Hello - this is for an iPad3 with GPS. I want to use the iPad as a navigator. Download a mapset, go to a city, and navigate without using a wireless network. Totally autonomous. Let's use Rome, Italy a an example. There are third party applications that use OSM datasets. I'm OK with that. There are three questions: (1) What is the size of OSM files needed for street-level detail for a city like Rome, (2) would downloading this dataset impact the operation of OSM, and (3) is there a time of day that is most appropriate for downloading a file like this?

thanks,

Bob

Bainbridge Island, WA (US) --- Close to Seattle

asked 15 Apr '12, 06:00

BainbridgeBob's gravatar image

BainbridgeBob
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accept rate: 0%


There are different ways of downloading OSM data. One way is using ready-made "map tiles" like you see them in your browser. The other is downloading the raw data and using a suitable vector rendering software on the device. Existing applications on the iPad use either of these methods (see Wiki for a list of applications) but those downloading tiles are not well liked for reasons given below.

Question 1:

If you are talking tile data, this depends on what you mean by "street-level detail" and what area exactly you want. Let's assume you mean zoom level 17:

zoom level 17

Let's assume you want the area inside the motorway circle in this tile:

zoom level 10

The tile above is a zoom level 10 tile. You want approximately half of a zoom level 10 tile. Since each zoom level means 4 times the number of tiles, you can calculate like this: 0.5 tiles on z10 is 2 tiles on z11, 8 on z12, 32 on z13, 128 on z14, 512 on z15, 2048 on z16, 8192 on z17. Since you will probably want all these zoom levels, you're looking at roughly 10k tiles altogehter. Now the size of the PNG files varies depending on how much is on them, but for this area, I guess you'll end up with an average of about 8 KB per tile, so you're looking at a data volume of about 80 MB.

The map on the Geofabrik web site is a good tool for getting familiar with tiles and zoom levels because you can display a tile coordinate grid on top of the map there.

If you are talking vector data, then - depending on how densely the area is mapped - the same area should take about 20 MB in our raw "osm.pbf" format but you will not be able to simply download it; you will have to download a larger file (there are ready-made files for various countries or continents) and then cut out the region of interest from that file. You will then be responsible for rendering the data yourself which will usually encompass converting from .osm.pbf to something usable for map drawing. Existing vector drawing software like e.g. Navit does exactly that.

For raw data download sources, see the Planet.osm wiki page.

Questions 2 and 3:

If you are talking tiles, then downloading this data set would violate OSM's Tile Usage Policy. While our tile server usually is less busy when it is night in Europe:

tilesever munin graph

we would still ask people who need that amount of tiles to either produce them themselves (by downloading the raw data and setting up a rendering engine) or get them from another source. Teh aforementioned policy has links to alternative sources.

If you are talking vector data, then it does not matter when you download it.

permanent link

answered 15 Apr '12, 10:28

Frederik%20Ramm's gravatar image

Frederik Ramm ♦
71.3k846451113
accept rate: 24%

edited 16 Apr '12, 19:51

1

Frederik, you start your answer assuming what he wants to download is tiles. He asks for a map: why not propose him the vectorial map, that he'd use with an appropriate viewer? That's what Lambertus is doing weekly outputting Garmin versions of the whole world, region by region, so I presume there must be a way to just get the map, without the transformation into Garmin? (Indeed most probably there exist iPad apps that do manage it all, it's just I'm no iPad expert...) Hervé

(16 Apr '12, 19:37) Herve5
1

Indeed. Thank you Herve5, I will modify my answer accordingly.

(16 Apr '12, 19:39) Frederik Ramm ♦

Note that, at least for Navit, you can download vector data for just an area you choose (no need to download planet.osm). That's what http://maps.navit-project.org/ is for.

(17 Apr '12, 07:55) sleske
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question asked: 15 Apr '12, 06:00

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last updated: 17 Apr '12, 07:55

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