2009/12/28 Mark Burton <markb@ordern.com>:
Hi,
I'm trying to understand how the --generate-sea stuff works. I want to know how it decides whether an "island" is water or land. The code does not really contain sufficient comments for me to work out what it's doing. I would expect it to close coastline segments that reach the tile boundary in a direction that is consistent with the "water on the right" convention but I can't see that in the code. Can anyone help please?
I hesitate to whine about contributions to mkgmap as I don't want to stop them arriving but if we can't easily work out how the code works then it doesn't bode well for maintenance and bug fixing when the original author has moved on.
I have agreed to help produce a marine map of the Baltic so it would be really nice if the sea was filled in without any really major problems (flooded land, etc.) I realise that this is dependent on the performance of the MP stuff but it seems to me that generating the right polygons in the first place would be a good start.
Hi Mark! I can't help you with the code directly, but I'm facing a similar problem because i use elevation contours as polygons for visualisation of terrain shapes and have to do a _lot_ of manual fixing with JOSM just to close the ways of my rather small srtm tiles. What I do is the following (described how i think a program could do it): 1. find the end node of a way tagged with the lowest elevation level that is on the tile boundary. 2. as my srtm tiles are generated by "ground truth", contour lines always go clockwise around a peak (higher terrain is on the right hand side). So I follow the tile border clockwise until I either hit a corner node (i have to insert them manually), which I would then append to the way and continue, or hit a start node of a contour with the same elevation, which I would combine with my way and continue *at the new end node of the current way*(this is important because there can be other ways in between which should form a polygon on thier own). 3. If I hit the start node of the way i'm currently working on, i close the way and look for others I did'nt touch before 5. if there are none left, i add all counter-clockwise ways not touching the boundary to a multipolygon with role=inner and move on to the next elevation step I think the scheme is pretty simple and should work for sea, but I since I can't write code, I can't create a tool that does the job for me. If you work on the generate-sea-code, maybe you could consider to make the close-ways-(anti)clockwise-around-tile-borders-to-form-polygons functionality usable for other things, too - such as creating usable polygons from elevation contours? I'll upload a 1°x2° test area map of this stuff within the next days, in case someone wonders what the hell this guy is talking about. ;-) -Martin