Hi IL, thanks for your reply. Hopefully my explanations here can give those people on the community here who are working with these large map files a bit more understanding when dealing with their printer. Please understand I’m in no way wanting to come across as high-minded or a know-it-all; I simply have worked for 20+ years in the graphic design/print industry - and more often than not have learned the hard way! lol *By the way, ppi = resolution on a display, dpi = resolution on a device that reproduces an image (printer) - let’s keep it simple and use dpi… it’s not technically the same thing, but let’s not overcomplicate what can be a confusing subject!
It’s important to first of all understand the difference between vector and bitmap (rastor) graphics. Vector graphics are resolution-independant, meaning as long as they are kept in their vector format they can be scaled to any size without loss of detail or clarity. Many vector graphics are created in programs such as Adobe Illustrator… Common vector formats are .ai, .eps, and .pdf.
Bitmap graphics often originate from a camera (continuous tone photograph), scanner, or from bitmap software programs such as Adobe Photoshop. Bitmap graphics always have a fixed size at a specific resolution, and the two factors have a dependant, inversely correlating relationship. So, when size goes up, resolution goes down. When size goes down, resolution goes up. Common bitmap file formats are .jpg, .gif, .png, .tiff, etc. Even a .pdf might actually be a bitmap image… you can always convert a vector image to a bitmap image (rasterize), but never the other way around.
Bitmaps can be either expressed as a certain size @ a specific resolution, or at their pixel size (which expresses neither the size or resolution, but simply the amount of detail in the image). The key difference with a bitmap is that if you try and upscale the image, you keep the same amount of detail, but due to the larger size (same detail over a larger area), you lose clarity. The net effect is a loss in quality.
So, in the map file we are referring to… the size is 72x33 inches at 300 dpi, or 21600 x 9900 px. Just now, I opened that image in Photoshop, and when I changed the dimensions to 96"x44", the resolution dropped to 225 dpi - but it’s important to note that the pixel size stays the same - it’s the EXACT same image. It would be pointless for me to lock in that size, and up the resolution back up to 300, because there would be no visible increase in image detail or clarity. You can’t create resolution (detail) from nothing… it’s either there, or it’s not.
With this map from Ambilzi, I don’t know what resolution his source file was at - it would depend on the resolution that he did the original scans in. But the image YG/Ambilzi have provided us is 21600 x 9900 px, and so we won’t ever get a more detailed map without redoing the scans - no thanks!!! However, as I mentioned in my earlier post, 200+ dpi is still sufficient for this process. In this case we have a working resolution of 225 dpi.
Example: If I was to send a magazine advertisement to off to be printed by an offset printer on high-quality, gloss stock, I would never provide a bitmap graphic at a resolution higher than 300dpi, because that printer is not capable of printing more detail than that resolution provides. In fact, most people would not be able to tell the difference between a continuous tone image (photo or artwork with gradients) printed at 300dpi and one printed at 200dpi. When asked to make a side-by-side comparison, sure - but they would never pick it out on their own accord when browsing a magazine.
So, this brings us back to the map. In my experience with flat bed printers on vinyl, anything over 200dpi is pretty much superfluous. In fact, I’ve included bitmap images in tradeshow banners that were only 100dpi (because that’s the best source image I had to work with), and no-one ever knew. Having said that, if I have a 300 dpi image to send to the printer, that’s what I’ll use! And if I have to convert a vector image to bitmap, I’ll convert it to 300dpi at the right size - no point going higher in resolution.
IL, you mentioned “…I can easily tell the difference in losing 100 DPI. Its almost worse than going from PDF to Jpeg.” I need to point out that I never said it was hard to tell the difference in losing 100dpi, I said it was hard to tell the difference between 300 and 200 Even my grandma would be able to tell the difference between a 72dpi image and a 150dpi image (if she were alive…!)
The bottom line: the map file that I edited and sent back to YG is the same size that I received it in - and we won’t ever be able to make it bigger (add detail) - but I think it’s big enough. The only other major factor on our side of the equation when sending to a printer is choosing the best file format for the job… and that’s my next post.
*Hopefully I’m not disseminating any technical heresy here, I’m sure there’s someone on this forum who will be able to debunk something that I’ve shared, lol.