GPS is one of those technologies that have gone from being a luxury commodity to a default feature on most mobile devices. As a result of that, many people no longer feel a need to buy a dedicated GPS receiver, be it for the car or for hiking/hunting. With so many well known GPS manufacturers now offering smartphone software that often has more features than a proper receiver, you can't really blame them.
The truth is though that you don't really need a very good GPS module to be able to figure out where a car is. Chances are it's on a road, which normally means that there aren't a dozen trees directly overhead. When it comes to hunting and hiking though, it's a whole different ballgame both when it comes to software availability and GPS accuracy.
I've been using both a smartphone and a proper handheld hiking GPS side-by-side for a year now, and in that time I've discovered a lot of things about both units. They both have their strengths and their weaknesses, and the question whether a smartphone can replace a proper GPS receiver is more complicated than you might think.
I originally got into "off-road" GPS receivers when I started Geocaching last year. Geocaching involves navigating the environment to find hidden boxes that other players have hidden, using a GPS and coordinates to the box. At first I used my iPhone with the official Geocaching application, but a few trips outside the bounds of normal road maps made it clear that a proper GPS receiver that was designed for such use was needed. I did some research and ended up with a Garmin Oregon 450, one of Garmin's (then) new touchscreen GPS receivers.
Software and maps
One of the first things I noticed with the Oregon was that not all devices are as smart as we have come to expect from being around smartphones and MIDs all the time.
First of all, the 450 had no usable maps preloaded, which meant that I had to go out and find some. When you're used to giving your billing information to a company and then having every piece of software you could ever need thrown at you, then having to manually load a device felt a bit 2005. Quite frankly, it made me wonder how non-geeks are able to use these devices at all. I've later discovered that quite a few of them aren't able to use them, and there are a lot of requests for help with loading files on Geocaching forums (and, I would assume, other forums where GPS receivers are discussed).
Loading maps is certainly one area where the smartphone wins, but ironically it also wins when it comes to map availability, at least here in Norway. Garmin basically just takes the maps that the Norwegian Mapping Authority have made and turns them into topographic map packs, where Norway is divided into 20 map packs each costing $120. Access to these maps in non-Garmin form is however free, and I have an app on my iPhone that can access the same maps as long as I have an internet connection.
On top of that, my Garmin is limited to just the maps I load onto it and has no real time access to various map services. My iPhone on the other hand can access satellite photos from services like Google Maps and Bing Maps, and even some incredibly detailed airplane photos from a national mapping service. If you have a smartphone you don't really need to have an accurate GPS lock all the time, as you can access overhead photos that do a much better job of showing where you are than any normal map and location could.
Unfortunately, that's pretty much it for the smartphone's advantages. While you might very well be able to navigate using overhead photos from satellites and airplanes, a hiking GPS needs to be able to navigate you through woods and mountains that are low priority for those who make the overhead photos. When it comes to actual accuracy in difficult situations, smartphones simply don't hold up.
I've had instances where I've been in the middle of a thick forest and the discrepancy between my Garmin and my iPhone has been as much as 100 meters, with the Garmin being 2 meters off the actual target and the iPhone pointing to a whole different area of the forest. Most of the time the iPhone is closer to 15-20 meters off in heavy forests, but that difference might mean the difference between the right and wrong side of a river or the right and wrong end of a cliff, both of which are situations I've experienced myself.
That being said, smartphones are often a lot faster at getting a GPS lock than dedicated units because they have assisted GPS (aGPS). This technology uses data from cell phone towers to determine the rough position of the device, and then uses that to locate satellites for a proper GPS lock. Most GPS receivers don't have this feature, and while it normally only takes about a minute for my Garmin to lock on, I've had instances where it's taken up to 30 minutes for the initial lock.
Durability and battery life
The accuracy issue is however not the biggest downside of using a smartphone as a replacement for a proper hiking GPS. Durability and battery life is what really separates the two devices in terms of usability out in the wilderness, and it's on this point that the smartphone really loses.
I've dropped my Garmin straight onto asphalt, it has fallen off rocks I've placed it on, dropped to the ground and gotten soaked from rain more than once; and it has the scars to prove it. It still works just as well though, as it's not just waterproof but quite shock proof as well. The USB port is hidden under a rubber flap, the power button is rubber and part of the casing, and the memory card and battery compartment is all sealed up as well. The resistive plastic touchscreen is recessed about 3mm and the entire device is certified to be water proof.
Smartphones on the other hand are normally plastic and glass, and are neither water proof nor shock proof (or even resistant). If you're going to rely on a device to show you the way through a mountain, you don't want it to short circuit at the first sign of rain.
Battery life is also a huge issue, and in the end this is what made me realize I needed a proper GPS receiver.
When a smartphone is used for navigation, you're not only using the screen all the time but also using the GPS module, data network (for quick positioning as part of the assisted GPS standard as well as for downloading maps), and virtual compass. That drains the battery quite quickly, and you can forget about using it for an entire day. Charging it is also a problem, and you can't just pop in a new set of AA batteries like you can with most GPS receivers. While my Garmin can go 15-20 hours on a pair of AA batteries, I'm lucky to get 3-4 from my iPhone.
I can't even begin to count how many times I've found myself in the middle of nowhere with my iPhone in one hand and my Garmin Oregon 450 in the other, all the while thinking that combining the two would create the most powerful navigation tool ever. Smartphones clearly win when it comes to ease of use, software/map availability, and initial GPS lock speed. However they lose when it comes to accuracy, durability, and battery life.
What this means is that the choice between them depends on what scenario the user has in mind more than anything else.
Smartphones are excellent GPS units for cities and high population areas, but I would strongly advice against running into the Amazon Rainforest with one. As a geocacher I see more and more people who get into the hobby by having access to smartphones, but pretty much every single one upgrades to a proper GPS receiver once they've decided to stay with the hobby.
I think the same principle would apply to any hobby or sport where a GPS is needed, and I don't think smartphones can replace hiking GPS units as well as they can replace in-car navigation systems.