How to Convert a .PDF Map to GISby Tammy James
You just received an interesting map and want to use it as a GIS layer, but it is in .PDF format and won’t open in ArcMap. It would be easiest to get your hands on the GIS source data that the map was made from, but if that is not possible, there is still a way to use the map. Georeferencing is the process of assigning spatial reference information to a data layer so that it will be projected at the correct location and line up with your other spatially-referenced data in ArcMap. This can be accomplished in a few simple steps.
Convert Your .PDF Map to a JPEG
Open your .PDF map in Adobe Acrobat.
Click “File” on the menu, then “Save As.”
Choose “JPEG” from the “Save as Type” drop-down box. Click “Save.”
Preparing to Georeference Your Map
Open ArcMap 9.3 and click “OK” to open a new empty map.
Click the “Add Data” button and add your projected reference imagery or other data source.
Click the "Add Data" button and navigate to the new JPEG file. Click “OK.” Click “No” when ArcMap asks if you want to build pyramids, and click “OK” when it informs you that the file has no spatial reference information.
Click “View” from the menu, then “Toolbars,” then select “Georeferencing.” This brings up the Georeferencing toolbar.
Drag the Georeferencing Toolbar to the top of the screen to dock it.
Georeferencing Your Map
Click the title of your map file to select it, then right-click and choose “Zoom To Layer.”
Click the “Add Control Points” button on the georeferencing toolbar. Click a location on your JPEG map that corresponds to a location on your reference data layer that is easy to locate.
Click the title of your reference layer to select it, then right-click and choose “Zoom To Layer.”
Click the location on your reference data layer that corresponds to the location you just selected on your JPEG map.
Repeat Steps 1 through 4, choosing locations that represent the entire map region, until your JPEG map sufficiently matches your reference data layer.
Click the Georeferencing drop-down menu and choose “Rectify.” Choose the cell size, resample type, file name, file type, and compression type, or accept the defaults and click “Save.”
- The quality of your georeferencing job depends on the quality of your reference data set, your ability to accurately match locations on the reference data set with locations on the .PDF map and the number and spatial distribution of matching locations you use. Be sure to take these factors into account when determining the quality of spatial reference information that you need.
- You can apply a transformation to the map by clicking on the Georeferencing drop-down menu and choosing “Transformation.”
- Use reference data sources that match a data layer used in the .PDF map, if possible. For example, if the map has township, range, and section on it, then use a similar ArcGIS data file and match line intersections during georeferencing.
- In addition to JPEG, GIF and TIFF formats are also supported. The procedures for georeferencing these formats are the same as those listed for JPEG.
- chinese map image by yang xiaofeng from Fotolia.com