[Xastir] Map projection (Re: Breaking up shapefiles)

[Tom Russo]

On Sun, Dec 19, 2004 at 09:40:46AM +0200, we recorded a bogon-computron collision of the <oh2kku at iki.fi> flavor, containing:
> Curt Mills wrote:
> >Makes me really look forward to the day when we can pre-process all
> >of our map data as we import it into our datastore, then pull it out
> >at lightning speed in the datum/coordinates we like for drawing.

> This reminds me to ask about the "natural" map projection in Xastir. If 
> I were to construct raster maps, what would be the ideal projection for 
> Xastir? I.e. the map would not have to be rotated or twisted or turned 
> to be seen properly in Xastir, only linearly scaled up or down to fit 
> the zoom level. I assume the datum is WGS-84.
> 

Xastir uses an "unprojected" coordinate system --- lat/long.

If you construct raster maps and want them to be displayed in xastir with
no distortion, make them such that the horizontal coordinate is longitude
and the vertical is latitude.  

Xastir has a limited capability to display UTM rasters in GeoTIFF format, but
it doesn't use an exact transformation and the errors can be quite large if
the image is bigger than 7.5minutes square.  The approximation it uses is
good enough for that size, but not good for the larger rasters like USGS 
1:250,000 scale maps.

The ".geo" method of importing rasters in transverse mercator projection is
not accurate at all.

So your best bet is to generate rasters that are already in lat/long 
unprojected coordinates.  Those will register well with shapefiles and other
vector layers that are in unprojected lat/lon.  These work Just Fine in both
GeoTIFF format and the .geo georeferencing of ImageMagick-readable images.
 
-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 http://www.qsl.net/~km5vy/
 "When life gives you lemons, find someone with a paper cut."