Hi Jukka, On Jun 21, 2008, at 3:21 AM, Rahkonen Jukka wrote:
> Hi, > > +1 for having an option of generating world files automaticly. > However, I am not sure if your approach would give correct world > files in all cases. My hunch as well, that's why I'd like some feedback. > I have undestood that Mapnik does not respect the bounding box or > envelope that is given to it if the dimensions of bbox does not suit > the output image size. Frederik Ramm is working on the same problem > when building the Mapnik WMS server. > Right, I'd be noticing the thread discussing this and it will be interesting to resolve it. > I think there is also a minory error in your code in this: > upper_left_y_center = extent.maxy+(scale/2) > Thanks, I'll take a look. - dane > In northern hemisphere I believe it should be upper_left_y_center = > extent.maxy-(scale/2). But the main problem at the moment is that > Mapnik may change your envelope without giving any information of > the geographical extents it is really using for rendering the output > map. It is possible to control the extents accurately by > precalculating the requested geographical extents and output image > size to suit each other but it is a bit complicated for my mind and > I consider it as error prone. > > I suppose that developers will correct me if I am giving wrong > information. > > -Jukka Rahkonen- > > > Excerpt from Mapnik users mailing list from March 30, 2008: > >> For example, if I give these ll extents in generate_image.py: >> ll = (24.0,60.0,25.0,61.0) >> >> the resulting map actually covers this area: >> ll (23.4841,60.0,25.5117,61.0) > >> I have taken the latter coordinates from Information Freeway >> coordinate display and verified them with GPS. >> >> I have repeated this test by different ll values dozens of times >> and the result is always the same. Latitude values in the >> resulting maps are the same I asked for, but not the longitude >> values. > > Artem Pavlenko answered: > > You get slightly different extent because mapnik internally adjusts > extent to fit map into provided image dimensions. > In your case 1x1 degree square bounding box in geographical > coordinate system translates into 'rectangular' shaped bounding box > in Mercator with width/height ratio about 0.49318281553588506. It is > slightly less then 0.5 and I suspect your Map object has width/ > height=0.5 > > There are few ways to address this. For example, you can ensure that > bounding box matches aspect ratio of the Map object (width, height). > Or initialize Map object with dimensions necessary to fit your > bounding box. > _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
