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I'm trying to georeference a picture using Latitude and longitude. The points are these:

top-left:  (116.30950927734372,39.970805680527725)  
top-right:  (116.35620117187497,39.970805680527725)  
bottom-left (116.35620117187497,39.93606595478707)  
bottom-right (116.30950927734372,39.93606595478707)

When I tried to calculate the area of a polygon,the result is very small(something like e-7).I realized to my horror that the QGIS had treated the Unit as meter.What should I do?

(QGIS1.8)

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1 Answer

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On georeferencing, you have to enter the Coordinate reference system. For lat/lon, it should be WGS84 (EPSG:4326).

When you use the measuring tool, check "ellipsoid". Then the measuring is done in metres, else in map units, which will be degrees.

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bold**I mean,to calculate polygon aear in the "Attribute table".I used the"field calculator",and the expression is"$area".The result from field calculator is very small.The measuring tool works well.**bold – user13278 Dec 4 '12 at 7:06
Besides,I wonder if I could adjust my scale to a proper one.The length measured from the picture is about half as it should be.For example,the length of the playground should be 400m,but the result from the measuring tool is only about 200m. – user13278 Dec 4 '12 at 7:12
The field calculator always uses map units. If you want meters, you have to reproject your data to a projected CRS in meters, like UTM. – Andre Joost Dec 4 '12 at 7:27
Should I modify "Layer Units" in "Project Properties"?I tried to set the Layer Unit to meter and pressed "OK",But nothing happened.When I open "Project Units" again,the Layer Unit was still "Decimal degrees".I don't konw why I failed to change the Layer Unit.Is this a bug? – user13278 Dec 4 '12 at 7:39
That won't work, maybe only for the measuring tool. Field caculator only looks at the coordinate data stored for the layer. After reprojecting (and saving to another file), field caculator has the values to do the calculation right. – Andre Joost Dec 4 '12 at 7:43
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