SketchAndCalc uses the ‘surveyors formula’ to Calculate Irregular Land Area once the Perimeter has been drawn.
Carefully defining the area’s perimeter is critical to the overall accuracy of the land calculation. As the process of drawing establishes vertices for which X and Y coordinates are plotted and entered into a mathematical table.
As you continue to draw, the table of X and Y coordinates builds until the shape has been closed (the starting vertex joins the ending vertex).
The order of vertices defines which XY coordinates should be cross multiplied to corresponding coordinates. This order is necessary to define the table and is why SketchAndCalc’s drawing behaviour enforces continuous ordered drawing. i.e. you can not draw lines out of order, as there would be no way to define the table.
With the table now defined the mathematical algorithm can be applied to calculate land perimeter and area in all common measurement systems, metric and imperial.
As for drawing scale, when using maps the drawing scale is informed by the map data itself. So unlike calculating the irregular area of images where an image can be uploaded to the drawing canvas, there is no need to set the drawing scale with a known length.