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Maybe this is standard across mapping engines, but in MapGuide if I set the scale to 7 million I can see about half the world. At about 4 million I can see a continent. What unit of measurement could this scale possibly be in?

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By definition, a scale is unitless, because it expresses a ratio of a distance on the map to a distance on the earth. See iant's reply to your related question and @alex markov's comment below that reply. – whuber Mar 7 '12 at 18:37
@whuber Thanks, it makes more sense now. – Garrett Hall Mar 7 '12 at 19:40

As whuber wrote in his comment: Scale is unitless. It expresses a ratio of a distance on the map to a distance on the earth.

e.g. A scale of 1:150,000 means that 1 cm on the map equals 150,000 cm or 1.5 km in the real world.

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The convention is 72 pixels to the inch. I couldn't say for sure what MapGuide is using.

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Unless exporting to WMS, 96 pixels/inch is the default for mapguide, and the only possibility when using tiled basemaps. – johanvdw May 6 '12 at 17:37

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