[GIS] gdal2tiles doesn’t use resampling method correct

gdalgdal2tilesmapniktiles

I'm using gdal2tiles to create tiles from MODIS satellite imagery (250 m resolution) and I have noticed that I get the same bad result if I use -r near or -r bilinear (or lanczos for that matter). See attached image.

gdal2tiles generated tile

If I do the same generating with Mapnik and generate_tiles.py the images looks so much better.

Mapnik generated tile

The source file has a smaller resolution than the output tile, but I guess that's what the resampling bilinear (or lanczos) should "take care of"? Now it seems like gdal2tiles only scales the image without adding any resampling method?

I've tried to generate the tiles from geotiffs and jpegs (with jgw) but it's all the same.

The reason I want to use gdal2tiles rather than generate_tiles.py with Mapnik is that gdal2tiles are so much faster. Look here: The fastest way of generating tiles from raster?

Best Answer

GDAL2Tiles has been my student project I did for Google during SoC programme. There are some shortcuts in the used workflow made because of limits of the Python binding to the GDAL core.

During the last years we have completely reimplemented GDAL2Tiles into C/C++, fixed all known bugs and added a ton of new features.

Try http://www.maptiler.com/ and see the difference - both in quality, size of the produced tiles as well as the required speed to render the raster. You can merge multiple files into a seamless layer, render into SQLite packages, upload efficiently to Amazon S3, host tiles as WMTS with open-source projects, etc.

MapTiler comes with the GUI as well as command line (in the Pro version). It has the option for resampling which runs just as it should.

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[GIS] The fastest way of generating tiles from raster

Instead of creating metatiles I have now tested different tiling scripts that I've found. They are all using the same source image which is a JPG (with a .JGW) in EPSG:3857 projection. Bilinear has been used in scaling and the zoom level was 9.

generate_tiles.py (slightly modified): 22 minutes.
gdal2tiles.py (patched for 2 cpu cores): 3 minutes.
gdal_tiler.py : 18 minutes.

So far gdal2tiles.py wins!

[GIS] use mapnik to read from multiple geotiffs

This sounds like a perfectly reasonable thing to do with mapnik. You have (at least) two options to do that.

The first and simplest option is to do something along the lines of (pseudo code):

create a mapnik map
create a style (potentially based on user input) # you can also create a style per layer
append the style to the map
for each file
    create a datasource (with the Gdal plugin in your case probably)
    create a layer
    attach the datasource to the layer
    append your style to the layer styles
    append the layer to the map
set the geographic extent of the map
render the map

You can have a look there: https://github.com/mapnik/mapnik/wiki/GettingStartedInPython and there: http://mapnik.org/docs/v2.0.2/api/python/mapnik._mapnik-module.html

The other option is to build your mapnik xml on the fly, load the map with the xml and then render the map. You can find the mapnik xml reference there: https://github.com/mapnik/mapnik/wiki/XMLConfigReference

Best Answer

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[GIS] The fastest way of generating tiles from raster

[GIS] use mapnik to read from multiple geotiffs

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