Tiling and projections¶
This tool was intended to be used to preprocess data like creating hillshades
which is used for web maps afterwards. Therefore the output projections
supported are Web Mercator (
mercator) and the geodetic projection based
on WGS84 (
geodetic). For the tiles within these projections, a tiling
scheme based on the WMTS-tiling is used.
It is also possible to configure a fully custom tile pyramid with a custom coordinate reference system.
Every tiling a web framework uses is based on a pyramid of multiple zoom levels (tile matrices). Each tile matrix consists of tiles arranged in rows and colums.
Every tile within a tile pyramid can be described by three numbers: the zoom level, row and column. As these numbers are used in the WMTS protocol, Mapchete can easily map them to the desired tiles to be processed.
Most web maps use the
mercator scheme (or pyramid), the second most used
scheme is the
geodetic scheme which also covers the polar regions but shows
distortions especially in higher lattitudes which are unfamiliar to most users.
Properties of supported pyramids¶
|zoom 0 rows||1||1|
|zoom 0 cols||2||1|
If you want to dig deeper into the topic, there is a more detailed description on the ẀMTS Simple profile standard by the OGC.
Internally, Mapchete uses tilematrix, a python tool handling tiles and tile pyramids. It is not planned to support other projections or tiling schemes yet, but if there is demand and resources, tilematrix would be the starting point.
Depending on the process it sometimes makes sense to increase the tile size.
metatiling setting of 2 means that a metatile consists of 2x2 WMTS tiles.
Increasing the metatile size often also increases performance as some overhead
is reduced. However, having huge metatiles could also mean RAM is running out
at some point.
Mapchete lets you to specify which metatile size is being used when processing the data and which size is used when writing the outputs. Please note that the output tile size cannot be bigger than the process tile size.
Sometimes there can be artefacts on tile borders. This can be if the process requires neighbor pixel values for every pixel (e.g. the hillshading algortihm) or when reading raster data in some different projections.
In these cases, a
pixelbuffer value can be set. This will enlarge each tile
on each side by the numbers of pixels specified. This option can be set for both
process tiles and output tiles. By default, the output tiles pixelbuffer is set
to 0, which means that when writing the process output, the buffer will be
Please note that the output
pixelbuffer cannot be larger than the