Easy. A vector file.
The answer used to be more simple, when billboards were printed only. But today billboards can be digital too. Let me elaborate on these two options.
Large format printing is a four-color process. So providing a logo file in CMYK format is most likely preferred. The size of the billboard must also be considered. Vector art files (.eps for example) can be scaled to any size without any degradation. So as a general rule, you can’t go wrong with a vector .eps file. We’ve had clients say their billboard looks bad and after further investigation, we found out they provided raster art to the billboard company. Raster art, such as .jpg, .png or .tif files can not be scaled up in size without losing quality. So if you send a .jpg file that is 1 inch in size and scale it to 12 inches, it is 12 times larger than it was meant to be…so of course will look bad.
A more complicated answer, but similar to web, digital billboard files should be raster art, such as a .jpg or .png file. Digital billboard vendors often provide their own guidelines on art file preparations — including size and resolution — and following them generates better results. But, as a general rule, you should create raster files at actual size, or a ratio of the actual size, with an end resolution of 72 dpi. For example if you have a 48×14-foot billboard, build a jpg file that is 576 inches by 168 inches at 72 dpi. If you build that file at 50% scale, 288 inches by 84 inches you need to increase the dpi to 144…so that when it is scaled up to 100 percent it projects correctly.
Managing your brand and logo at large sizes can be challenging when you are most often focused on print, collateral and web sizes. Understanding the correct file formats and when and how they should be used is an important step that is often overlooked.