[CWBP2] World Map construction thread

[Redrobes]

Well, I am taking a back seat in this CWBP so its all up to you but it would seem odd to me to have a world map that is broken up into chunks where the plots are then of different scales.

I thought the idea would generally be that you could walk off of one plot and onto your neighbors and that the roads and terrain would be at the same scale. Its as though the underlying world is there as one thing but you all go in a map it differently - not that the underlying world is different per plot.

Seems to me that if you had different scales per plot then you might as well have 50 random plots that don't butt up at all.I thought that the scale was 1 pixel is 4 km at the equator dropping to 0 at the poles in the usual Mercator style. The problem with mapping a globe is that you have curved space. If the plots are small enough then you can select the scale from the middle point of the plot and then make it that all over the bitmap and accept the slight distortion. Or as mentioned before, you can reproject it (gdal etc) to something of equal area and then transform it back again. Note, I wouldn't do this as its too much hassle but its an option. So choose small plots and the issue goes away. But yes, a house at the pole will stretch all the way across the world map since the pixels there map near to a point at the poles.

I think one of the big differences between the old and new coop discussions are that here it seems that people believe that once all the plots are taken its all done. I think what was understood more clearly - well ok maybe just discussed more frequently - is that other people would come along and take a plot regional map and then map something of larger scale within it, like a city for example. Maybe it would help if I pointed people at my vid I made of the last CWBP showing the state of play near to the end. You will see that its just not possible or sensible to map cities within the plots in the top most plot map and that you will necessarily have to go down a level and make more maps within it. So there are only going to be regional maps at the plot level since any city at the equator of a size less than 4Km will be mapped with one pixel.

https://www.youtube.com/watch?v=qELcbKXdRm8But
or
http://www.viewing.ltd.uk/Temp/CG/VD...gDale_x264.avi

I say again - you don't have to follow what happened before. I'll try to fit all the maps together with my app if they are supposed to fit together. And I would suggest to people to have a little think about what one pixel means at the world scale on their plot. If you took 200 pixels of world map plot size at the equator area and then made a 4000 pixel map from it then your 4000 pixels mean 200 * 4km or 800km or 1 pixel is 200m. So features should represent that kind of scale - so not really suitable for houses etc.

Do you know I have reread the thread again and got a different impression here about what is being asked. Ummm - the scale is changing as you go vertically up and down the world map but that they are the same scale at a border between neighbors. So its the same scale in a world projected map but since the world is global and curved its changing across the map vertically. And yes you should deal with that so my opinion is to choose small plots and pick the center pixel as the scaling. I'll so another post with the scaling math...

[Falconius]

The way I'm looking at it is that this is a world, and we are merely mapping/developing portions of it. The same as if we were just mapping or recording history of places on earth. You don't make a map of China the same scale as a map of Jamaica, you don't even necessarily use the same projection. We are not trying to map every inch of the surface, just the places that interest us. If plots abut then they have to match borders but there is still no reason to match scale, if you know a road comes in 4 km from the bottom edge it doesn't matter that bordering map uses 1km:4pixels and you use 1km:6pixels, all that matters is that what is represented is a road at 4km above the bottom edge.

[Redrobes]

Oh I see what you are saying now.

There are two types of scale being used here interchangeably. You have mapping scale which is 10000 pixels = 40,000 km from the original bitmap and also the bitmap scaling which is just resolution.

Ahh in that case, no I dont think it matters at all what resolution you make the images for the maps just that the scale of the maps is consistent. The resolution could be that you choose a 4000 pixel map for your plot region and then within that make a 4000 pixel map of the city within that plot. Now ignoring for a second this curved space stuff and just assume your at the equator then your map scale for the region is 200m per pixel, your city map is going to me something like 1m per pixel or maybe even less but the map scale is consistent in that the features on the two maps are the same size relative to the map scale.

The reason I am offering to do the world map is that I write ViewingDale which was an app to handle many bitmaps of maps at different resolutions and to adjust them all so that they make one seamless map at a given scale. Ok it doesnt handle curved space like say GIS packages or google earth might like to do but it has realtime pan and zoom and has lots of useful features to handle changes to a world map. That is most unlike most bitmapped based world mapping apps like say google earth where you would have to push the one giant bitmap through a script to break it all up into its tiles again. Most other GIS apps are vector based and so dont care about bitmap resolution. Unfortunately, very few people like to map in vector. The bitmap - vector argument rages ever on within these forums.... so lets not go there.

So my opinion and what happened last time is that people need to relate their chosen location to a map scale and then map it at that scale. But the image resolution was not important - as was the mapping style. They were up to the mapping individuals, computer capabilities, preference.To make one giant bitmap of the results would normally be a bit of a chore since one would normally have to rescale all the bitmaps and layer them up. But I have my app which does all of that in real time so thats why I offer to do it. Its hard for everyone but me.

[Redrobes]

For info, with the image posted at the top of the thread and world size specified, then the scale of the map at Y pixel positions are as follows:

At north pole there is no scale.

