[Xastir] Can I use MrSID files in Xastir?

[Tom Russo]

> -------- Original Message --------
> Subject:      Can I use MrSID files in Xastir?
> Date:         Wed, 01 Apr 2009 14:16:29 -0500
> From:         Jim Tolbert <RiverRidge at Centurytel.net>
> To:   XASTIR <xastir at xastir.org>
>
>
>
> Hi, all...........
>
> I have access to the aerial photos of our county.  They are in MrSid
> format (Multi resolution Seamless Image Database) and are used in the
> Arcview system by the county land information department.  The mapping
> technician tried to output it a few times, but got no where.

No, you can't use MrSID format images directly in Xastir. 

You *can* possibly get a MrSID SDK to link in with GDAL, and use gdal_translate 
to pull them in to regular GeoTIFF format.  Last time I checked, though, 
Lizardtech's SDK for Linux was so old you couldn't link it to GDAL with any 
current generation version of GCC.  If anyone had ever followed through on 
the promise to support GDAL rasters in Xastir, then having the MrSID 
SDK linked in with GDAL, and building Xastir with GDAL support would have 
covered this situation, but then again if we had some ham we could have 
ham and eggs if we had some eggs.

On the upside, you can usually use the free tool from Lizardtech called 
"mrsiddecode" to uncompress the images into GeoTIFF format.  I've done this 
with some of our county's high-res aerial photos and it usually works well.
On occasion, though, I've had the durned mrsiddecode tool simply segfault on
an image with no path forward.  Sometimes, too, the mrsiddecode tool doesn't
properly georeference the GeoTIFF file and you have to make it produce a 
TFW file, then use gdal_translate with that TFW file to produce a properly
georeferenced file.  I haven't figured out why it does that sometimes.  It
could be related to how the MrSID file was produced (or what tool was used
to produce it).

Be warned:  MrSID is a *phenomenally* efficient compression algorithm.  Once
you use mrsiddecode or gdal_translate to make a GeoTIFF file that Xastir
can use, the file might easily be 50 times larger than the original.  High-res
aerial photos tend to produce *enormous* GeoTIFF files, even when you remember 
to specify options to enable PACKBITS compression on the Tiff file.  This fact
is what pretty much prevents me from bothering most of the time.  

Oh --- the other thing that bites you is that usually the GeoTIFF you get
will be an RGB image (three bands) which Xastir also can't use directly.  You
then have to dither the thing into a 1-band colormapped file.  You can do this
pretty easily with the GDAL tool rgb2pct (Red Green Blue to PseudoColorTable).
This will decrease the file size somewhat, but it'll still be enormous.  You
might want to invest in a terrabyte USB drive or two if you're gonna make a 
sizable collection of these images.

-- 
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 

-- 
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