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I've decided to give this map design thing a go - I downloaded the USA counties shapefile from National Atlas, exported them to SQLite from QGIS, and then linked the SQLite file in Tilemill, and this is the result I get:

Stretched Out Map

I know that something with my SRS string is off, but when I selected WGS84, it still looks too wide. What should I be doing to make sure my map projection remains proper?

(The reason I exported to sqlite was that there are like 3000 nodes in that shapefile, and mapbox would only show 500 when I imported the shapefile directly)

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    Welcome to gis.SE. Per the answers below, its a matter of how you choose to distort (some part of) of the ellipsoid / geoid to show it in 2D. You think it looks "too wide", but it would help if you can tell us what you want it to look like. Alternatively, you're trying to make a map for a reason - to communicate something or perhaps just to look nice. What is that reason? – BradHards Dec 29 '13 at 0:05
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There are a very large number of map projections to choose from.

Which ones are "proper" really depends on what you mean by "proper". Every single one of them introduces some kind of geometric distortion (such as distance, direction or area), yet each also usually preserves some kind of geometric quality. What quality/quantity do you wish to keep "proper"?

WGS84 is a spatial reference system, as you say, but is not actually a projection. Any time you have geographic coordinates (as you seem to have) and have them plotted out, the default "projection" is usually Plate Caree (equirectangular or equidistant cylindrical -- it has many synonyms) projection because the maths are trivial.

The fact that your output is "too wide" suggests to me that you wish to preserve "shape" and that tends to be a quality of conformal map projections. A common conformal projection for the USA is Lambert conical conformal projection. That will give you good shape, including curved parallels.

Experiment with different projections. (Details on how to do that could be given by folks who know the details of Tilemill, SQLite from QGIS.

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TileMill displays all spatial data in EPSG:3857 / Spherical Mercator. This is why your projection looks "too wide." I know there is a way of changing the projection TileMill uses but am not entirely sure how, believe it's a bit of a hack. However, the folks at MapBox are very nice and if you contact them I am sure they would offer you advice on how to use a different coordinate system such as the Lambert Conformal Conic for your map.

update: this looks promising: http://blog.kartena.se/using-tilemill-without-spherical-mercator/

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WGS 84 (a Lat/Long reference system) is not strictly speaking a projected coordinate system. For a more "as usual" rendering of your dataset, I recommend that you use the Albers Equal Area Conic Projection. But it depends more on what properties (area, angles, orientation...) you want to preserve on your map.

For more choices, see http://www.epsg-registry.org/ where you will find a list of commonly used projection for the different parts of the World.

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