Zone File Format (.zfc)

Overview

The zone file format was created to store zonal attributes without needing to load everything into memory. It was originally designed during the Pentium 3 era when RAM was extremely limited and serves very little purpose today. It had a sister format Origin Destination Cache (.odc) that was used for storing OD matrices of different types for different times of day, caching the results to limit memory usage.

Format

The zfc file starts with the following header.

Field	Type
Version	Int32
Types	Int32
Segments	Int32

Version gives the version number of the zfc, typically 2. Types gives the number of different types of data that are stored for each zone. The segments field gives us the number of zonal segments that are contained. Each segment is a range of continuous zones allowing for the zone numbers to have large gaps without using any disk space.

Each segment is then stored with a record in the following format.

Field	Type
Start	Int32
Stop	Int32
DiskLocation	Int64

Start contains the starting Zone number for the segment. Stop contained the last zone of the segment. DiskLocation stores the starting location of the data for this segment.

After all of the segments the data is then stored in order from the first zone for each type to the last as a float32.