To extract a geographic area from a set of ADSBExchange JSON files got to the Utils menu and choose 'Extract from JSON files'. This will create a new set of JSON files with only the aircraft in the selected geographic area. You can use this new set of JSON files with FlightMirage's replay function. That will save a lot of disk space, rather than having the whole world's flights on your PC you can keep just the local area you want. To save space further you can zip the new JSON set into a new zip file and then just unzip when you want to use it.
The extraction process takes about thirty minutes for a full days JSON files. There is no progress indicator, you can open the destination folder in Windows File Explorer to see the new JSON files being created. If you are short of disk space then unzip only part of the zip file that you downloaded from the ADSBExchange website. Extract from those files. Delete those files. Repeat the process until you have extracted all the files that you want.
For the FlightMirage main web page see http://www.stevematt.f9.co.uk/flightmirage/index.html.
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Last updated 29 October 2020