PCB-Investigator offers many ways to measure distances depending on the individual application. They can be found under Start » Measurement and are listed below:
For all methods a selection history gets added at the bottom of the dialog which allows you to return two every measure you took by simply clicking on it. The visualisation will change and zoom accordingly.
The Calculate Area function either calculates the area of just the selection or all active layers. The second set of radio buttons allows you to choose if you want the algorithm to treat overlapping objects as one surface or calculate the area for each object even if this leads to duplication of overlapping areas. The result is stated in the unit you choose under Start » Change Units and can either be mm2 or mils2. Furthermore, the percentage is stated in relation to the area of the whole board which is important for one of the following use cases: