Hey Bill, thanks for posting. I’m a consultant (civil engineer) in Brevard so it’s good to see SJRWMD getting into the use of drones. They’re a great tool, and we use them for almost all of our current engineering projects. We use a Phantom 4 Pro and process in Pix4D. Since almost all of our projects involve a surveyor, we have the benefit of using very accurate GCPs for processing our imagery.
As you might know already, the altitude readings are based on barometric pressure. They’re going to be relative readings, meaning that even if you calibrate the drone before flight, the altitude is still going to be some “random” value that doesn’t correspond to any real NAVD88, NGVD29, etc. value. Until you use an RTK model (or something along those lines) or input some type of ground control information, you’re not going to be able to achieve any real level of vertical accuracy. Your horizontal accuracy may also be off by a decent amount (5-10 ft in our experience). Your imagery may also “tilt” in one direction since there’s no vertical control to orient it correctly.
To achieve what you’re looking for, your best bet (for having a small budget) is to input some type of ground control in your Pix4D processing. There are some pretty cheap GPS units out there that could give you a decent measurement of x, y, and z. By setting a few targets and inputting some “good enough” values into Pix4D, it will greatly improve your results. They won’t be perfect, as the GPS units have limited accuracy, but it would be a big improvement over no control at all.