adds a small dictionary to the page that says, in effect: “this rectangle on the page is a map; its corners are at these latitudes and longitudes; it was drawn in this coordinate system.”
- The viewport. A rectangle on the page that contains the map. A page can have several, for example a main map and an inset, while the title block and legend sit outside them.
- The control points. Pairs of points that tie positions inside the viewport to latitude and longitude, usually the four corners.
- The coordinate system. An EPSG code, a full definition (WKT), or both, so software can show coordinates in the grid the drawing uses, such as a UTM zone, MGA zone, State Plane zone or British National Grid.
From those three things a viewer can fit the page to the ground. That gives coordinates for any point inside the map frame, and it gives the scale: how many meters on the ground one millimeter on the paper represents. Measurements inside the frame are then real lengths with no calibration.
The two encodings
There are two ways of writing this into a PDF, and they are not interchangeable.
| Encoding | Where it comes from | Written by |
|---|---|---|
| ISO 32000 geospatial PDF | Adobe’s extension, now part of the PDF 2.0 standard | GDAL and QGIS by default, and most current GIS exports |
| OGC best practice | TerraGo’s original GeoPDF format, published through the OGC | Older TerraGo files and some government map series |
Most files you will meet today use the ISO 32000 encoding. If a viewer reads one encoding and not the other, files in the other one open as plain PDFs. Swyvl’s free viewer reads the ISO 32000 encoding; the older OGC encoding isn’t supported yet.
GeoPDF vs GeoTIFF
Both carry coordinates, but they solve different problems. A GeoTIFF is one raster image with a coordinate system, the standard for orthomosaics and elevation models. A GeoPDF is a document: vector linework that stays sharp at any zoom, text you can search, several pages, several map frames on a page, and it prints like any PDF. Survey and engineering drawings are usually better as GeoPDFs; imagery is usually better as GeoTIFF.
How to check if a PDF is georeferenced
- Open it in something that reads geospatial PDFs. The quickest is the free GeoPDF viewer. QGIS works too: add the PDF as a raster layer and it lands on the map. On the command line,
gdalinfo file.pdfprints the coordinate system and corner coordinates for the first page. - Look at each page. A drawing set can have coordinates on some sheets and not others. The free viewer marks each page that has them.
- Check the coordinate system and location. Is the footprint on the right site, and is the grid the one you expect?
- Check the scale. Measure a dimension printed on the drawing. If the coordinates are right, it will match.
What to do with a GeoPDF
- Read coordinates off it. Hover over a point in the free viewer to get its latitude, longitude, easting and northing.
- Measure at true scale. The scale comes from the coordinates, so distances and areas inside the map frame are real. See how to measure a PDF drawing to scale for drawings without coordinates.
- Deliver it with your other site data. When you upload a GeoPDF to a site in Swyvl, next to your orthomosaics, scans and 360° photos, its georeferenced pages are detected and measure at their own scale, and your client opens everything from one link.
- Open it without GIS software. See how to open a GeoPDF without ArcGIS.