Find a Geospatial File's SRID Using Scala and GDAL
On occasion, you might find yourself working with a new spatial data set where it would be really handy to figure out what the current SRID is. Luckily, the smart people at GDAL.org have done the hard work for you. Here’s how you do it in Scala using the GDAL Java Bindings.
GDAL Setup
First, you have to set up GDAL with the Java bindings on your machine. It’s not as simple as just having GDAL installed. There’s instructions to do this here for various systems, and here for mac, but in my experience these aren’t very up to date and you may have a date with StackOverflow to get through this part. The following worked for me on macOS High Sierra:
- First, disable System Integrity Protection. The rest of this won’t work if you don’t do that.
- Run:
- Follow the instructions that show in your terminal in the output of the previous command to update your .zshrc or .bashrc or whatever file it is your terminal uses.
- Add the following to the same file as in the previous step:
- Restart any terminals you have open or run
source [the file from above]in your terminal
Project Setup
Add the following to your project’s build.sbt file to get the Boundless Geo resolver and to load the GDAL library (I’m running Scala 2.12.4 and SBT 1.0.3 for the record):
Download this sample file and save it in your project under the /src/main/resources folder, creating it if it doesn’t already exist.
Blocking Approach
The simplest way to do this is to write it without caring about whether your code blocks. To do this, add the contents of the Main class below to whichever class you’re working with, following along with the comments:
That’s it! Build, run, and enjoy. If it’s possible to identify your SRID from the file you have, there’s a good chance the AutoIdentifyEPSG method will do it.
Non-Blocking Approach
To achieve the same thing without blocking, wrap everything in a function that returns a Future when it is called. Then, call the function, and use pattern matching to choose what to do with the function returns successfully or not. Make sure you import the global ExecutionContext at the top and that you block the main thread at the end to prevent the JVM from terminating, as shown below.
This approach has benefit of being able to complete other work while the file is being loaded, which is handy when you’re dealing with large geospatial files.