Calculate polygon area in QGIS 2.18

Question

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:

1. a new layer been created named "added geom info"- why the data isn't created in the polygon attribute table?
2. 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.

Answer 1

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