I'm flying a very large tract of land of about 1 mile long and 1/3 mile wide. I may not get permission to center myself on the site.
Your swath of land is roughly 200 acres. To answer this question even remotely intelligently, I need more information:
* Single Grid or Double Grid?
* Overlap? I assume 75/75, yes?
* AGL and/or GSD??? Most clients are good with 1"/pixel GSD, but some want as much as .6"/Pixel.
* Connectionless flight???
* Terrain aware???
Assuming 1" GSD, single grid (based on your three battery assessment) and 75/75 overlap (industry standard), and a Phantom 4 (P4P is the best drone for mapping)...
Like you, I generally try to get as close to the center of the job as possible. But most times that is not possible. The good news is that, almost regardless of your launch point, you shouldn't be more than a mile from your drone at any given time. In the mapping game, this is child's play. My drone often flies out 2 or 3 miles, often maintaining 100% connection throughout the mission. But for corridor type missions, I have been known to fly 8 or 9 miles out, which requires a connectionless mission...but I digress...
I am unfamiliar with Pix4D, but have used other mapping programs, like GS Pro. If you're not flying Terrain Aware, GS Pro is a great program. I would consider using that one for simple grid missions. That said...
There is a highway along one side, so I thought I might start at one end and walk the long end as I fly while having a safety car following.
This is a very bad plan for many reasons.
1) This is a very small mission. No need to move your launch point or actively follow the drone.
2) While I also launch from either permitted or public land, I would never launch from a highway. Believe me when I say that the farther you get from the public eye, the better off you'll be. Wear your Dayglo green dork vest to look official.
I know I need three batteries...
Then make sure you have 4 or 5 batteries and/or a charger on hand.
...and I am concerned that if I start to land downstream of my starting point and the battery goes too low, will the drone attempt to return to it's original starting point while I'm landing it.
I don't understand the question. So instead of trying to answer this, I'll just give you a work flow.
Set up your mission so the start point is furthest from you and the end point is closest to you. You want the bird to work towards you, not away from you.
Every mapping app has the ability to "Pause" the mission. This is so you can change your battery. Most apps will ESTIMATE battery usage, but nothing is more efficient than actively monitoring your mission and managing it according to battery needs. If you're flying a single grid mission, the bird is going to fly right by you many times. When you see the battery get to 40%, wait until the bird is at its closest point to you, pause the mission, bring her back (either manually or RTH - I prefer manually), change the battery and relaunch. The bird SHOULD resume at its stopping point. That said....
The only way to predict the behavior of your software is to PRACTICE. Create a small grind mission and practice stopping the mission and changing your battery. The last thing you want is to learn how your software works and/or behaves on the job site.
I'm not sure how the algorithm work.
Practice. Create scenarios. Learn your software.
I'm using PIX4D and Phantom 4.
Does the Phantom 4 have a mechanical shutter? If not, I would consider getting a P4P. The mechanical shutter is all but necessary in the mapping game.
Any ideas? I can short cycle the flights and land at 30%, but I may not have enough total battery time if I do that.
Read above. You better have AT LEAST 50% more battery than you need. Most times everything goes as predicted, but there are times that the software bugs out and you have to relaunch, essentially wasting 10% of your battery or more. You should be prepared for that scenario.
If you're not comfortable allowing your bird to fly a mile out, you can break up the grid mission into 2 or 3 grid missions (being sure that there is 75% overlap between missions). Once you complete a mission, simply relocate and start your next mission.
Here's a screen shot of a mission I did a couple weeks ago roughly the same size as your mission. The swath of land was 1.4 miles from end to end, so we were never more than like .8 miles from the bird.