Storing WMTS rasters into GPKG database🔗

WMTS is well supported under QGIS. But sometimes you want to use it offline. Why (ab)using online resources while you can download it and use it later ? Furthermore, since about 10 years, we can store raster data into a gpkg file. What is cool with this format is that it is (nearly) universally supported by any modern GIS software. And you can store multiple raster and vector data into a single file. Pretty convenient for exchange and use it as a centralized database (for local stuff of course).

This article introduces a method to capture WMTS raster data into a gpkg file.

First things first, you got to find a WMTS source of data. Be careful to read the conditions about accessing this data: most of the time, you will have to use it with a fair usage. It means that if you try to download all of tiles at high level zooms, you can be expelled from the service (most of the time IPv4 banning) for abusing it.

You have to be careful.

Once data found, you need to build a GDAL WMTS definition file. It is an XML file which does handle WMTS parameters for requesting tiles. Here is a sample for the official SIA OACI maps stored on the french Geoportail web site:

<GDAL_WMTS>
  <GetCapabilitiesUrl>https://wxs.ign.fr/an7nvfzojv5wa96dsga5nk8w/geoportail/wmts?Service=WMTS&amp;Request=GetCapabilities&amp;VERSION=1.0.0</GetCapabilitiesUrl>
  <Layer>GEOGRAPHICALGRIDSYSTEMS.MAPS.SCAN-OACI</Layer>
  <Style>normal</Style>
  <TileMatrixSet>PM</TileMatrixSet>
  <ZoomLevel>11</ZoomLevel>
  <Format>image/jpeg</Format>
  <DataWindow>
    <UpperLeftX>-592403.0</UpperLeftX>
    <UpperLeftY>6645000.0</UpperLeftY>
    <LowerRightX>1470650.0</LowerRightX>
    <LowerRightY>5050000.0</LowerRightY>
  </DataWindow>
  <Projection>EPSG:3857</Projection >
  <BandsCount>3</BandsCount>
  <DataType>Byte</DataType>
  <Cache>
    <Path>/home/mribreux/gdalwmscache/</Path>
  </Cache>
  <UnsafeSSL>true</UnsafeSSL>
  <ZeroBlockHttpCodes>204,404</ZeroBlockHttpCodes>
  <ZeroBlockOnServerException>true</ZeroBlockOnServerException>
  <UserAgent>Mozilla/5.0 (Windows NT 10.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.7113.93 Safari/537.36</UserAgent>
</GDAL_WMTS>

You need to provide the following:

• GetCapabilitiesUrl: the URL to the GetCapabilities interface of WMTS.
• Layer: the name of the layer you want to display.
• ZoomLevel: This is where you need to be careful: values more than 10 or 11 involve a large number of tiles to download.
• DataWindow: This is where you define the extent of the stuff you want to download.
• Cache: you can cache already downloaded tiles into a directory. I urge you to use a cache directory whenever possible because if the translate operation fails at one point (access to server failed for example), cache directory will be used instead of re-downloading every tile from scratch.
• Path: The cache path directory. Be careful, it doesn't support special characters like '~' for example. You should use full path.
• UserAgent: Some services (it is the case for geoportail.gouv.fr) discard downloads where the User-Agent is not a "regular" web browser one. Fill it with your web browser User-Agent and it will be Ok.

This XML definition can also be opened and used directly under QGIS. But most of the time it is much more convenient to reference the WMTS server under QGIS interface directly because QGIS can explore all of the layers of the service and much more information (metadata).

Once written, you simply use the XML file as a datasource under gdal_translate. Every parameters is set in the XML file so gdal_translate parameters are not so complicated to use:

gdal_translate -of GPKG ./geoportail_oaci.xml ./vfr.gpkg -co RASTER_TABLE=sia_oaci -co TILE_FORMAT=JPEG -co APPEND_SUBDATASET=YES
gdaladdo -r cubic -oo TILE_FORMAT=JPEG -oo TABLE=sia_oaci vfr.gpkg 2 4 8 16 32 64

Then you just open the gpkg file in QGIS and choose a layer.