On Fri, Sep 7, 2012 at 11:30 AM, Tom Russo <[email protected]> wrote: > On Fri, Sep 07, 2012 at 10:07:14AM -0600, we recorded a bogon-computron > collision of the <[email protected]> flavor, containing: >> Why not just >> re-project the APRS stations and any shapefile overlay to match the >> raster tiles? Wouldn't this be much faster? > > Yes, it would, but then you'd have to deal with the issue that many rasters > aren't in the same projection, and the vector overlays are often in something > else, too.
AFAIK, all the web tile services are Mercator. The issue here is finding a way to make the web tile services render quickly. My hunch was that vector overlays would be less computationally expensive to reproject, so leaving the tiles in Mercator would be most efficient. > So the approach is that the Xastir coordinate system is basically Platte Carre > (equidistant cylindrical) with a scaling so that the entire globe is > represented > by long integer coordinates. Everything is done in that coordinate system, > and maps have to be reprojected. > > >> I normally use Xastir with a USGS topo tiles base layer (from ESRI, >> using the OSM tile loader) with an OSM Cloudmade shapefile overlay. > > If you really want topo maps as a base layer, you're better off getting > USGS GeoTIFFs and doing the reprojection once, converting them to WGS84 > lat/lon instead of NAD83 or NAD27 UTM. This way, no projection is done at > render time. You can do the reprojecting using gdalwarp. I occasionally use USGS DRG GeoTIFFs, but I prefer the scanned topo tiles. The DRGs don't look as nice because they're limited to ~5 colors (or something like that), not millions of colors, meaning there's no antialiasing on the text and contour lines. This makes a big difference in readability, at least for me. Where would be a good place to improve efficiency? My two ideas so far: 1. Have Xastir reproject each tile before caching it (rather than caching the original download), so they do not need to be reprojected on each cache read. I don't know if this idea is compatible with how Xastir stitches tiles for display. 2. Add a "fixed zoom Mercator" mode to Xastir where tiles are displayed in their native size/shape and everything else gets reprojected. This sounds really difficult to implement :-) Tom KD7LXL _______________________________________________ Xastir mailing list [email protected] http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
