GeoPDF is a unique data format that brings the portability of PDF to geospatial data. A GeoPDF document can present raster and vector data and preserve the georeference information. This can be a useful format for non-GIS folks to consume GIS data without needing GIS-software. While GeoPDF is a proprietary format, we have a close alternative in the open Geospatial PDF format. GDAL has added support for creating Geospatial PDF documents from version 1.10 onwards. In this post, I will show how to create a GeoPDF document containing multiple vector layers.
Get the Tools
OsGeo4W is the best way to install GDAL on Windows. The default installation gives your GDAL tools with PDF format support. You can use the GDAL tools via the OsGeo4W Shell included in the install.
Once installed, add the path to GDAL library to your .bash_profile file to be able to use the commands easily from the terminal. Launch a Terminal and type in the following commands.
echo 'export PATH=/Library/Frameworks/GDAL.framework/Programs:$PATH' >> ~/.bash_profile source ~/.bash_profile
Installation instructions will vary with the distribution. On Ubuntu, you can install the gdal-bin package.
sudo apt-get install gdal-bin
Verify GDAL Install
If you already have GDAL installed, or just installed it, run the following command in a terminal to verify that your GDAL installation is working and has support for GeoPDF format.
gdalinfo --formats | grep -i pdf
If you see Geospatial PDF printed in the output - you are all set. If you do not get any output or get an error, your install is not correctly configured.
Get the Data
For this example, I chose to use OpenStreetMap Metro Extracts from MapZen. Download the shapefiles (OSM2PGSQL SHP format) for the city of your choice. I am using the extract for Bangalore city in this example. Unzip the downloaded file to a folder on your computer.
The process for creating a GeoPDF file from a bunch of shapefiles is the matter of running a single gdal_translate command. But we need to prepare the data and figure out the correct command-line options. So follow along to understand how you can arrive at the final command - or simply scroll to the end to see the final command-line.
latuviitta.org has a comprehensive overview of all the options available for GeoPDF creation via GDAL. The follow steps are adapted and simplified version of that guide.
- First step is to create a .vrt file that can hold all the vector layers we want in the PDF. If you just need a single layer in the PDF, you can skip creating the .vrt file and directly reference the layer in place of the VRT. Note the <SrcSQL> tag in the VRT file. This is for filtering out all features where the 'name' field is empty. You can leave that out or modify to suit your dataset. Name this file osm.vrt and save it on the same folder with your data.
<OGRVRTDataSource> <OGRVRTLayer name="roads"> <SrcDataSource>bengaluru_india_osm_line.shp</SrcDataSource> <SrcLayer>bengaluru_india_osm_line</SrcLayer> <SrcSQL dialect="sqlite">SELECT name, highway, geometry from bengaluru_india_osm_line where name is not NULL</SrcSQL> <GeometryType>wkbLineString</GeometryType> <LayerSRS>WGS84</LayerSRS> </OGRVRTLayer> <OGRVRTLayer name="pois"> <SrcDataSource>bengaluru_india_osm_point.shp</SrcDataSource> <SrcLayer>bengaluru_india_osm_point</SrcLayer> <SrcSQL dialect="sqlite">SELECT name, geometry from bengaluru_india_osm_point where name is not NULL</SrcSQL> <GeometryType>wkbPoint</GeometryType> <LayerSRS>WGS84</LayerSRS> </OGRVRTLayer> </OGRVRTDataSource>
- GeoPDF is a raster format that can overlay vectors on top. So we need a raster layer as the base. If you have some satellite imagery or scanned raster for the area, you can use it as the base layer, or we can create an empty raster for the extent of the vector layer. ogrtindex command creates a bounding box polygon from the given input layers. gdal_rasterize command then fills this polygon with the given value and creates a raster. The -tr option specifies the pixel resolution of the raster in degrees. You can tweak that to get the output size you need. cd to the directory where you have extracted the vector layers and run the following commands.
cd Users\Ujaval\Downloads\bengaluru_india.osm2pgsql-shapefiles ogrtindex -accept _different_schemas extent.shp osm.vrt gdal_rasterize -burn 255 -ot Byte -tr 0.0001 0.0001 extent.shp bangalore.tif
- Now we can convert the empty bangalore.tif raster to a PDF - overlaying the vector layers from the osm.vrt file.
gdal_translate -of PDF -a_srs EPSG:4326 bangalore.tif bangalore.pdf -co OGR_DATASOURCE=osm.vrt -co OGR_DISPLAY_FIELD="name"
- Once the conversion finishes, you can open the resulting bangalore.pdf file in any PDF viewer. Opening it in Adobe Acrobat viewer, you can see the map data layers. You can browse the features in the layer panel, search for any attribute value and zoom/pan the map.
- Another popular use of GeoPDF files is to use it as offline base maps using programs such as Avenza PDF Maps. Loading the bangalore.pdf file on Avenza Maps on your mobile phone, you can use the GPS to view your current location or trace a GPS route on top. Search also works across layers in the PDF.
You can download the sample bangalore.pdf Geospatial PDF format file for exploring the format yourself.