GPS Units come in many levels, I have done a lot of surveys, and spending £2,000 on a GPS mobile computer, gives you sub meter readings. But to get the most out of them you need the correct software, such as arcpad, which costs hundreds.
The difference in practice is remarkable, between a garmin and a real GPS computer system. For example, using a garmin to find a lost observation borehole that has not been seen in 15 years, it took 7 people 3 days, and still didn't find it.
I used my magellan mobile mapper field GPS computer, and found that borehole in 15 minutes.
With the correct kit you can find anywhere or anything. The added benefit is with a good GPS system and software, you can load any map from any source, for anywhere in the world. You don't have to buy maps, spend the money on the unit.
For example, I can geo reference a photo of a map, loaded on to my GPS unit and navigate by that.
For a few hundred pounds you can get slightly older units GPS computers that will beat most commercial GPS units on the market, and the best bit is that you can calibrate them. I do this everyday when surveying.
The current level is RTK units, centimetre accuracy, and even millimetres in the open by the sea....a bit more pricey £10,000, plus subscription, sim card, mobile data, etc.
This thing is that everyone uses their mobile now, the accuracy is rubbish, but no one cares. I have been a lecturer and have failed masters students on geo referencing errors, he was using his phone and got 300m error.
In summary a high end garmin will give you 15m error as standard in the open, 100m error or more in woodland. Maps are extras on top of the unit.
Same money will buy you an older GPS computer, 5m or less accuracy and you can use open source maps for free.