Resolution

[Gudern]

As a newcomer, I am currently thinking about trying a continental scale map.
The first question I have - what is the resolution people take for a continental map ? Not assuming it will be printed because that would dictate resolution. Just done for a screen.
Is a thought given to scales of the depicted objects or is the resolution whatever one wants ?

[johnvanvliet]

map scale and printing size / resolution really are different things

how BIG of a file size do you want ?
i am currently sending a planetarium a copy of my 25 gig venus map and 8 gig heightmap for venus
the big one is 364 pixels per degree

also keep in mind the square rule
if you double the size you SQUARE the surface area

moving from a 2048x1024 px map to a 4096x2048 map is 4 FOUR times the work


so a continent ????
like Australia ?
Or North America ?
Or Eur-Asia ?

think about 90 to 120 degrees of longitude
and 45 to 90 degrees of Latitude

[Azélor]

It depends on how much detail you want on you map.

It can be any size you want. One thing that might dictate the size of the map is the size of the smallest object that you want people to be able to see clearly.

[Chick]

For a computer screen, a resolution of 300 is adequate to show small details. My advice is start with 300, make some image with details as small as you expect to use in your final map, and then see what it looks like, If it's too blurry, go for higher resolution.

[Gudern]

Thanks for these insights.
But I realize that I asked my question badly.
The map I imagine on continental scales is something like 3 000 x 3 000 km.
Yes I know that an area is a length squared and that (2x)² = 4x² (I am an architect)
My question was what resolution in pixels I should take for such a map ? Or what is usually used ? Or standard ? Or convenient ?

Is it right that if I take Chick's 300 (dpi ?) then it leads to a 3 600 x 3 600 pixels on a screen around 30 cm x 30 cm ? And that then the 3 000 x 3 000 km area would have a resolution of 1 pixel = 850 m x 850 m in reality ?
This would be a pretty high resolution - details like the Central Park in New York or a tree copse will be seen. High mountains will have a height between some 6 and 12 pixels.
Is this kind of calculations done here ?

[Chick]

Read what John VanV said, resolution and pixel size are independent. You can have a 3000x3000 pixel image at 72 dpi or at 600 dpi, and it is still 3000x3000 pixels.

1000x1000px @ 72dpi


1000x1000px @ 600dpi


Can you see any difference?

72 dpi is generally enough for something to show on a computer screen. 300 dpi is needed for printing to avoid the eye being able to see the pixels.

[Redrobes]

I just popped in to say that you might have a look at my tut on this most confusing of topics as well as a whole load of other similar stuff:
http://www.cartographersguild.com/showthread.php?t=2596

The image size you use is enitrely down to your preference, how much detail you want to hold and hob much memory you want to burn on it. There are no standards. Personally I like to use many images of many scales to allow a seamless blend from continental small scale down to battle board large scale maps.

[waldronate]

http://alvyray.com/Memos/CG/Microsoft/6_pixel.pdf is a fun read for those curious about what this "pixel" thing that makes up our images might be (and what it might not be). 72dpi was based on the classic printer's point and was only a good approximation for the original Macintosh screens. The default Windows resolution is 96dpi, but modern hardware is pushing that number higher and higher ( https://msdn.microsoft.com/en-us/lib...(v=vs.85).aspx is some background on that issue). The notion of visible size is very different from the notion of how much detail can be shown for a visible size (dots of hardware display per inch of physical space adjusted for viewed distance). It's a complex problem that really requires a good knowledge of how you expect to use the image...

[xpian]

- The Difference Between Dots Per Inch and Pixels Per Inch -

Strictly speaking, when we're discussing the resolution of digital files here--TIFFs, JPEGs, GIFs, PNGs, Raster EPS, etc.--we're talking about Pixels Per Inch, or PPI. This is true for all of the raster graphics files (a raster is a grid, like a grid of colored squares called pixels). Vector graphics are a separate topic.

When it comes to printing your files, whatever kind of files they might be, your printer's resolution is measured in Dots Per Inch, or DPI. A pixel in your image can be many colors...those pixels that make up the forest on your map are all kinds of beautiful, subtle shades of green. A dot printed out from your printer can only be a handful of colors: cyan, magenta, yellow, or black. Some printers have a "light cyan" and "light magenta", but generally speaking, a printer DOT is an all-or-nothing affair. Either the dot is there on the substrate (paper), or it is not.

If you think about it for a minute, you'll see that there's no way your Pixels in your image are ever going to be equivalent to your Dots from your printer. Take those green forest pixels I mentioned above, for instance. Let's zoom in on just one of those dark green pixels. You can see it on your monitor, if you zoom in close enough, as a nice evergreen square. When your computer sends the map image to your printer, the printer RIPs the file (Raster Image Processing). The printer knows that it can't lay down a "dot" of pretty dark green. The printer only has Cyan, Yellow, Magenta, and Black to work with. It figures out how to make that shade based on color tables stored within it, and prints it out. When you get a magnifying glass and go to look at the forest in the map you just printed...and you zoom in on that one little green pixel you were looking at on your monitor...you'll find that the pixel is now represented by (perhaps) 5 dots of cyan, 1 dot of magenta, 7 dots of yellow, and 3 dots of black. All of those dots seem to be randomly (in a stochastic screen) scattered in the little square of the pixel on the piece of paper. When you put the magnifying glass away and pull your eye back from the page, the dots all blend together into what looks like a little green square. It's an illusion, of sorts, but it's a convincing one and something we deal with in every moment of our lives when we look around us in the world. Grab a handful of sand from an ocean beach somewhere, look at it closely, and realize how many different colored grains of quartz and other rocks are blending together to make that yellow-beige, sandy color.

For this reason, professionals in the printing industry know that your raster graphics don't need to be the same resolution in PPI as the printer has DPI. Especially if we're talking about full-color graphics. A good rule of thumb is that you can get away with your PPI being about half your DPI, at the same size. For instance, if you know that your printer does 600 dpi on a US LTR size page, you can easily set your Photoshop file to be 300 ppi at 8.5 x 11. You'll know that each of those little PS pixels is going to have to be represented by several of the printer's dots, so it makes sense for the DPI number to always be higher than the PPI of the image. Making your image any higher resolution than that is almost certainly just wasting file size, and because computers and printers slow down when handling big files, wasting time. I've personally done many, many tests of this on lots of different kinds of printers over the years. You'd be surprised how low you can drop the resolution of your digital file and still have the print look *identical* to a higher-resolution digital file.

The main exception to this is line art, or pure black-and-white graphics with sharp edges and hard lines. Generally speaking, you might as well make those the same resolution in PPI as your printer has in DPI.

NOTE: there's a whole discussion to be had about different types of process color screens, specifically the difference between the standard 4-color process you see in magazines, newspapers, packaging, most posters, and most books, versus the stochastic screen used in inkjet or dye-sub printers. And the standard 4-color process is measured, not in DPI, but in LPI or lines per inch. But the actual guidelines are basically the same: you can use graphics that are about half the LPI of the printing press in PPI...and maybe up to the same LPI to PPI ratio for really fine, sharp lines.

[Vhey]

Put in easy speak:

Pixel count is just how much of the medium you have to work with. DPI doesn't matter here, or anywhere electronic. (computer screen, internet, etc.)

DPI (dots per inch) is just to tell the printer how many of those pixels to print in one inch on the paper. You'll need to know this if you plan to print it.