Gimp script: Selection area finder

[Redrobes]

Sounds like you should at least start by upping the color palette from 8 bit greyscale to full 24bit RGB. Not sure about the other one but maybe they are related.

[blocus]

Thx a lot for your fast answer, I selected the RGB mode, and then it assest the perimeter as well as the area , however i found that the value are wrong compare to the reality.
Indeed I'm interesting in the area, and when i compare the number of pixel from the histogram option and the percentage of area ( %*nm of pixel) the value are similar. But when i converted with the real scale , in my case i use 150 micrometers , so I multiplicate the number of pixel per 150*10e-3 and it gives wrong estimation , do you have an idea to assess the superficie with the number of pixel ?
Merci Beaucoup
:=)