[mkgmap-dev] Overview map: tiny patch/review series

Wed Jun 17 00:02:55 BST 2009

Please try to compile the attached "empty" map image as overview map 
with mkgmap. maptk compiles it nicely, cgpsmapper too, mkgmap produces 
crap only (and actually needs to split it before it can process it). 
Currently I therefore use maptk for overview map compiling, and gmaptool 
to create the overview map. I must say I only use empty overview maps, 
is there actually any speed advantage of a distinct real overview map, 
compared to putting in a level 14 consisting only of points of country 
capitals, or maybe country boundaries?
If there is maybe it would make sense to only render 18-20-22-24 levels 
in default mkgmap style, and create a nice overview image for layer 16 
and below.

So I think there is definitely somethings where mkgmap still fails. Or 
is this only because mkgmap cannot correctly compile overview map images 
from mp sources?

The overview image is from: 
http://familiehuzzel.spacequadrat.de/GPS/Typefiles/index.html
Dunno if for something this simple any copyrights may apply, so I add 
the source.

Thilo Hannemann wrote:
>
> Am 16.06.2009 um 18:33 schrieb Marco Certelli:
>
>> Just speculation:
>>
>> I've a and b that defines a line. I look for the distance between a 
>> point p and the line.
>>
>> Given the triangle p-a-b where p is the vertex and a-b is the "base", 
>> the area of the triangle can be calculated from the lenght of its 3 
>> sides (pa, pb, ab).
>>
>> After that, since the area is also base x height / 2 we can calculate 
>> the height = area / base * 2
>>
>> well, the height is exactly the distance of the point p from the line 
>> a-b
>>
>> Maybe...
>
> You are right. It is Heron's formula. And from what I see the 
> implementation is correct. Even the coordinates on the sphere are no 
> problem, because the formula itself doesn't use lat and lon and we can 
> assume the triangle to be flat locally. So I think the implementation 
> is quite clever indeed.
>
> Okay.
>
> But that still doesn't explain why the map looks better if I reduce 
> the error distance setting for the Douglas-Peucker filter. The error 
> should be already invisible at the default setting.
>
> Regards
> Thilo
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Hintergrund Weiss Europa.zip
Type: application/x-zip-compressed
Size: 10200 bytes
Desc: not available
Url : http://lists.mkgmap.org.uk/pipermail/mkgmap-dev/attachments/20090617/aaecd42d/attachment.bin 