Custom georeferenced charts can be great addition for use in ForeFlight Mobile EFB. Many chart files are commonly in PDF format. However, a standard PDF may not contain geospatial data to allow that procedure to be displayed as a map layer. A geospatial PDF file contains geospatial data to allow the chart to be displayed on ForeFlight's moving map.
In order to create a geospatial PDF, geospatial data will have to be added to the existing file. GIS software, such as QGIS, Avenza MAPublisher, and TerraGo Composer are available to accomplish this task. ForeFlight supports both the Adobe and OGC geospatial standards for geospatial PDF files.
Example Georeferencing Process Using QGIS
QGIS is a free, open-source program that allows users to georeference terminal procedures. The process described here can be applied to any map in a Spherical Mercator Projection with a Coordinate System using WGS84:4326.
NOTE: Projection error is greater toward the edges of large maps. Consider limiting the map to an East/West size of 7 minutes latitude and a North/South size of 5 minutes longitude, 1:500,000 scale.
GeoReferencing a File
STEP 1: To begin a new project, select Layer > Georeferencer. The Georeferencer window will open.
STEP 2: Within the Georeferencer, click the Open Raster button to choose the PDF to georeference. The file explorer window will open.
STEP 3: Select the PDF to be georeferenced.
STEP 4: Select the Transformation Settings button from the Georeferencer window, verify the following settings, and select OK.
- Transformation Type: Linear
- Target CRS: EPSG: 4326 - WGS 84
- Resampling method: Nearest Neighbour
- Load in project when done is checked
STEP 5: Use the Add Point button to bring up crosshairs instead of a cursor.
STEP 6: Position the crosshairs over a point for which coordinates are known and click in that location. An Enter Map Coordinates popover will appear.
STEP 7: Enter the coordinates for the location selected. West (X) and South (Y) coordinates should be entered with a negative (-) sign preceding the numbers (Example: enter -103.37 for 103.37W).
TIP: If using published waypoints as points of reference, to find the Lat/Long value for the waypoint, enter the waypoints in ForeFlight Mobile's FPL editor on the Maps page. Tap on the waypoint in the FPL editor and choose Show on Map. A slide over will appear showing the coordinates for that waypoint. Decimal Degrees (DD:dd°) is the preferred format for entering waypoint coordinates.
STEP 8: Select OK in the Enter Map Coordinates popover.
STEP 9: Repeat steps 7-9 to define at least four identifiable points on the PDF using Lat/Long values. The precision of point definitions will result in more accurate georeferenced PDFs.
Coordinates Entered for Five Waypoints
STEP 10: Select OK.
STEP 11: Click the Start Georeferencing button. Once georeferencing is complete, the PDF will be copied from the Georeferencer to the QGIS home page.
Cropping the File
A file may be cropped to show a particular area. Skip this section if no cropping is needed. To crop the file, follow these steps:
STEP 1: Go to Raster > Extraction > Clip Raster by Extent.
STEP 2: Click on the dropdown menu button in the Clipping Extent row and choose Draw on Map Canvas.
STEP 3: Within the project window, select the area to remain in the file.The gary area drawn will be the extent of the file boundaries.
STEP 4: From the Clip Raster by Extent Window, select Run.
STEP 5: Once the process is done running, select Close in the Clip Raster by Extent window. The Project window will now show two layers in the layer section. One will end with the text "modified" and the other end with "(extent)".
STEP 6: Click on the layer that ends in "modified".
STEP 7: Select the Remove Layer Group button to remove the layer that ends in "modified".
STEP 8: See Step 14 in the prior section above to save the file.
Saving a PDF File
STEP 1: From the home page, select Raster > Conversion > Translate (Convert Format). A Translate (Convert Format) window will open.
STEP 2: Select the Converted "..." button and choose Save to File. A popover will appear.
STEP 3: Specify the file name and destination, and select PDF files (*.PDF) as the file type.
STEP 4: Select Save.
STEP 5: Select Run in the Translate (Convert Format) window (The window may be behind another QGIS window). The PDF will be exported as a geospatial PDF that can be used in ForeFlight.
NOTE: Mac users may see a warning saying that the action could not be completed, but the PDF file should be located in the location you selected above.
Additional Resources: QGIS User Guide Georeferencer Chapter