Y=125, Scale=313 in m/pix
Y=250, Scale=625 in m/pix
Y=375, Scale=933 in m/pix
Y=500, Scale=1236 in m/pix
Y=625, Scale=1530 in m/pix
Y=750, Scale=1815 in m/pix
Y=875, Scale=2089 in m/pix
Y=1000, Scale=2351 in m/pix
Y=1125, Scale=2597 in m/pix
Y=1250, Scale=2828 in m/pix
Y=1375, Scale=3041 in m/pix
Y=1500, Scale=3236 in m/pix
Y=1625, Scale=3410 in m/pix
Y=1750, Scale=3564 in m/pix
Y=1875, Scale=3695 in m/pix
Y=2000, Scale=3804 in m/pix
Y=2125, Scale=3889 in m/pix
Y=2250, Scale=3950 in m/pix
Y=2375, Scale=3987 in m/pix
Y=2500, Scale=4000 in m/pix

And it reverses again down to the south pole. So treat Y as distance in pixel from the top or bottom of world image.

[Falconius]

I was going to try and figure out a scale like that to put in the front post. Thank God you saved me the trouble.

[Azélor]

The resolution in DPI is not important, only the number of pixels is?

I understand that with the equirectangular projection, things at the poles appear much bigger than what they are in reality.
If we decide that a plot should be X10 times larger than the world map for the specified area at the equator, does it mean that this multiplacator will need to change according to the latitude ?

[Redrobes]

The DPI is irrelevant and is another kind of scale.

The map has a real world size and the images for it have a bitmap number of pixels we call resolution. For every image there is a scale from pixels to real world ratio and this changes from the equator to the poles as given in the table above because of the spherical shape of the globe and rectangular shape of the bitmap representing it.

DPI is used when you print an image to a piece of paper which also has a real world size of about 12 inches which is a lot less than 40,000km so there is a scale in pixels to inches which is DPI for paper and a scale of pixels to km, miles, leagues etc for the map. Once you print the map onto paper there is now a new scale of km, miles, leagues ratio to inches of paper length. This is the more normal miles per inch that you might see on a bought map.

So in order to get a scale you need TWO units and the scale is the ratio between the two. It doesnt matter what one of the values is alone, the scale only matters when you have two values. It matters what size your mountains and cities are on the bitmap as a ratio to the size of the landmass. That is fixed by the CWBP map world and that spherical adjustment that you need to make for it. But for every bitmap you can choose the resolution and map it at whatever pixels per km you want so long as the mapping is matched to the landmass sizes. You can print the final maps out onto whatever size paper you feed into your printer and then you must pick your DPI to make it fit.

This link might help explain scale, resolution and DPI.
http://www.cartographersguild.com/tu...explained.html

[Azélor]

I already know about DPI. My point was that if you try to merge 2 files with a different DPI, you will need to adjust the size/DPI manually.

But does that mean that the same object will have a different size depending where it is on the map?

Here's an example, if I take two different latitudes:
y=875 (more or less the 60th parallel) and y=2500 (equator)

the second is twice as big as the first
if a mountain is 20 km wide at the equator (5 pixels), it is still 20 km at y=875 but it appear larger at 10 pixels since the map is stretched?

But I'm not sure if you answered my second question or maybe I was not clear.
I mean, regional maps or plots should show more details than the world map. In order to do that, they need to be bigger (more pixels). But how big should they be? 5, 10, 20 times bigger?
If we have two map at the equator, next to each other, is it important to use the same size (x5,x10,x20...) or not?

I was imagining that a desert could be mapped with a different multiplicator since it's less interesting but I'm not sure it's a good idea.

[Redrobes]

Yes, to the degree that a house at the pole will take up all of the top row of pixels as the scale goes to 0 or infinity depending on how you measure it. Our normal Mercator style Earth map does significantly distort the areas of countries leading to a very false sense of land mass size.

I don't know if you have to have a fixed scaling multiplier for larger scale maps especially since it also depends on the latitude, but you could specify that as a requirement if you like. If you want to make a single map of all the plots then you do need to resample all the map images together. That's normally a nightmare which is why I wrote my app.

You will find that city maps will probably need to be more than 20x resolution on the original bitmap scaling. Its likely that you will need a bitmap scale of one pixel to about 20cm so that is more like 1 pixel on the original map is 20,000 pixels of city mapping at the equator ! Or to put it another way - one pixel of the original map is not likely to cover a whole city in a normal fantasy demographic. So you wont be able to make one bitmap to hold all the world showing anything with city like detail.

You are bound to have discreet mapping levels or to use a zoom style map browser.

[Azélor]

Discreet mapping

I don't know if you have to have a fixed scaling multiplier for larger scale maps especially since it also depends on the latitude, but you could specify that as a requirement if you like. If you want to make a single map of all the plots then you do need to resample all the map images together. That's normally a nightmare which is why I wrote my app.

So it's not really a requirement and I guess the level of detail depend on the artist?