For accurate map measurement the selection have to go through the center of the map border. But if your map border is thick then you have to grow the selection for setting it through the center line of the border. By measurement palette you can grow or shrink the selection separately width or height or both and you will get the area, width, height of stretched selection same time.
For example, if your border is 2px thick and after selection and clearing noise width is 129px in Result palette. Then new width in measurement palette have to be 130px. If you are measuring map then don’t forget to enable the “Calculate with border” option.
Whoitam Image Measurement is a special tool for measuring length with angle in degrees. It can measure length between two points with automatic calibration or custom Calibration. It shows degrees at the same time. It also shows the length of a point where nearest color of that laying under first point is present.
Whoitam Image Measurement is a special tool for measuring area on a map or any image. After automatic selection it shows perimeter, width, height, area of selection. Enabling “Analyze” option is accurate but, it shows sometime slightly different two result when clicking on different position in a same region. Then you have to choose the bigger one as result. Calibration is available. It has other features like clearing noise in selection automatically. You can also measure area by polygon tool.