use Cartesian when GeoTiff missing SRS

in this case, the user is expected to know how the grid should be
referenced.

Applying Cartesian makes it clear enough that karta doesn't know, while
the previous behaviour (WGS84) could be misleading since it's a proper
reference system.

fixes #37

karta/raster/_gdal.py

@@ -239,10 +239,10 @@ def read(fnm, in_memory, ibands=ALL, bandclass=CompressedBand):
                           "flattening": sr.GetInvFlattening(),
                           "name": sr.GetAttrValue('PROJCS')}
         else:
-            hdr["srs"] = {"proj4": "+lonlat +proj=WGS84",
+            hdr["srs"] = {"proj4": "",
                           "semimajor": 6370997.0,
                           "flattening": 1.0 / 298.257223563,
-                          "name": "unknown"}
+                          "name": "NA"}
 
         max_dtype = 0
         rasterbands = [dataset.GetRasterBand(i) for i in ibands]

karta/raster/read.py

@@ -1,7 +1,7 @@
 """ Functions for reading raster data sources as RegularGrid objects """
 import numpy as np
 from .grid import RegularGrid
-from ..crs import ProjectedCRS, GeographicalCRS
+from ..crs import ProjectedCRS, GeographicalCRS, Cartesian
 from . import _gdal
 from . import _aai
 from ..errors import GridError
@@ -50,7 +50,10 @@ class of band used by returned grid (default karta.band.CompressedBand)
          'sx'       : hdr['sx'],
          'sy'       : -hdr['sy']}
 
-    if proj4_isgeodetic(hdr["srs"]["proj4"]):
+    if len(hdr["srs"]["proj4"]) == 0:
+        # invalid or missing SRS information
+        crs = Cartesian
+    elif proj4_isgeodetic(hdr["srs"]["proj4"]):
         geodstr = "+a={a} +f={f}".format(a=hdr["srs"]["semimajor"],
                                          f=hdr["srs"]["flattening"])
         crs = GeographicalCRS(geodstr, name=hdr["srs"]["name"])