[GIS] Calculate polygon area in QGIS 2.18

geometrypolygonqgis-2shape-area

I'm trying to calculate a polygon area. This is the polygon and the attribute table:

When I open field calculator and use the Geometry–> area$, the area result for ID 1 is (in square meters) 1809477.67:

but when i use the Export/Add geometry column tool, two things happen:

a new layer been created named "added geom info"- why the data isn't created in the polygon attribute table?
the area result is (in square meters): 1799292.92 – why I get different results?

When I measure the area in ArcGIS 10.3 – the result was 1799292.92 square meters.

I also read

How to calculate polygon areas and perimeters using QGIS?

but didn't find any suitable answer.

Best Answer

The Export/Add geometry column tool states:

This algorithm computes geometric properties of the features in a vector layer. It generates a new vector layer with the same content as the input one, but with additional attributes in its attributes table, containing geometric measurements. Depending on the geometry type of the vector layer, the attributes added to the table will be different.

You can calculate the area directly from the attribute table. Crate a new decimal/real field, and use the $area expression.

The values will correspond to the layer CRS

Related Solutions

[GIS] Calculate polygon area inside extent

I realize that this an answer to an older thread but if you've upgraded to 10.1, check out the Tabulate Intersections tool in the Analysis toolbox (ArcInfo level).

I don't think it handles empty spaces but in theory you could create a bounding box polygon, clip out your original data, merge the clip with your original data and assign the clip with dummy attribute(s) that are easily filtered.

Python OGR – Calculate Area of Polygons Using OGR in Python Script

I ran your script (slightly modified) at the Python Console of QGIS:

from osgeo import ogr

vlayer = iface.activeLayer()

provider = vlayer.dataProvider()

path = provider.dataSourceUri()

tmp = path.split("|")

path_to_shp_data = tmp[0]

driver = ogr.GetDriverByName("ESRI Shapefile")
dataSource = driver.Open(path_to_shp_data, 1)
layer = dataSource.GetLayer()
new_field = ogr.FieldDefn("Area", ogr.OFTReal)
new_field.SetWidth(32)
new_field.SetPrecision(2) #added line to set precision
layer.CreateField(new_field)

for feature in layer:
    geom = feature.GetGeometryRef()
    area = geom.GetArea() 
    print area
    feature.SetField("Area", area)
    layer.SetFeature(feature)

dataSource = None

and it worked (see next image).

However, the precision of values (0 decimal) at the field "Area" is different to values printed at the Python Console:

1062218109.64
1241319130.43

As you are pointed out that your printed areas are very small (0.00000x) and likely do not reflect square meters, this is the reason for your resulting "Area" field contains all 0's. Probably, you have a projection problem in your shapefile. It is not in meters.

Editing Note:

I included the code line to set the precision (2 decimals) of 'Area' field and it worked.

Best Answer

Related Solutions

[GIS] Calculate polygon area inside extent

Python OGR – Calculate Area of Polygons Using OGR in Python Script

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