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I have several polygon shapefiles, both set to WGS84 CRS. I have been using the google basemap with the Openlayers plugin, using the mercator CRS. On the fly CRS transformation is enabled. Should my shapefiles be set to the same thing as the google basemap?

When I use the measure area tool (View>Measure>measure area), the areas of the polygons in my two shapefiles are accurate. However, when I try to calculate the area using the Area function of the field calculator, the values are incorrect.

I have read posts saying that the openlayers plugin distorts the areas. I tried saving the shapefiles as wgs84, opening them in a new project, and calculating the areas, but the values were the same. I am in Pennsylvania, USA. If not the mercator CRS, what should I use? Should find an alternative basemap?

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Either Pennsylvania North or Pensylvania South State Plane will be the best (depends on where you are exactly). There are variants in feet if that is your prefered measure.

You will need to use a different base map as well as Google, Bing etc are only available as Web Mercator which is pretty distorted by Pennsylvania. Have a look at PASDA in particular their PAMAP WMS layers of 45cm orthoimagery for the whole state.

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