Superb - so you use a selection wand in the area and then ask it how big this is. Cool and probably clearer than using the histogram window.
I just wrote a little GIMP script inspired by the discussion here: http://www.cartographersguild.com/showthread.php?t=2279
It will tell you the perimeter of the current selection in pixels and the area as a percentage of the total image area.
Installation and use: Once you've installed the .scm file in your gimp scripts directory, "Area and perimeter" is accessible under "Select" from the image menu.
UPDATE: Version 0.2.0 - added perimeter-finding
Last edited by isomage; 12-13-2008 at 02:37 AM. Reason: New version
Last edited by isomage; 12-12-2008 at 08:45 PM.
For a test, I computed the area of Texas using this image.
The image is 750 x 733 pixels. According to its scale, 96 pixels = 100 miles.
Selecting Texas and running the script gave 47.2%. The selected area was then (100 / 96)^2 * 750 * 733 * 0.472 = 281556 square miles.
Wikipedia says that the area of Texas is 268820 square miles. If the map and its scale are perfectly accurate, then we've overestimated by a little less than 5%.
That's not too bad an estimate, considering that the map may not be perfectly accurate; a scale of 98.2478 pixels per 100 miles, instead of 96, would have given a correct result -- a mere two pixels of difference.
Last edited by isomage; 12-13-2008 at 02:43 AM.
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Note that the histogram function doesn't work on color-indexed images. The script currently will only work in RGB mode, though I may add support for grayscale later.
I follow your instruction isoman but once i installed the file and i select perimeter and area , Gimp heralds me 2 errors
such as : Calling error for procedure 'gimp-image-add-layer'
and execution error for Area and perimeter : error procedure execution of .... failed on invalid input arguments : image '... ' is of type 'gray', but an image of type 'rgb' is expected
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 ?
In theory it should be right or somewhat close. All I can suggest is that a micro meter is 10^-6 not 10^-3 which is millimeters. So if you were a 1000 or a 1000000 out in area then that could be the case. Otherwise I am not sure.