That viewingdale software looks awesome
I'm working away on a 27"x41" map at 200dpi (so, 5400x8200px), and I've found that it works best to break up the map into several different component .PSDs that each have a specific element being worked on.
So, I've got seperate files for:
1. Projection Lines
2. Landmass Design
3. River Systems
4. Heightmask / Topography
5. Geography & Land Coloring
6. Map Background Texture
7. Map Border
8. Icons & Symbols
10. Top-Level Texturing & Weathering
Now, sure, that's a lot of files to work with, but it keeps even the big land color, border, and typography files much, much smaller (even though they are huge in and of themselves) than a single master file with all elements would be.
Ack. Double post! Sorry, guys.
It is actually. In geography and cartography, the area covered is the "extent" while the size things are drawn (specifically the ratio between how big they are drawn, and how big they really are) is the "scale". So a "large scale" map has things drawn larger than a "small scale" map.
Originally Posted by Falconius
map a city = large scale
map a world = small scale (The confusion comes because the map is bigger but the objects appear to be smaller)
PokealypseNow: I usually keep only the most complex layers such as the texture in another file. There is no need to get rid of the rivers since it's not that big. But separating the file generally means that future modifications will become much more complicated. Sadly, at some point, it might become necessary and it's better this than not being able to work on the map at all.
CraigV, what is the style you're shooting for? If it's some faux hand-drawn ancient parchment thing, then you're needing raster work with irregularity and textures and shadings and such. If it's more like a modern atlas view, you have at least a possibility of using vector graphics, which takes care of some of your scaling issues (and filesize issues). If you have a world map, your linework had better all be several miles wide, or they won't print. If you use a vector app like Inkscape, that's less of a problem; the stroke applied can be changed based on what scale you're outputting.
Face it, you will never have a single map file that serves for everything between world map and city map*. At some point you have to use work you've done toward one end of the scale range as *basis* for separate maps at other scales. BTW a simpleminded mnemonic for large vs. small scale is "large scale = small area". After a while you look at a world map and think "small scale" without having to derive it....
Did you try Azelor's suggestion about altering Gimp's filesize limits in its settings? Can you give us the basic specs of your computer? Ram, processor, clock speed, operating system? From that we might be able to guess "there's yer problem!". Still - the advice on not keeping overmuch detail in one file is valid. If you want one file to suffice for at least SOME range of output maps, it's fine, say, to have one layer of all your close-up labels - maybe region-sized material including road names. A separate layer might be intended for world size work, where roads aren't even indicated, and ocean names span the entirety of the ocean basin. Once you do the work to create each set of labels, the intermediate steps of which (for me) involve a gazillion different text layers or shape layers and enough file complexity to bog my computer, you can flatten *A COPY* of that version to one raster layer, and either keep it off to the side in another file or at least get the benefit of having but a single layer (pair of single layers) to clutter you master mapfile with. When I've tried to maintain modest filesize by doing such flattening early and often, right in the master file, I invariably decide later I've done something wrong and I wish I could go back and alter stuff -- say, fix dozens of city labels, which would be super easy if they were still text objects, but which became a major chore of re-creation since I'd rasterized the whole bunch. Hence the "...A COPY..." advice above.
There's categories of layers you want live in the master file you're drawing in, even if the particular map version your active layer is destined for doesn't need them - example being the purely political map you're needing next, and for which you're putting in a bunch of extra towns and villages, may not NEED your climate nor landcover info, but it matters that you scatter more towns in that farmland than in the impenetrable forest. Some such reference layers I put up with in my working file, others I copy elsewhere and display as though I had a physical reference map on my desktop.
My current machine has 8GB of ram and 64-bit Win7 to access all of it - I haven't tried anything ambitious since I got it, and I'm rather looking forward to not having to do so much shuck-and-jive with separating layers into files.
Going back and forth may still be most efficient in some cases - like if it takes entirely different programs to accomplish different parts of your workflow. Ex: I've recently added Inkscape to my toolset just because it makes text-on-a-curve so simple, where fighting with my raster PhotoPlus to get curved text is a pain. So PhotoPlus gens the base map to some point, I output maybe a png at full size, I import that to Inkscape, lay out my labels there, save JUST the bare labels, and reimport that to PhotoPlus. So - you want to use the Gimp because you already know it, or because it seems an approachable new tool? Are you willing to consider learning multiple new tools at once? If your timeframe for this project is open-ended, leave yourself the option of branching out when you hit a step that the Gimp struggles with.
* unless your name is Google and you have several spare megabucks in your pocket...