Map File Formats & Performance
Picking appropriate file formats and compression rates for map tiles is vital to keeping maps fast. We've researched the topic and found a surprising amount of diversity in implementation - formats like JPEG, which is rarely used for text, is used for roads and road labels on some maps, and GIF, which is believed to be superseded by PNG, is still used on some maps.
Here's a chart of several providers' choices:
Note that bing's maps are served as .png files but contain image/jpeg data, and that Google serves different formats based on detected bandwidth.
Given that JPEG offers continuously varying image quality, we analyzed how the 'Quality Number' indicates filesize:
