Read/Write driver for TMS tiles

This work is continuation of the SoC 2007 project GDAL2Tiles. The goal is to create a new driver for reading and writing maps in TMS tiled format.

Data format

TMS tiled maps

The driver will operate on data in the ​OSGeo TMS TileMap Resource format used already by GDAL2Tiles. In short, the map is described in one XML file. Image data is divided into several zooming levels represented by directories containing tiles as individual files.

Tiles

The TMS specification doesn't prescribe the format of tiles but PNG and JPEG are the only useable formats in web setting. Both have existing drivers in GDAL. The TMS driver will use only PNG tiles at first. Extending the support to JPEG tiles as well is possible next step after implementing both reading and writing functionality for PNG.

Work plan

The work is divided into several steps. First two are essential, all others are optional.

Read driver

First step is to create a read-only driver. The Open method will be called on the main XML file describing the map. The <TileSet> URLs will be expected to be paths relative to the main file. This is the case with data produced by GDAL2Tiles. When this expectation is not met, the driver could try to guess the names of directories using the last part of the URL and the base directory of the file being opened. Another approach is to use auxiliary file describing the layout, this is similar to the way the WMS driver works.

Although the tiled map has several zooming levels, only the lowest one will be used. IReadBlock requests will be handled by loading data from the files representing the selected area using existing PNG driver.

Write driver

Next on plan is the writing part. The driver will support the CreateCopy method returning the created dataset as ReadOnly (implemented in previous step). This will allow to export existing datasets in tiled format. Creation options will include the dimensions of tiles, number of zooming levels and the zooming ratio between them.

Implementing Create method and Update mode dataset is questionable. The PNG driver has this mode controlled by compile time option, so the TMS driver would have to do the same. Even then, the PNG driver imposes some restrictions on the calling side: "Note: callers must provide blocks in increasing Y order.", from pngdataset.cpp. This would translate to the TMS driver itself and its users too. JPEG drivers have CreateCopy method only.

Performance tuning

The reading strategy at this point will be probably quite simple -- creating new datasets for appropriate tiles in the IReadBlock method and getting data from them. This could prove to be inefficient. There are several ways to solve this issue:

• Holding all tiles opened -- could be wasting too much memory.
• Managing a cache of recently used tiles.
• Prefetching adjacent tiles into the cache.

These and other should be explored and the final solution selected based on measured performance.

HTTP Read driver

The TMS tiled maps are mostly used on web, so the ability to read the map from http connection could be quite useful. This step is somewhat related to the preceding one because without intelligent caching, the performance could very well become unacceptable.

Student info

My name is Václav Klusák and I am a graduate student of Applied Informatics at ​Masaryk University in Brno, Czech Republic. I can be reached by email (keo@keo.cz) or ICQ (247 160 335).