# BLX -- Magellan BLX Topo File Format

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.

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.

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.

## Georeferencing

The BLX projection is fixed to WGS84 and georeferencing from BLX is supported in the form of one tiepoint and pixelsize.

## Creation Issues

Creation Options:

- ZSCALE=1: Set the desired quantization increment for write access. A higher value will result in better compression and lower vertical resolution.
- BIGENDIAN=YES: If BIGENDIAN is defined, the output file will be in XLB format (big endian blx).
- FILLUNDEF=YES: 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.
- FILLUNDEFVAL=0: See FILLUNDEF