Hello Bart! I took your picture and analysed it with ImageJ. You can do a particle analysis with the image which measures the required diameter.
I used the "Find Edges" method Process->Find Edges and then adjusted the Brightness/Contrast/maximum Image->Adjust->Brightness/Contrast. After this i converted the image to 8-bit Image->Type->8-bit and thresholded the image Image->Adjust->Threshold until i've got closed circles in red. In the Analyse->Set Scale and Analyse->Set Measurements dialog I selected the attributes to measure (Ferrets Diameter or Bounding Rectangle! You have to adjust the appropriate scale!). I then did a particle analysis which excluded particles < 1000 Analyse->Analyse Particles. If you select the option "Add to Manager" all measured Particles are added to a "Region of Interest Manager" ROI in which you can select and measure your Particles seperately, if you wish. You can also fit a Ellipse to the selected Particle in Edit->Selection->Fit Ellipse and then measure the Particle. If you want to see a Particle measurement in Action see my application which uses ImageJ and R and the Flash tutorial in which i demonstrate a Particle measurement which is analog to the ImageJ application. http://www.uni-bielefeld.de/biologie/Oekosystembiologie/bio7app/flashindex/index.html http://www.uni-bielefeld.de/biologie/Oekosystembiologie/bio7app/flashindex/index.html I highly recommend you to use a new version of ImageJ because of this capabilities, downloadable at http://rsb.info.nih.gov/ij/ http://rsb.info.nih.gov/ij/ I hope this information is a help for you. With kind regards Marcel -- View this message in context: http://www.nabble.com/Calculating-diameters-of-cirkels-in-a-picture.-tf4319669.html#a12307704 Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.