areaqgisqgis-plugins
Is there any way I can automatically measure/calculate the area of a shapefile (a polygon for instance) without using the qgis default measure tool and avoid to manually have to draw the area to be measured/calculated?
As an option, perhaps a tool that allows me to enter coordinates and have the area information as a result.
For the Project coordinate reference system, you have to choose a projected CRS, which uses real metres as units. Openlayers Plugin uses Google mercator, which is only corresponding to real meters at the aequator. The further north you come, the more distorted the length units are (look at Greenland in Openstreetmap, it is not so big in reality).
By creating a blank project, you have most probably WGS84 as CRS, wich will give you distances (and areas) in degrees, not meters. Any UTM CRS will do, as long as you are within the 3-degree zone of the CRS. Many countries have their own projected CRS to enable most precise surveying.
Qgis seems to measure distances in most projected CRS correct, but this might not be the case with area measuring.
EDIT: If you check "Ellipsoid" in the measurement toolbox, the area is always computed in metres. At least for Qgis Lisboa.
Activate edit mode, and open field calculator. There, use the Geometry -> $area function on existing field, or create a new field. Note that the units are the same as your CRS, so if it's in square meters, multiply by 0.00024711
Best Answer
This plugins might help you. Not exactly by giving you the area by enetring coordinates, but it makes measuring areas faster.
1-CalcArea: this plugin calculates the area of a polygon while it's being created or edited
2-AutoFields: Automatic attribute updates when creating or modifying vector features This plugin allows you to configure vector fields to be automatically calculated for you when digitizing new features or modifying existing ones. Think about it as triggers in a database or as an automatic Field Calculator.
I personally use the second one. When I import a shapefile, i activate the plugin to automatically generate an Area attribute (new or existing column), and automatically calculates the Area of all existing polygons. If i add or edit poligons in that shapefile, the area is re-calculated automatically.
When there is an "Area" attribute in your shapefile, then the "identify tool" can be used to see the polygon´s area with one click.