[gdal-dev] Reading S57 Extents from Python

[Mike Slinn]

I realized after looking at the C++ code that entity extents are 
returned as a tuple and not as a method call. In Python this means that 
the extent coordinates can be accessed as slices:

print extents[:2], extents[2:]

Printing them out in Python is a bit tricky due to character encoding 
issues, so here is an example that builds on my previous posting, in 
case anyone ever has need of it:

def degreesNS(deg):
	hemi = "N"
	if deg<0:
		hemi = "S"
	return "%.3f%c%s" % (abs(deg), u'\N{DEGREE SIGN}', hemi)

def degreesEW(deg):
	hemi = "E"
	if deg<0:
		hemi = "W"
	return "%.3f%c%s" % (abs(deg), u'\N{DEGREE SIGN}', hemi)

extentStr = "      Extents: (%s, %s) to (%s, %s)" % \
            (degreesNS(extents[0]), degreesEW(extents[1]), \
             degreesNS(extents[2]), degreesEW(extents[3]))
print extentStr.encode('utf-8')

Unfortunately, it looks like the layer extents are computed on every 
call to layer.GetExtent() by reading every coordinate on that layer.  If 
one wants to know the extents of the entire data file, the call must be 
made on every layer that has data.  This doesn't scale well.

Section 7.4.1 of the S-57 spec (IHO Transfer Standard for Digital 
Hydrographic Data, Edition 3.1, November 2000 - Main Document - 
http://www.iho.shom.fr/publicat/free/files/31Main.pdf) discusses 
Catalogue directory record structure.  CATD(12) is the Catalogue 
directory field, and it contains the extents for the entire chart, 
referenced by subfields with labels (SLAT, WLON) to (NLAT, ELON).  How 
can this information be retrieved from Python?

Mike