Hi Brad, Inline...
On Sep 8, 2010, at 7:24 PM, Simpson Brad-C-Lockheed wrote: > Dane, > > I’m the one who found a Mapnik error when running on big-endian > architectures. Yes, I remember. > I’m hoping to get a little advice regarding projections. Sure. > > I want to use Mapnik on our system to render maps in a standard > longitude-latitude grid. I couldn’t use the unmodified OSM shoreline_300 > file since it is in a Mercator projection, so I tried to convert it. I would use ogr2ogr to reproject the shapefile to epsg:4326. I just ran a workshop that included how to do this: http://dbsgeo.com/foss4g2010/html/getting_stylish.html#reprojecting-shapefiles > I started with the shp2pgsql utility, loaded the result into a PostGIS > database table, then tried to transform the table (loRes) with the commands: > > SELECT updateGeometrySRID(‘loRes’, ‘geom’, 4326); > UPDATE loRes SET > geom.=ST_Transform(SetSRID(geom.,900913),4326); > > To get this to work, I had to remove the ‘+nadgri...@null’ verbiage from the > proj4text field of the 900913 entry in the spatial_ref_sys table. This could > potentially be the cause of why I’m seeing coastlines in the wrong location > (mainly a y offset, but I suspect there may be even a smaller x offset as > well). yes, small offsets seen in data moving to or from spherical mercator does indicate a problem with nadgrids=null. > > Here are some questions I have: > > Where can one find a discussion on the ‘+nadgrids…’ specifier? > not sure. if proj4 wiki does not have one I would recommend creating the wiki page, and poking the proj4 list to contribute to it. I bet it has been discussed on the proj4 list if you can find the archives. > I’ve found references on the web that the OSM files are in a spherical > Mercator. How is a spherical Mercator projection specified with regard to > Proj4? http://spatialreference.org/ref/sr-org/6/ this is also the new epsg:3857 (which we need to update spatialreference.org with...) > When using QuantumGIS to plot shape files, processed_p and shoreline_300, > then plot on top of each other, but world_boundaries_m plots offset from them > in the y axis. Aren’t all three of these shape files supposed to be in the > same projection? well they should be, but are not. world_boundaries_m is in standard mercator, I think epsg:3395 (which will likely lack the nadgrids=null and therefore will look shifted if you assume it is in spherical mercator). > > However, my main question is how to I convert the shapefile shoreline_300 > into a standard longitude-latitude grid? ogr2ogr > > Any advice would be appreciated. Thanks, > > Brad Simpson
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