The Field Calculator performs simple and complex calculations for both text strings and mathematical operations. Note that the modifications affect all records (if none is selected) or only the selected records. To calculate the percentage of each polygon with respect to the total surface area (e. g. plant_cover layer), the value of the total surface area must first be determined. To do this, open the attribute table, right click on the field header (Area), select Statistics and in the Sum line you will find the total area of all polygons in this layer. Then right click on the header of the created field (Percentage), select the “Field Calculator” tool and use the following expression: [Area] * 100 / total area value. The expression [Area] in the equation corresponds to the name of the field, which can be selected directly by double-clicking on the “Fields” section (Figure 30).
Equations can contain different mathematical operations (sum, subtraction, division and multiplication), including parentheses. Calculations can be developed using VB Script or Python. When accessing geoprocessing functions is required (including entity geometry calculation), Python is recommended. In addition, this language provides greater opportunities for scripting calculations. Further information about the field calculator can be found in “About calculating fields“, where a comprehensive list of examples is available (Figure 30).
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