[Xastir] Maritime Charts on Xastir

[Tom Russo]

On Tue, Sep 29, 2009 at 10:11:02AM -0700, we recorded a bogon-computron collision of the <archer at eskimo.com> flavor, containing:
> On Tue, 29 Sep 2009, Jason KG4WSV wrote:
> 
> > A bit of googling around leads me to suspect you can download the
> > "RNC"s (raster files), then use an apparently free utility to
> > re-project and convert them to geoTIFF (I'm guessing GDAL would
> > convert them, too).
> 
> Xastir only handles geoTIFF in certain formats:  Those used for USGS
> DRG's.  The coordinate system has to be in lat/long, and it can't be
> tiled, it must be raster lines.  There are some other considerations
> as well but I don't recall them off-hand.

You misremember.

The coordinate system may be in UTM or lat/lon.  Originally, it was just UTM,
but the lat/lon was an extension added so that you can use gdalwarp on
all sorts of odd projections (e.g. Lambert Conformal Conic, which is used in
aviation charts), per instructions in README.MAPS.

> There were tweaks at one point done to be able to handle the DOQQ
> aerial image files.  I think that had to do with the skipping of the
> border chopping and skipping stretching the projection into a
> rectangle (originally needed to tile the UTM-projection DRG's
> together).

DOQQs in GeoTIFF format with UTM projection are handled with no modification,
so long as they're B&W single-band.  Color DOQQs generaly require that the 
3-band color is composited into a single-band colormapped TIFF.
Just don't create or install an associated FGD file.  The FGD file is used only
for collar clipping.  The GeoTIFF projection information is stored in the
tiff file itself and is used for approximate warping from UTM to the Xastir 
coordinate system (scaled lat/lon).  Any GeoTIFF in UTM projection or lat/lon 
will work, but the problem comes when you use maps of smaller scale than USGS 
Topos.  The approximation used for warping is not correct enough to handle 
small-scale maps like 250k topos.  When you try, it'll look like it works, but
there will be significant distortion in the interior of the map --- features in
the image won't line up with overlaid vector data correctly.

The best way to deal with small-scale maps (100k and 250k topos) in UTM or 
maps in other projections is to use gdalwarp to get them into lat/lon (which 
needs no approximate warping in Xastir).  Instructions are in README.MAPS.

-- 
Tom Russo    KM5VY   SAR502   DM64ux          http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236        http://kevan.org/brain.cgi?DDTNM
  In some cultures what I do would be considered normal. 
                                  -- Ineffective daily affirmation