[mkgmap-dev] Overview map

Tue Jun 19 17:02:52 BST 2012

Well it's a long time since the last commit to the overview branch, but 
it would be very great if we could get a working overview map...
Why is there a need for it?
- Well currently most people build mkgmap maps down to resolution 16, 
using map information, else in Mapsource/Basecamp/Qlandkarte there is 
simply not enough zoomout/speed possible for maps covering bigger regions.
However, on the GPS this means, that the much quicker loading and better 
suited overview map will be used too late (with mkgmap trunk default 
style, I'm not sure if it is used at all, because there is no "empty" 
resolution).

Also this would help everyone who is running out of levels, and hence 
has to make a compromise in the number of levels - as the max number of 
8 levels (0-7) is not really enough if you would like to show a nice map 
when zoomed out very far on the desktop.

The old overview branch, had, IMHO the main problem, of not being 
configurable. It would be great if the overview map could be created 
using the following scheme:

1.
a)Create a completly new lines, polygons, points, relations and options 
style-file so one can specify what goes into the overview map.
e.g. lines.overview; polygons.overview;

or
b) alternatively, just change the format of the options file to 
something like:
levels = 0:24, 1:22, 2:21, 3:20, 4:19, 5:18, 6:16
levels_overview= 0:16, 1:14, 2:12, 3:10

I think b) is more practical, as it allows to more quickly change the 
level of detail for the overview map as it shouldn't become too big. So 
e.g. if you create a map of Europe you use the above figures, but if you 
create a map of a small country/region like e.g. Austria, you could 
change it to
levels = 0:24, 1:22, 2:21, 3:20, 4:19, 5:18
levels_overview= 0:18, 1:16, 2:14, 3:12, 4:10

2. Add an additional parameter for the douglas-peucker algorythm. As for 
the overview map, there neeeds to be very strong filtering - though I 
don't know the size limit for an overview map.

3. Maybe, as for the coastline/sea and boundary information, mkgmap 
could use a drastically  reduced dataset, to help speed up the map 
creation, and make it easier to fix things.

------------

A nice example of an overview map, is included in the Topo Hispania. It 
has the following levels:
0:16, 1:15, 2:14, 3:13, 4:11, 5:10
(with level 5 being empty) -- the normal map files have 0:24, 1:23, 
2:22, 3:21, 4:18, 5:16, 6:14 (with level 6 being empty)
Even though it is only 186KB big, it has quite a lot of information of 
Spain in it.  For everyone not wanting to download the full topohispania 
(v2) I uploaded it to openmtbmap.org/topohispania.img
It would be great, if mkgmap could produce something similar (though of 
course it will be more difficult, with changing data - though point 3 
could help to give us the same quality).
(note tophispania.img may not be used commercially/sold)

On 25.07.2010 13:35, Steve Ratcliffe wrote:
> Hi
>
> I have started to implement adding features into the overview
> map.  You can find ready built jar files on the download page
> and in the 'overview' branch in subversion.
>
> It creates the overview map from the .img (as before) and so it
> is very quick to try out even on a set of tiles covering a
> whole country.
>
> There is no configuration yet, it just takes everything (lines and
> polygons) that is in the least detailed layer of the detailed maps and
> adds it into the overview.
>
> You will immediately notice a big problem in that the overview map
> shows up even when the detailed maps are being displayed, so you
> get ugly jagged lines across the top of the map.
>
> I remember this happening the last time we has an overview map
> and I wonder if anyone can remember if there was a solution then?
>
> I'm going to start by trying the (so-called) POI flags, as one of those
> has been noted to be set differently on detail and base maps before.
>
> ..Steve
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