After I download OSM data from CloudMade.com, I use the C# code here (http://wiki.openstreetmap.org/wiki/Mercator#C.23) to transform the longitude and latitude into X and Y coordinates. Then I convert them into ArcGIS shapefile, and use the tool to calculate the area and length. To my big surprise, the results are not correct. For example, the actual area of Sweden is only 0.45 million square kilometers, while the result is summed to 1.17 million square kilometers.
Am I doing something wrong? Any of your response and comment are highly appreciated.