Isohypse Binary File
Introduction
This page describes a new format, called isohypse binary file (or IBF in short). It is intended for storing isohypses (a.k.a. contour lines, relief contours) in a compact form. The format was created by User:Breki, author of several OSM tools (Srtm2Osm, Maperitive) which can generate isohypses using SRTM data.
Why A New Format?
Tools like Srtm2Osm can use SRTM data provided by Nasa to generate isohypses for a given area. Isohypses are then saved in an OSM XML file. Past experience has shown that this is not a good approach. The main deficiencies are:
- OSM XML is a very verbose format and the generated file can have a size in hundreds of megabytes even for a small country.
- In order to use contours from an OSM XML file, all the file data has to be loaded in memory. One of the reasons for this is that an isohypse is represented as an OSM way and a collection of OSM nodes, which are stored separately in the file.
Goals For The New Format
- To hold the data in a condensed non-compressed lossless form.
- To make fetching of the isohypse data for a particular geographical area relatively easy (i.e. without requiring the whole file to be loaded and processed in memory).
Current Status
Currently the C# library for IBF is being developed by me (User:Breki). I've rewritten the isopleting algorithm and now I'm developing a new command line tool to generate IBF files from SRTM data for a given area (and other IBF-related functions). Additionally, GroundTruth will be extended to support the IBF format in the future. --Breki 20:46, 9 March 2009 (UTC)
Format Description
General Architecture
The IBF file holds isohypses for one or more rectangular (min lat, max lat, min long, max long) geographical areas. Typically these areas will be of the same size and will be disjunct, but this is not enforced by the specifications. Also, the optimal size of a single area still has to be determined by experiment - it should be small enough to enable normal processing of isohypses on a typical PC, but large enough not to cause the relief to be split into too many small pieces. The size will also depend on the resolution of the data - areas with isohypses generated from SRTM3 dataset will probably be larger that those generated from SRTM1, because SRTM3 uses a lower resolution grid.
IBF File Header
Length: 8 bytes
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 3 | IBF format marker | IBF |
| 3 | 1 | IBF format version (major) | 1 |
| 4 | 1 | IBF format version (minor) | 0 |
| 5 | 3 | currently not used |
Area Directory
Length: 4408 bytes
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 44 | area directory entry 1 | 0 if empty |
| 44 | 44 | area directory entry 2 | 0 if empty |
| 4356 | 44 | area directory entry 100 | 0 if empty |
| 4400 | 8 | offset (in bytes) of the next area directory | 0 if this is the last directory. Otherwise, it's calculated relative to the start of first area definition of this directory |
Area Directory Entry
Length: 44 bytes
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 8 | area definition offset (in bytes) | relative from the starting position of this entry in the file |
| 8 | 1 | type | 0 represents a raster-based isohypse area (like that from SRTM3) |
| 9 | 3 | currently not used | |
| 12 | 8 | area's minimum latitude (double) | |
| 20 | 8 | area's maximum latitude (double) | |
| 28 | 8 | area's minimum longitude (double) | |
| 36 | 8 | area's maximum longitude (double) |
Area Definition Header
Length: 12 bytes
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 2 | elevation entries count | |
| 2 | 1 | elevation units | 0 for meters, 1 for feet |
| 3 | 1 | currently not used | |
| 4 | 4 | latitudinal resolution | in points per degree. Example: SRTM3 has a resolution of 1200 points per degree |
| 4 | 4 | longitudinal resolution | in points per degree. Example: SRTM3 has a resolution of 1200 points per degree |
Elevation Header
Length: variable
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 2 | elevation (in units) | |
| 2 | 4 | isohypses count | |
| 6 | 4 | 1st isohypse length (in points) | |
| 8 | 4 | 2nd isohypse length (in points) | |
| n*4 + 4 | 2 | n isohypse length (in points) |
Isohypse Definition
Length: variable
| position (in bytes) | length (in bytes) | description | value (example) |
|---|---|---|---|
| 0 | 4 | isohypse points count | |
| 4 | 8 | starting X coordinate | |
| 12 | 8 | starting Y coordinate | |
| 20 | 1 | flags - the first two bits indicate initial direction of the isohypse. The third bit indicates whether the isohypse is closed or not (closed isohypses do not duplicate the first/last point) | |
| 21 | Math.Ceiling (points / 4) | isohypse movements | each 2 bits represent a single movement of the isohypse (0 for left, 1 for forward, 2 for right, 3 for initial point without movement) |
| x | points | elevation factor | from 0 to 255 |
Practical Implications
I did some initial tests on the new isopleting algorithm which uses IBF structure. I created isohypses with 20 meters interval for the area of Sicily. Stored on the disk, the IBF file is about 4.5 MB of size. Compare this to the 450 MB for the file containing isohypses in the OSM format for the same area - the IBF file is thus around 100 times smaller, which makes it much more suitable for storing isohypses of the whole world. --Breki 20:19, 7 March 2009 (UTC)