[mkgmap-dev] help needed

Sat Aug 2 17:00:57 BST 2014

Hi Gerd,

 > Reg. Precision:
 > "Correct" results are an illusion.

I don't know what exactly the problem is, so my idea was more generic. I 
mean that program speed is only a secondary requirements, I wouldn't 
hesitate to use a suitable algorithm even if it makes program slower.

 > I am now going to find out if Garmin draws the line a-c
 > (saved in the img) as a great circle or not, depending
 > on that we have to use different algos.

MapSource and BaseCamp display line the same way as JOSM, you can see it 
on picture from my previous mail. Orange lines are highways from your 
OSM file.

Is it a problem of simplification of long lines that distorted 
geographical grid? I think you won't get an easy solution here, since it 
could depend on source data.

If source data assume that 2 points create line on great circle, then it 
won't be displayed correctly in Mapsource or JOSM. If source data 
contains multiple intermediate points on great circle, than 
simplification which use great circle distance, will remove intermediate 
points and damage correct presentation of lines.

My first idea is to simply leave points, that are far enough form next 
points on line. Far enough could be for example 1km, or 1000 times of 
simplification distance. This won't increase much map size but preserve 
long line shape, regardless of calculation method used for simplification.

Other problem could be splitting of objects, when truncating them to 
tile area and TRE subdivisions. Probably here should be used great 
circle algorithms.

-- 
Best regards,
Andrzej