#mapnik log: Friday 27, March 2009

2009 | 03

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00:17:53 <nikq> Mapnik Trac: Changeset [1021]: - added paths.py.in -> a template to generate paths.py with automake -  ... | http://trac.mapnik.org/changeset/1021
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02:29:56 <dukeku> gis conference prices are nuts
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09:22:25 <navy> hello ! how can i see Artem Pavlenko?
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13:44:16 <synax> what unit of measurement does Evelope() store its coords in?
13:45:53 <Berteun> Depens on the projections.
13:45:55 <Berteun> projection*
13:50:13 <synax> latlon
13:50:21 <synax> it's not outputting lat/lon
13:50:33 <Berteun> What projection string do you use?
13:50:34 <synax> I'm getting values like 448198.52
13:51:31 <synax> ah, I guess I never specified a projection
13:55:35 <synax> brb
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14:13:35 <synax> Berteun: I've set my layer.srs to "+proj=lonlat +ellps=WGS84" but layer.envelope() is still issues these kinds of coords: 445310.12,4951192.32,448198.52,4954812.56
14:16:08 <synax> Berteun: I'm trying to get the bounds of my shapefile to feed into generate_tiles.py
14:18:08 <Berteun> I don't get it then…
14:18:32 <synax> what can I clariy?
14:18:36 <synax> *clarify...
14:19:01 <Berteun> What shapefile are you using?
14:19:25 <synax> as in, where did I get it?
14:19:55 <Berteun> Yeah, just an example?
14:20:03 <Berteun> Or your own shapefile?
14:20:07 <synax> I don't know what projection it's using... I have been able to generate a map w/ all the shapes it contains using lonlat projection
14:20:25 <synax> We acquired these shapefiles from our local goverment
14:20:36 <synax> they don't have accompanying .prj files...
14:21:34 <Berteun> I don't really know how this works actually.
14:21:52 <Berteun> My guess would be that if the .shp files gives you coordinates like you say, that it isn't actually in lat/lon format.
14:22:23 <Berteun> What's the area of the shapefiles? Whole world, or just a specific country?
14:22:58 <synax> much smaller - it's a city
14:23:06 <synax> less than a city actually
14:25:47 <synax> are there any utilities/tools out there that can read my shapefile(s) and give me the projection info?
14:26:11 <Berteun> That's a good question!
14:26:24 <Berteun> I would be inclined to say no.
14:26:34 <Berteun> But, what country are you in?
14:26:45 <Berteun> For example the Dutch government (where I am) always uses 'Amersfoort new'.
14:27:17 <synax> Canada
14:27:23 <Berteun> So if you know that, you could lookup the probable projection, and try to convert those coordinates you get from the shapefile (445310.12, etc) to lan/lot.
14:27:27 <Berteun> And see whether it makes sense.
14:28:34 <synax> how would I convert the coords, assuming I can figure out the original projection?
14:30:32 <Berteun> You're using Python?
14:30:37 <Berteun> You can do something like:
14:30:42 <Berteun> from mapnik import *
14:30:50 <Berteun> p = Projection("+proj ....")
14:30:57 <Berteun> p.inverse(Coord(X, Y))
14:31:08 <Berteun> That gives you lat/lon
14:31:27 <Berteun> Or, p.forward(Coord(X, Y)), which takes you from lat/lon to projected coordinates.
14:31:38 <Berteun> You might want to try one of these:
14:31:38 <Berteun> http://spatialreference.org/ref/epsg/3347/
14:31:41 <Berteun> http://spatialreference.org/ref/epsg/3348/
14:33:07 <synax> ah, so that's what those functions do!
14:35:21 <Berteun> Yes, and normally, if you specify your target projection correctly and your layer.srs correctly, mapnik will do the conversion automatically.
14:36:13 <Berteun> But as far as I understand now, de .shp files just store an X and Y coordinate without extra info, so without the .prj you'll just have to guess...
14:36:14 <synax> "specify correctly" ?
14:37:02 <Berteun> Well, if you would tell mapnik your canadian shapefiles have coordinates in the German system, most probably your city would end up somewhere completely wrong if you would combine that with OSM data.
14:37:19 <Berteun> So you have to figure out the /exact/ projection string (which is sometimes not so easy).
14:37:38 <Berteun> Perhaps it's easiest to ask your local government anyway.
14:38:04 <synax> Both of those projections you linked to seem like they'd work, but I'm not seeing the proj string for them
14:38:09 <synax> proj -l
14:38:35 <Berteun> You can click on the 'proj' on the page.
14:38:46 <Berteun> *slow site*
14:39:01 <Berteun> http://spatialreference.org/ref/epsg/3347/proj4/
14:39:06 <Berteun> And the same for 3348
14:39:57 <synax> aha
14:40:09 <synax> I'm learning so much already ;)
14:40:29 <synax> I've been frustrated with this stuff all week... I'm very green to the whole mapping scene
14:40:53 <Berteun> It's tough. :)
14:41:08 <synax> I *hate* that Mapnik's API is not documented at all
14:41:10 <Berteun> Those projections strings are more or less magic to me too.
14:41:18 <synax> looking at code fragments and trying to guess is annoying as hell
14:53:34 <synax> does inverse() output long, lat or lat, long?
14:55:50 <Berteun> Depends whether you input lon/lat or lat/lon :)
14:56:40 <Berteun> In shapefiles X = usually longitude, Y = latitude.
15:09:38 <synax> you're a great resource, Berteun ;)
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21:10:26 <nikq> Mapnik Trac: PluginArchitecture edited | http://trac.mapnik.org/wiki/PluginArchitecture?version=12
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21:38:10 <nikq> Mapnik Trac: Changeset [1022]: changed logging | http://trac.mapnik.org/changeset/1022
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23:39:12 <nikq> Mapnik Trac: XMLGettingStarted edited | http://trac.mapnik.org/wiki/XMLGettingStarted?version=8
23:44:47 <ser> "OSM Mapnik database is updated with hourly diffs so that most data changes should get rendered within an hour."
23:44:50 <ser> where can i find these scripts?