Offline WMS – Benchmarking raster formats for QField

What are we looking for?

We would like to use WMS offline on QField. For that, we need to figure out what is the best way to get a raster from a WMS and which format is the most efficient (size and performance).

In this post we’ll show you is how to generate the ideal raster file from a WMS and the results of our efficiency tests for the the different raster formats.

WMS to GPKG

The simple way

If there is no limitation on the WMS or you need only a small region, here is the easiest process.

1. Request the WMS and store a description file in XML:

gdal_translate "WMS:url" file.xml -of WMS

2. Create a Geopackage from the information in the description file.

gdal_translate -of GPKG file.xml file.gpkg -co TILE_FORMAT=JPEG

That was quite simple, right?

The larger datasets way

If the command takes too much time, it means that it is trying to download too much data and could be caused by downloading higher resolution data than required.
The command might even completely fail if it contains a request for bigger data blocks thant the server allows.

Here is the process to get larger datasets in a simple way. Let’s use a real example:

1. Use gdal_translate "WMS:https://www.gebco.net/data_and_products/gebco_web_services/web_map_service/mapserv?request=getmap&service=wms&crs=EPSG:4326&format=image/jpeg&layers=gebco_latest&version=1.1.0" test.xml -of WMS
2. Open the test.xml file for editing, here you’ll find the parameters of the WMS. We change the „SizeX“ to 3600 and „SizeY“ to 1800. By changing these parameters we lower the resolution. It is important to keep proportionality.
3. Another thing we need to change are „BlockSizeX“ and „BlockSizeY“ that define the size of the tiles. We change both to 2048.
4. Finally, use gdal_translate -of GPKG test.xml test.gpkg -co TILE_FORMAT=JPEG
5. To make a Geopackage pyramid use gdaladdo GPKG:test.gpkg:gebco_latest. It will replace the Geopackage, if you want to keep the original one, you need to copy it first.

Now you have a raster Geopackage that you can use in QField.

Testing raster formats

Preparing the files

As first step we exported our test orthophoto WMS to a plain GeoTIFF using QGIS‘ default behaviour.

Format	gdal_translate	gdaladdo
gpkg JPEG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_JPEG.gpkg“ -co TILE_FORMAT=JPEG
gpkg PNG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG.gpkg“ -co TILE_FORMAT=PNG
gpkg PNG_JPEG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG_JPEG.gpkg“ -co TILE_FORMAT=PNG_JPEG
gpkg PNG8	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG8.gpkg“ -co TILE_FORMAT=PNG8
gpkg WEBP	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_WEBP.gpkg“ -co TILE_FORMAT=WEBP
gpkg pyramid_JPEG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_JPEG.gpkg“ -co TILE_FORMAT=JPEG	gdaladdo GPKG:C:\test\test_JPEG.gpkg:test_gpkg_JPEG
gpkg pyramid_PNG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG.gpkg“ -co TILE_FORMAT=PNG	gdaladdo GPKG:C:\test\test_PNG.gpkg:test_gpkg_PNG
gpkg pyramid_PNG_JPEG	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG_JPEG.gpkg“ -co TILE_FORMAT=PNG_JPEG	gdaladdo GPKG:C:\test\test_PNG_JPEG.gpkg:test_gpkg_PNG_JPEG
gpkg pyramid_PNG8	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_PNG8.gpkg“ -co TILE_FORMAT=PNG8	gdaladdo GPKG:C:\test\test_PNG8.gpkg:test_gpkg_PNG8
gpkg pyramid_WEBP	gdal_translate -of GPKG „C:\test\ortho_test.tif“ „C:\test\test_WEBP.gpkg“ -co TILE_FORMAT=WEBP	gdaladdo GPKG:C:\test\test_WEBP.gpkg:test_gpkg_WEBP
JPEG2000	gdal_translate -of JP2OpenJPEG „C:\test\ortho_test.tif“ „C:\test\test_jpeg_2000.jpg“
COG DEFLATE	gdal_translate „C:\test\ortho_test.tif“ „C:\test\test_cog.tif“ -co TILED=YES -co COPY_SRC_OVERVIEWS=YES -co COMPRESS=DEFLATE
COG_JPEG	gdal_translate „C:\test\ortho_test.tif“ „C:\test\test_cog_JPEG.tif“ -co TILED=YES -co COPY_SRC_OVERVIEWS=YES -co COMPRESS=JPEG
tif	In QGIS right click on the layer > export > save as > (see the details in the picture under the table)
MBT	gdal_translate -of MBTILES „C:\test\ortho_test.tif“ „C:\test\test_mbt.mbtiles“

Rendering test results

We have tested many formats, here is a table with the results of the size and rendering speed in QGIS and QField.
To analyze the speed we used qgis_bench.exe -i 10 -p "C:\test\test.qgs" >> "C:\test\test.log.
Qgis_bench is a tool that renders a QGIS project a number of times to get performance measurements. The parameter -i is to define the iterations and -p is the project used which contains only the generated raster.

Format	Extent [m]	File size [GB]	Total_avg	Total_maxdev	Total_min	Total_stdev
gpkg JPEG	52’880/29’230	0.4	250.242	255.781	5.539	244.984
gpkg PNG	52’880/29’230	2.9	412.002	490.328	152.142	259.859
gpkg PNG_JPEG	52’880/29’230	0.4	250.125	256.875	6.750	245.172
gpkg PNG8	52’880/29’230	1.4	283.875	296.406	12.625	271.250
gpkg WEBP	52’880/29’230	0.3	330.238	348.109	73.534	256.703
gpkg pyramid_JPEG	52’880/29’230	0.5	1.009	3.406	2.397	0.688
gpkg pyramid_PNG	52’880/29’230	3.0	1.208	3.281	2.073	0.688
gpkg pyramid_PNG_JPEG	52’880/29’230	0.6	1.491	4.344	2.853	1.016
gpkg pyramid_PNG8	52’880/29’230	1.6	1.508	4.375	2.867	0.969
gpkg pyramid_WEBP	52’880/29’230	0.4	1.333	4.906	3.573	0.766
JPEG2000	52’880/29’230	1.1	13.888	136.109	122.222	0.219
COG DEFLATE	52’880/29’230	3.6	264.427	273.094	25.411	239.016
COG_JPEG	52’880/29’230	1.0	14.778	131.172	116.394	1.734
tif	52’880/29’230	6.4	2.367	6.734	4.367	1.672
MBT	52’880/29’230	4.4	0.469	4.641	4.171	0

Analysis

File size

The Geopackage WEBP (with and without pyramid) has the best result for file size, but it is not yet supported by QField (from 1.6) and is only slightly smaller than the JPEG variant.

Plain GeoTiff, MBTiles, Cloud Optimized GeoTIFF (COG – DEFLATE mode) and Geopackages with PNG generate by far the largest file sizes (up to 20x larger) and are thus not recommended.

Rendering speed

MBTiles are on average double as fast as JPEG Geopackages with pyramids which in turn are more than double as fast as GeoTIFF and 15x faster than COG.
Geopackages without pyramids are 200 to 400 times slower.

Conclusion

Even though MBTiles render faster than the Geopackage pyramid JPEG, they come with an almost 10x bigger storage requirement which makes us say that the best offline raster format supported by QField is Geopackage pyramid JPEG or if you need transparency and slightly smaller files Geopackage pyramid WebP.

If you need transparency before QField 1.6, the best results are achieved with Geopackage pyramid PNG_JPEG.