HERE Map Data Download
Some key functionality offered through HERE SDK depends on HERE Map Data being downloaded and cached on the device. Rendering a map on the screen, for example, is not possible without first downloading map data to the device. Similarly, it would not be possible to provide accurate turn-by-turn navigation without downloading map data to the device. Offline operations such as offline routing and search also require map data to be downloaded in advance to the device. This section describes different approaches you can take to manage map data download.
Passive Download Approach
The passive approach is where you allow the SDK to download map data as needed. A typical example is when a user pans the map and triggers an on-demand map data download to render the map.
Map data downloaded in this way is stored in a persistent cache with a default size of 256 MB. Cached map data can be used for offline operations in cases where a network connection is not available or not desired, such as when the device is in roaming mode. However, there is no way for you to know if sufficient data have been downloaded to enable all offline operations, e.g. offline search or routing.
NMAMapLoaderAPIs to update to the latest version.
[NMAApplicationContext setDiskCacheSize:NSUInteger]to change the default disk cache size.
Active Download Approach
HERE SDK provides two alternatives to actively fetch map data:
- Map data may be downloaded in the form of map packages for a predefined region or country.
- Map data may be downloaded for an arbitrary bounding box or a radius around a route.
The first active approach is where you request the download of map data packages which cover an entire country or region using the
NMAMapLoader APIs. You do this by selecting from a list of map packages. A map package may be a state (such as California), region, or a country (such as Belgium).
The second active approach is where you explicitly trigger a fetch of map data through the
NMAMapDataPrefetcher APIs by specifying a bounding box or a radius around a route. The resulting downloaded map data is stored in the same cache used in the passive download approach, where the cache size by default is 256 MB. The SDK does not place a limit on the size of the area requested for download but it is expected developers are aware of this cache limit and only request areas that result in map data download with size under this limit. You can get a size estimate of the map data that will be downloaded through the
To illustrate how much data may be downloaded, consider a bounding box covering an area of 200 km by 200 km in New York City as illustrated in the following screenshot. In this case approximately 250 MB of map data is downloaded.
The next example shows a 160 km route from New York to Philadelphia with a radius (route corridor width) of 500 m. The map data downloaded is about 100 MB.
A comparison of the different approaches for downloading map data is shown below:
|Passive Approach||Active Approach|
|On-demand||Map Packages||Bounding Box / Route|
|Downloaded map data can be used for offline operation||Limited (1)||Yes||Yes|
|Complexity involved in managing downloads||None||Medium||Low|
|Size of map data downloads||Medium (10s of MBs)||Large (100s of MBs)||Medium (10s of MBs)|
|Upper size limit of cache where data is stored (2)||256 MB - 2GB (2)||None (3)||256 MB - 2GB (2)|
|Option to check areas for which map data has previously been downloaded (4)||No||Yes||No|
|Option to check in advance the size of map data to be downloaded||No||Yes||Yes|
|Option to selectively remove downloaded map data||No (5)||Yes||No (5)|
|Can perform incremental updates between map data versions||Yes (6)||Yes (6)||Yes (6)|
- (1): Map data downloaded on-demand may support some offline operations such as rendering and search while others, such as routing, do not work correctly as some essential data are missing.
- (2): The default value of disk cache upper limit is 256 MB. To change the upper limit of disk cache size use
- (3): The number of map data packages which can be downloaded is only limited by the space available on the device.
- (4): For example, if you want to display to the user what map data has been downloaded and is available for offline use.
- (5): Only option is to completely clear the cache using
MapDataPrefetcherAPIs removing all map data downloaded on demand and by specifying a bounding box or route. Downloaded map packages are not removed.
- (6): Incremental updates are available when updating to the latest map data release from the two previous releases. Incremental updates are typically small downloads as only the changes are downloaded. For example, when updating to the Q1 2018 map data release from the Q4 2017 or Q3 2017 release, an incremental update or patch is used. Where a patch is not available (such as updating from Q2 2017 to Q1 2018), all map data packages are re-downloaded resulting in a much larger download size, and map data that was downloaded on-demand or by specifying a bounding box/route is removed.
Many applications may use a combination of all three approaches. For example, a UI may be provided to allow users to select map packages to download. Also, an option may be provided to download map data for a route prior to starting turn-by-turn navigation. Map data download on-demand would always be available as a fallback for the case when map data was not downloaded using the other approaches.