I use a DJI Phantom with a multispectral camera coupled with Agisoft Metashape to produce an ortho-photo-mosaic for final determination of vegetation indices but will work for mapping. If you are into serious mapping this is what you need. As far as a magnetometer for gravity readings you will have to hope someone else can reply. You may be pushing feathers uphill with a blunt stick on that one......Hi, I'm in my final year of university and I am using drones to create a mapping tool as my final year project. So having a GPS and a magnetometer would be really helpful when it comes to stitching different pictures together. Hope that answers your question.
You could use one controlled by Ardupilot- several kits out there, I’d have to search for examples. Start here:My supervisor said that it needs to be an open sourced drone so I'm guessing I do need access. Thanks for the advice on the CAA, I've been looking at this too.