<html> <head> <title>BLX -- Magellan BLX Topo File Format (available from GDAL 1.6.0)</title> </head> <body bgcolor="#ffffff"> <h1>BLX -- Magellan BLX Topo File Format</h1> <p> BLX is the format for storing topographic data in Magellan GPS units. This driver supports both reading and writing. In addition the 4 overview levels inherent in the BLX format can be used with the driver. </p> <p> The BLX format is tile based, for the moment the tile size is fixed to 128x128 size. Furthermore the dimensions must be a multiple of the tile size. </p> <p> The data type is fixed to Int16 and the value for undefined values is fixed to -32768. In the BLX format undefined values are only really supported on tile level. For undefined pixels in non-empty tiles see the FILLUNDEF/FILLUNDEFVAL options. </p> <h2>Georeferencing</h2> <p> The BLX projection is fixed to WGS84 and georeferencing from BLX is supported in the form of one tiepoint and pixelsize. </p> <h2>Creation Issues</h2> Creation Options: <p> <ul> <li> <b>ZSCALE=1</b>: Set the desired quantization increment for write access. A higher value will result in better compression and lower vertical resolution. <li> <b>BIGENDIAN=YES</b>: If BIGENDIAN is defined, the output file will be in XLB format (big endian blx). <li> <b>FILLUNDEF=YES</b>: If FILLUNDEF is yes the value of FILLUNDEFVAL will be used instead of -32768 for non-empty tiles. This is needed since the BLX format only support undefined values for full tiles, not individual pixels. <li> <b>FILLUNDEFVAL=0</b>: See FILLUNDEF </ul> </p> </body> </html>
