[OpenLayers-Users] Cache maps with OSM

[Stephen Woodbridge]

On 9/24/2012 4:18 PM, hz hanks wrote:
> Hey, Stephen:
>
> Downloading the completed data is feasible but, as you mentioned, need
> much larger disk to store the data. My idea is just to simulate the
> internet user's process and cache the tiles that are received during
> that preview process. So the cache may not be perfect but can be used
> for offline easily. So from where I can manipulate those tiles I want,
> if I'm using OpenLayer? Or the idea is not feasible?

This is exactly what I am suggesting and mapcache handles all that for 
you and it is transparent. There are only two things that you might want 
to do.

1. if you know you are going offline and you have some area of interest 
that you want to have tiles locally for, then run the seeder to populate 
just that area of interest.

2. manage the cache disk space, by deleting tiles that you do not need, 
however you determine that.

What is nice about this solution is that you do not need to make any 
changes to OpenLayers when you change from online to offline.


browser -> mapcache -> OSM
               |
               V
             cache

So when you are online, and request a tile, mapcache uses the local copy 
if it exists otherwise if gets it from OSM and caches it.

If you are offline, and request a tile, mapcache uses the local copy if 
it exists otherwise tries to fetch it and fails and return a default 
NO_TILE tile.

Later when you are online again and if you request what was a NO_TILE 
before it will fetch and store the new tile.

So the cache gets populated as it is used unless you decide to run the 
seeder for some area of interest like a bounding box and you have 
control of what zoom levels etc that you want to cache with the seeder.

-Steve


> Best,
> Hanks
>
> 2012/9/21 Stephen Woodbridge <woodbri at swoodbridge.com>:
>> On 9/21/2012 8:53 PM, hz hanks wrote:
>>>
>>> Hey, All:
>>>
>>> I have created a simple web based map application using
>>> OpenLayers.Layer.OSM. Now I'm asked to cache part of the map in
>>> advance. ( Say, cache it when it's accessible to the Internet and then
>>> we may move the laptop to a place without the Internet. )
>>>
>>> My idea is to use setCenter( center, zoom ) function to change the
>>> zoom argument and let the OpenLayer to cache the data. I went through
>>> the example of offline-storage.js provided by OpenLayer and now how to
>>> add cache control. But how can I make sure that all data is cached
>>> before I change the zoom argument again? I tried to register event
>>> "loadend" ( change the zoom argument after "loadend" is triggered )
>>> but it didn't work. Did I choose a wrong event? Or is this idea not
>>> feasible ( caching every tile that has been displayed is not
>>> guaranteed? ). Is there any other way to do that (e.g. create a
>>> offline map of a desired area) and how?
>>
>>
>> This is not a function or capability of OpenLayers, but that said you might
>> want to ask this question on the Mapserver.org list regarding mapcache.
>>
>> The idea being to set up apache2 on your laptop with mapcache installed and
>> configured to pull tiles from OSM, then configure your OpenLayers app to
>> pull tiles from mapcache. Mapcache will pull and cache the tiles locally as
>> they are requested or you can run a seeder that will pre-populate the tiles.
>>
>> There are two problems that I see with this setup:
>>
>> 1. unless you have a huge disk you will need to write a script to clean
>> tiles that you do not need.
>>
>> 2. you need to write a script to run the seeder and pull the area of
>> interest that you are interested in, before you go offline.
>>
>> Hope this helps,
>>    -Steve W
>>
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