PNG (Portable Network Graphics)

PNG is a raster format with lossless compression: the picture is saved exactly as it was drawn, without the smearing around thin lines and small labels that JPEG produces. For maps that matters: roads and text consist precisely of contrasting edges.

The format is chosen when the map is needed as an image and has to open anywhere — to drop into a presentation or a document, to put on a site, to send to a colleague, to attach to a post. No special software is needed to view it.

The map is rendered on the server in the chosen style and saved as an image; the sheet size and the resolution are set at export time, and the sheet may be picked automatically from the area of the region. The picture has a technical ceiling: if at the given resolution a side or the total number of pixels goes beyond reasonable limits, the resolution is lowered automatically — that way the export does not run into memory and always reaches the end.

A raster has one limitation, but a fundamental one: the picture cannot be enlarged without losing crispness, and the objects on it cannot be edited separately. If you need scaling and editing, take SVG, vector PDF or DXF.