Guys, this discussion of preparing items 10 meters wide, entirely in PS, at resolutions appropriate for small prints, completely misses the point. How in the world do you think billboards were digitally produced, say 10 or 20 years ago when the memory and storage capacities of computers was vastly smaller?
The first thing to realize is that even for artwork this large, viewers expect certain items, especially text, to be sharp when reproduced to any size, and when viewed from any distance. Doing the graphics design elements in a vector application accomplishes this perfectly. Text, text efx, lines, Bezier curves (eg, the pen tool), shapes, solid colors, color gradients, and any graphics that can be done in Illustrator (or other vector based program) are almost always done that way. Obviously, to reap these benefits, the artwork must be exported in a file format that your printer will accept, and which supports vectors such as *.AI, *.eps, etc., (ie, but certainly NOT *.jpg, *.tif, *.png, etc.).
In addition to the sharpness being preserved at any size scale, the second benefit to working this way is that you will be working on files of megabyte size or less, not tens or hundreds of GB. Even modest sized computers won't be taxed.
(BTW, note that some vector programs, particularly those designed for page layout (eg, InDesign), have limitations on the maximum real-world size you can make the file, but this is not really a limitation since you can always work at a smaller scale, eg, 1 inch = 1 foot, 1 mm = 1 cm, etc., and the resultant file will still produce ultra sharp edges even after the printer's RIP scales the file back up to full size for printing. )
If there are no photos or other bitmaps in the final product, your work is essentially done at this point.
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However, if the final product does include a photo or other bitmaps, the first thing to do ask the art director whether, by some chance, the images might be able to be converted to vectors. This generally imparts a drawing or cartoon-like feel to them, but in some cases, (eg,
Lichtenstein-like images) this is entirely acceptable, maybe even preferable. If so, you are in luck. Such conversions can usually be done quickly using AI's "Image Trace" followed by a little manual touch-up work, and your file still stays relatively small, and able to be scaled to any size without problem.
However, if one or more photos absolutely must be included, this can be done quite simply by placing a good quality, reasonably high resolution version of it (say, at least 4000 px in the long dimension) into the AI or PDF file, essentially without regard to the very low ppi this will become when scaled up to billboard size. This is because:
(a) billboards almost never are viewed up close; and,
(b) even if a viewer is able to stand a few feet away from something this large, they have no expectations of being able to see the bacteria crawling around every pore of the model, LOL. As long as the image isn't obviously pixelated and the graphics are sharp, they'll be happy, even if the photo is a bit soft when standing right next to it.
If you want to get into the issue of what final dpi (for bitmaps) are acceptable at what distance, just Google {"rule of 240" billboard}, or go to the websites of companies that produce billboards for a living, e.g.,
https://www.cbsoutdoor.com/outdoor101/Documents/OAAA_OOH Print Production Specifications.pdf :
"* Outdoor (billboards, Ecoposters, etc.): a
minimum effective resolution of 18-25 ppi at
full printed size.
* Transit (bus interiors and exteriors, subway
posters, commuter rail media): a minimum
effective resolution of 80-100 ppi at full printed
size."
HTH,
Tom M
PS - FWIW, just a month ago, I helped design (and have produced) a 60 foot (wide) by 3 foot (high) timeline / history of an organization. It was going to be viewed by attendees at a scientific conference, almost always standing at least 2 or 3 feet away from it. This allowed me to go down to 100 ppi for the images. For ease of transport and mounting, it was broken down into 10 sections, each 6 feet wide, and each representing 5 years of the organization's history.
Because there were a large number of photos, and only simple text, text efx, and other hard-edged graphics, once the text content was agreed upon, I knocked it out in about 3 days using ... guess what ... of all things ... ... ... Yeah, Photoshop to prepare the photos, but then dumped them all into ... of all things ... Powerpoint! A dumb business graphics program like this was all that was needed for this task. It produced great looking output, small file sizes, and was much faster to use (since no complicated graphics were involved) than using one of the vector programs I mentioned earlier. I could have used any of them, but overkill serves no purpose when there are deadlines and budget constraints. You use the best tool for the job and move on.