Thread: GPS Advice
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Old 10-26-2016, 10:08 AM   #15
Trail Boss
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@bioguide

That's a very valid question! I imagine most forum members intend to use the apps in areas with no cell phone reception so it's very important to be able to download maps to the phone and use them "offline" in the woods.

The short answer is: Yes.



Here's the long answer.

There are two ways to get online maps onto the phone for offline use, via auto-caching or explicitly downloading them.

1. Auto-caching online maps
Most maps are displayed using a connection to an online map server (via Wi Fi or cellular data). When the app displays an area, it stores it locally in the phone (it "caches" the map). Maps are usually comprised of "tiles" and the area you displayed may be comprised of one or more tiles. The app automatically caches the tiles for future use.

The next time you display the same area, the app simply retrieves tiles from local storage. This is true with or without a connection to a map server. It speeds the display of the maps (gets it locally from the phone and not remotely from the map server) and saves bandwidth (good when you're on a cellular connection).

HOWEVER, without a data connection, it can only show you the cached portions of the map. If you pan the map to something it doesn't have cached, you won't see it. What you will see depends on the app. Most will show some sort of error message for the portions of the map it cannot display.

There's another catch to "auto-cached maps"; it only caches the map at one magnification level. If you zoom into a cached map area, the app must retrieve new map tiles. Without a data connection, you won't see a magnified map. What you will see depends on the app. Some apps (like AlpineQuest) will take the previous zoom level and "blow up" the pixels (create "fat pixels"). The result is a grainy map, with no additional details of course, but it's aesthetically better than looking at a blank map.


Clearly, "auto-caching" is handy but has its limitations. There's a simple solution but it's a bit tedious. While connected (i.e. before entering the woods) pan the map so it displays the entire area you plan to visit. Magnify the map and repeat the panning operation. Magnify and pan however many times you feel you need. Now the app has cached all parts of the map you want and all zoom-levels. Clumsy and tedious, but it works just fine for an area of a few square miles.

2. Downloading online maps
This is the fastest and most efficient method to ensure you have a local copy of a desired map area with full coverage. You draw a rectangle around the area of the map you want, indicate how many zoom levels you need, and the app will retrieve it from the map server and store it on the phone. Done and done!

You'll notice the common theme is to draw some sort of rectangle or "bounding region" to identity the portion of the map you want to download "in bulk".

For completeness, I should mention there's one more way to get a map onto a phone and that's to buy it and download it. These are the so-called "premium maps" sold by some app vendors. They are typically sold by state or country. They are typically "raster maps" (scanned from the original; large file-size) or can be "vector maps" (pure map data; compact file-size).

The Nitty Gritty
Most of the apps have some means of letting you clear the map cache. Why would you want this? Maps change (trails re-routed, details added, etc) but you may not see those updates if you're using a map that was auto-cached two months ago. To address this, some apps set an expiry date for cached map tiles and will refresh them when the time comes.

Maps you download typically have no expiry date. Think of it like a paper map, fixed and eventually out-of-date. Some apps do offer the ability to refresh a downloaded map (it saves you the step of having to define the "bounding region" all over again).

Some online maps disallow being downloaded for offline use (auto-caching is OK but not explicitly requesting to download a map region). This is an agreement between the map provider and the app developer. The app will disallow "bulk downloading" for map limited to online use only.


Last but not least.
If you have an Android phone and want offline road (and trail) navigation, consider "OSMand" (iOS version doesn't do navigation). It uses a vector-based version of OpenStreetMap. All of New York state fits into ~290 Mb and that includes roads, trails, and Points of Interest (POI) so you can search for a POI or street address completely offline. The free version lets you download several maps, enough to cover several states. http://osmand.net/features

Last edited by Trail Boss; 10-26-2016 at 10:39 AM.. Reason: Typo.
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