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The method of determining ETA depends both on the manufacturer, as well as the data they have available for the specific roads you are going to be on. So without knowing which GPS model and which roads, the answer will unfortunately vary.
Generally speaking through, roads will have a classification which is the mapping companies starting point for an estimated speed when no other data is available. Then, as additional information is available it is applied to the ETA to give better estimates. The exact formulas are not something the GPS companies give out.
For example, you can additionally factor in current traffic conditions coming from live traffic services, traffic magnets, etc. You can also use current traffic conditions from other drivers based on cell phone signals, or even data coming from their GPS.
Other data sources include historic traffic conditions such as being able to say "along this stretch of road, on Thursdays at 4:30pm traffic is typically traveling at 37mph".
Still other models will try to apply some personal logic and determine that you typically drive 2mph faster than most of their estimates, so adjust future estimates accordingly.
Often though, the best ETA will come down to user location. For example where I live TomTom devices provide the best ETA for me, hands down. I can never drive nearly as fast as the Garmin expects me to. On the other hand a couple states away in places I often travel, the Garmin ETAs are much better. (And this is all while ignoring the accuracy of live traffic reporting which doesn't really exist in my local area.)
I'm sure many of us could nitpick on some parts of the methodology, but I would be surprised if anyone actually goes out and does this and improves on what you've done. So bravo!
From what I recall, at least many years ago, Garmin used a 50% benchmark to determine the accuracy estimate coming out of the GPS. For those who haven't heard about this before, basically they were (and maybe still are) tuning their estimated accuracy field to be accurate 50% of the time. So if the GPS says it thinks it is currently accurate to within 5 meters, that will be true 50% of the time and false 50% of the time.
Given that 50% of your points were within 4 feet of the margin of error from the images themselves, Garmin might therefore suggest that the GLO achieves 4 foot accuracy. Not too shabby.
While I have no idea what is being used for a processor in the GLO versus other Garmin devices, I wonder if the fact that the processor is dedicated to crunching GPS data only and not doing anything else helps contribute to the great accuracy being displayed here.
I have a potential mapping project coming up next summer... sounds like GLO it will be.
And no good if it doesn't perform in an emergency. In my experience, a smart phone's GPS is nearly worthless when there is no carrier signal.So I still reckon phone is his best bet. No additional space. No additional cost. No additional power needed.
I can't say as though either of those models specifically will-- perhaps they will not. As we know the manuals are not always a perfect indication of the devices capabilities. I have an older Forerunner as well as an older Edge, both of which will show lat/long. Perhaps it has been removed from some more recent models.
I disagree a little bit with the smartphone approach. Without access to a cellular signal, the GPS in smartphones are typically very low power and struggle to acquire a fix even in open areas. Battery life of the phone will also tank in that scenario.
I'd look towards something like a cycling GPS or a running/watch GPS. Something like the Edge 20 or Forerunner 10 would seem to be a better fit for this situation. Good battery life, no reliance on cellular (even to assist with getting a fix on location), very small, and durable. Those devices will probably get a GPS fix faster and more accurately than a smartphone without a cellular connection. You won't likely get it under $100, but you would be close.
When out hiking, often I decide I'd like to carry a GPS as an emergency backup to determine my exact location. I don't need to lug around a full handheld GPS and I always have paper maps with me. My phone will never get a signal and I don't want to burn the batteries trying to get a GPS fix. So I'll often carry around one of my Garmin Edge devices in a similar fashion. If I need it to determine my exact location, it is there quickly and reliably and takes up minimal space and weight.