I don’t think our current tools are able to identify the roads that cross different tiles. So currently you could use GPSMapEdit to add the necessary information to the map images. This is described in the wiki.
That’s an old posting before liosha’s script was available 
My solution to the routing across tiles problem is more of a hack, rather than a solution. I use osmosis to split my OSM data into tiles, and the connecting nodes of the splitted roads are manually tagged as “garmin:is_external_node=true” (they are currently not that many in OSM Malaysia). In liosha’s script I added a conditional statement that will change the nod values for these connecting nodes (see http://forum.openstreetmap.org/viewtopic.php?pid=13274#p13274)
My previous tests shows that it works when the tiles are square (the connecting nodes are all at tile borders). I’m not sure how it’ll work if the tiles are irregular as in the shape of a state or country. Had a plan to experiment on this but haven’t got started yet 
It’d be great if osmosis or any tile splitter tool can automatically tag the connecting nodes as such, rather than doing a major overhaul of your current tools.
Oh ok, but it’s about the same method as described on the mkgmap/routing page.
Tagging nodes as routing node manually is out of the question for me. It should be done automatically or it doesn’t work 
If the tools are good at detecting edge nodes then irregular shapes should also work imho.
Yes, that would be awsome, but until now neither Osmosis as well as Osmcut are able to split tiles properly (let ways continue right onto the edge and insert dummy nodes on the edge). That’s why every Garmin map created with OSM tools shows gaps in the roadnetwork near tileborders.
Kevin Lindsey has an excellent intersection routines example done with javascript/svg. Could be useful in finding locations of the dummy nodes.
I think the problem with the current planet splitting tools like (osmosis and osmcut) is that it requires a lot of preprocessing or RAM to gather all the road polylines. Determining the intersection with a bbox when you have the polylines available is CPU intensive (but doable), but I guess the OSM XML format isn’t very suitable to read polylines from.
I see three possible solutions:
Load all ways into memory (lots of RAM needed) and check each node to see to which way it belongs (CPU intensive), then save the road/node combination as polyline. Finally check each polyline agains every tile bbox to determine if crosses the border. Then add a node on the border position. Then split the way on the border node and save the resulting ways in the individual tile files. That is a lot of work.
Do as described above, but convert the planet file into a shape file first using osm2shp (dunno what how much RAM/CPU time that requires).
While writing this I forgot the third option I had in mind 
Edit: I cannot find any OSM to Shapefile converter in the SVN repository, so this leaves us at the first option…
Hi all!
I’ve updated osm2mp in svn.
Now it supports maxspeed and access=private
Do you mean you add new nodes to the way where it crosses the edge and tag those as “garmin:is_external_node=true”, or do you tag the nearest existing nodes to the edge that way? If it is the 2nd option, how does the Garmin know which route nodes connect to each other?
I’m looking at osmcut currently to do the job. Osmcut already knows which ways cross a tile edge so adding some more functions to enable routing shouldn’t be very difficult.
Yes, the new nodes should be right at the edge of the tile.
Ok, should the node have the same id in both tiles?
Doesn’t have to be the same id. Here’s an example (in polish format) of a way that extends to another tile. The most important thing is that the coordinates should match up, and it is exactly at the edge of the tile.
Tile 1
[POLYLINE]
Type=0x01
EndLevel=4
Label=~[0x04]E2 NSE
DirIndicator=1
Data0=(3.0391936,101.7062182), … (2.6274582,102)
RoadID=1299
RouteParams=6,4,1,0,0,0,0,0,0,0,0,0
Nod0=0,1046,0
Nod1=
…
Nod16=224,2649,1
[END]
Tile 2
[POLYLINE]
Type=0x01
EndLevel=4
Label=~[0x04]E2 NSE
DirIndicator=1
Data0=(2.6274582,102), … (2.1308003,102.7412188)
RoadID=285
RouteParams=6,4,1,0,0,0,0,0,0,0,0,0
Nod0=0,1143,1
Nod1=56,462,0
Nod2=97,754,0
Nod3=215,375,0
Nod4=223,769,0
[END]
Some info from Rottweiller Auto-Routing guide (via http://cgpsmapper.com/route.htm))
All of the above are if we are using the gpsmapedit method. It seems that maproute can detect and join the matching nodes automatically, which I think is the best way to work with irregular-shaped maps.
Saved by the bell for me 
http://www.mkgmap.org.uk/page/tile-splitter
Now only Mkgmap needs to be adapted to set the correct edge routing information.
The following is a copy from the IRC talk about this, contains some additional info:
Great stuff indeed 
Could someone point me to wiki article or similar describing the tiles you are referring to?
What defines the tile edges? I create map from southern half of finland. Looking at the link http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=http:%2F%2Fwww.mkgmap.org.uk%2Ftmp%2Fareas.kml&ie=UTF8&z=3
seems that my map contains 3 tiles. Is that the case?
The tilesplitter has some kind of tile optimisation algorithm which aparently produces these tile bboxes. Luckily you can define your own bboxes as well, placing the while of Finland in a single tile and have the algorithm determine the other bboxes.
This is my result when working with half the planet (Europe, Asia, Africa and Oceania)
That’s nice, prolly a picture of the month… And also fun seeing Google maps being slow.
Hi all!
I’ve added basic cropping functionality to osm2mp. It also creates boundary road nodes.
This time available in svn only.
Use --bbox <minlon,minlat,maxlon,maxlat> to specify boundary box.
Does this mean that I can take one large file (say country.osm), split it into smaller tiles via multiple runs of osm2mp with several matching --bbox’es and routing will work across tile borders?
weolvi,
yes, you can. And routing will work 
You can also use tile splitter first, and then osm2mp with bboxes from areas.list
I’ve successfully built map of Russia such way: http://gpsmapsearch.com/osm/mp/
Note that turn restrictions disabled this time
Awesome! It looks like we now have a full chain of software capable of generating routable Garmin maps without human intervention for the first time.
Very cool and good work liosha!
/me is going to implement an automated chain this weekend