====== Common HF2/HFZ extended header blocks ======

^ << [[l3dt:formats:specs:hf2|Back to HF2 spec page]] ^

===== Georeferencing =====

==== georef-extents ====

The "georef-extents" block describes the real-world extents of the file. It is largely copied from the georeferencing data in the [[http://www.vterrain.org/Implementation/Formats/BT.html|VTP Binary Terrain]] file header. The block contains 58 bytes, the structure of which is described below:

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "bin". |
| Block name |  4  |  16  |  string  | "georef-extents" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | 34 (bytes following block length). |
| Extent units |  24  |  2  |  short int  | 0 = degrees, 1 = metres, 2 = feet (imperial), 3 = feet (U.S. survey) |
| Left extent |  26  |  8  |  double  | |
| Right extent |  34  |  8  |  double  | |
| Bottom extent |  42  |  8  |  double  | |
| Top extent |  50  |  8  |  double  | |


==== georef-utm ====

The "georef-utm" block identifies the real-world UTM zone of the file. Please refer to the [[http://www.vterrain.org/Implementation/Formats/BT.html|VTP Binary Terrain]] file header for more info. The block contains 26 bytes, the structure of which is described below:

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "bin". |
| Block name |  4  |  16  |  string  | "georef-utm" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | 2 (bytes following block length). |
| UTM zone code |  24  |  2  |  short int  | |



==== georef-datum ====

The "georef-datum" block identifies the EPSG datum code of the file. Please refer to the [[http://www.vterrain.org/Implementation/Formats/BT.html|VTP Binary Terrain]] file header for more info. The block contains 26 bytes, the structure of which is described below:

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "bin". |
| Block name |  4  |  16  |  string  | "georef-datum" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | 2 (bytes following block length). |
| EPSG datum code |  24  |  2  |  short int  | |

==== georef-epsg-prj ====

The "georef-epsg-prj" block identifies the EPSG projection code of the file. The block contains 26 bytes, the structure of which is described below:

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "bin". |
| Block name |  4  |  16  |  string  | "georef-epsg-prj" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | 2 (bytes following block length). |
| EPSG proj. code |  24  |  2  |  short int  | |

===== Miscellaneous data =====

==== app-name ====

The "app-name" block identifies the application that saved the HF2/HFZ file. The block has a variable length. 

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "txt". |
| Block name |  4  |  16  |  string  | "app-name" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | variable (depends on following string length). |
| Application name |  24  |  variable  |  string  | Null-terminated string containing application name, e.g. "Terragen 2", "L3DT", etc. |

==== precis-rel ====

The "precis-rel" block identifies that the vertical precision of the file, if re-saved, should be calculated from the product of the horizontal scale of the map (in metres) and the relative precision factor given in the block body. The block contains 28 bytes, the structure of which is described below:

^ Name ^ Offset ^ Length ^ Type ^ Description ^
| Block type |  0  |  4  |  string  | "bin". |
| Block name |  4  |  16  |  string  | "precis-rel" (padded with zeros). |
| Block length |  20  |  4  |  DWORD  | 2 (bytes following block length). |
| Relative precision |  24  |  4  |  float  | |

Typically in L3DT the relative precision is set to 0.001, which means that when the map is saved, the vertical precision used in the HF2 compression is 0.001 times the horizontal scale (vertex spacing) of the map. This scheme ensures that the precision of the height data scales appropriately with the scale of the terrain.