|Product||File Size||No. Files||Uncompressed||Optimised||Optimised+Overviews||Total Storage|
|OS VML Col||0.3-10MB||562||184MB||2-30MB||3.12GB|
|OS VML BW||1-3.5MB||562||184MB||0.3-5MB||0.3-5MB||1.05GB|
|OS VMD Col||0.2 – 2.2MB||80||67.2MB||0.8 – 2.2MB||1-4.5MB||231MB|
There are number of things to look at in terms of optimising images for web mapping:
- File size and total storage required
- Draw performance
- Image quality
|Product||No. Tiles||Tile Dimensions||Draw Time||Notes|
|OS 25k raster||80||4000×4000||GIF: 0.3-0.5s||Uses external overviews (1/2, 1/4)|
|OS VectorMap District||80||4000×4000||GIF: 0.3-0.6s||Uses external overviews (1/2, 1/4)|
|OS VectorMap Local||562||7874×7874||GIF: 0.5-0.7s, PNG: 1.0-1.6s||GIF twice as fast as PNG|
The original supplied image files were variously LZW, CCITTFAX4 and Packbits compressed resulting in minimal file sizes. However, they were not tiled internally and did not contain overviews. Using gdal_translate and gdaladdo and various creation options the optimised output was only slightly larger than the input files.
Using MapServer and some very unscientific tailing of log files it is apparent that GIF is faster than JPEG and then PNG. However, image quality improves from GIF to JPEG to PNG. MapServer uses GD to process the images from the data store and save them in the output folder ready for the client to consume them.