If you have a dji drone, and wants to be able to do survey flight plans for free (while still using DJI-GO), you might be interested in this: https://unite.openworlds.info/mj-saunders/QGroundControl-to-DJI-GO

You will need a rooted phone.
You can use QGroundControl on the phone, or laptop, etc.
And you'll need to be a little comfortable with using a terminal/console.

I'm not sure if there's a way around needing a rooted phone.
Any suggestions for improvements, let me know.