DPI (digital dots or pixels)/PPI defined:
300 dpi/ppi = 300 pixels used for every 1 inch line of ink coverage.
1000 pixels will yield a 3.3333 inch line @ 300 dpi of resolution
DPI and PPI have been used interchangeably (though not always accurately) since pixels entered the printing industry. DPI comes from halftone/screen dots in offset printing. If a greater number of dots can be reliably applied to a given media (paper) the finished print will appear more like a photograph (continuous tone) rather than a halftone (a series of fine dots).
Scale your web banner into a billboard (BB):
Your web graphic is 72 pixels per inch, and that would actually work - provided it was sized at 100%*. But a 1000px web banner is only going to print at about 3.5in wide @ 300 dpi - so you have to scale it up to a BB height/width. Scaling up the physical size will proportionately scale down the resolution. So a 3.5in graphic scaled to a 7in height would cause the DPI to drop to 150. Scale that to 14in...75dpi, 28in...32.5dpi, and so on. Note: a 32.5 dpi file can actually work for some BB production, but in your case the physical height will only be 28", which is hardly a BB. Time to build a new file.
Do some math and convert your web banner into a BB. Your current banner = 1000px by 98px. Simplify that to 1000px by 100px. To make the math easy, lets say you need a BB that's 100ft x 10ft.
- 10ft height = 120 inches, or 36000px - we need DPI not dots per feet (300/in)
- 100ft width = 1200 inches, or 360000px
Ideally, you'd use a 120" x 1200" file @ your DPI requirement, and be done with it. But most software won't allow you to build a 1200in wide file, so you have to scale your artboards down to accommodate. The trick to that is to think in proportions that keep your math simple (1/4, 1/8, 1/10 scale). So if I were trying to build this example, I would create a document that was 120in x 12in @ 3000dpi (1/10th scale). Build your BB according to the design of your web banner. Then pass the file to the printer with instructions to print @ 10X, which would produce a 1200in (100ft) x 120in (10ft) piece of art @ 300 dpi (for a BB, 300 dpi is overkill). Note: Placed photography is not exempt here. If you need a photograph that's 10' tall, you're going to need to pay close attention to its DPI as you start to enlarge it.
Honestly, your BB guy could be asking for 600dpi so he has plenty of pixel information to work with. If you ask what his final printed DPI actually is, he may be able to help you create a smaller (KB) sized file. Definitely, open a dialogue with your provider - helping you helps them as well. They would love to get a file that needed 0 rework on their end.
Billboards are in a league of their own. You can't compare them to a poster, or the bitchin' $25000 dry ink printer at the shop. It's not the same. Generally speaking, viewers are 10s (or 100s) of feet away from the finished piece. Lower resolutions aren't detected by our eyes at those distances, let alone when we're zipping by on the freeway.
Check out LAMAR's FAQ page on DPI, scanning for billboards, etc. it's an interesting read. http://www.lamargraphics.com/lgweb/CustomerService/Contact_us/FAQ/Faq.htm
*LAMAR requires 100 dpi for HIGH RES (at 100% size).