The area measurement

[Kate Salmon]

Hi All Image-J users,

I am using Image-J for measuring the size (area and Ferets length) of foraminifera shells (microscopic shells made of calcium carbonate).  However, I am unsure what the 'area' measurement on the summary of results is actually measuring...  For instance, I measured a completely spherical shell and compared the area measurement made by ImageJ with the area of sphere which I calculated by using the Feret's diameter/2 and they came out as completely different measurements!

To measure the shells, I 'colour threshold' the image and then 'analyze particles', by showing the 'outlines' so I can see which measurment is assigned to which shell.  This replaces the image with outlines of the shells and I have always assumed in the past that the area would represent these outlined shells.  However, now I am unsure what they represent as there seems to be a large discrepancy between the measured area and my calculated area.

I have looked in the user guide but all it says is that the area measurment represents 'area of selection in square pixels or square calibrated units'.  It should be in square calibrated units as I set the scale at the beginning.

Can anyone explain to me exactly what area is measuring and why it is different to my calculations?

Many Thanks,

Kate

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[BenTupper]

Hi Kate,

On Jul 8, 2014, at 12:27 PM, Kate Salmon <[hidden email]> wrote:

> Hi All Image-J users,
>
> I am using Image-J for measuring the size (area and Ferets length) of foraminifera shells (microscopic shells made of calcium carbonate).  However, I am unsure what the 'area' measurement on the summary of results is actually measuring...  For instance, I measured a completely spherical shell and compared the area measurement made by ImageJ with the area of sphere which I calculated by using the Feret's diameter/2 and they came out as completely different measurements!
>
> To measure the shells, I 'colour threshold' the image and then 'analyze particles', by showing the 'outlines' so I can see which measurment is assigned to which shell.  This replaces the image with outlines of the shells and I have always assumed in the past that the area would represent these outlined shells.  However, now I am unsure what they represent as there seems to be a large discrepancy between the measured area and my calculated area.
>
> I have looked in the user guide but all it says is that the area measurment represents 'area of selection in square pixels or square calibrated units'.  It should be in square calibrated units as I set the scale at the beginning.
>
> Can anyone explain to me exactly what area is measuring and why it is different to my calculations?
>

It is hard to tell from your explanation. You don't fully explain what you mean by 'area of a sphere' 'calculated by using the Feret's diameter/2'  You can compute the volume of a sphere, but your 2d image will be a projection of the sphere thus the area computed by ImageJ will not be the volume.

Are you approximating the area of the shells using  Area = pi * (Feret's diameter/2)^2 ?  If so, then you might very well might find that your calculation differs widely from the area computed by ImageJ depending upon the shape of your shells.  You can see more about Feret's diameter here ...

http://rsb.info.nih.gov/ij/docs/guide/146-30.html#sub:Set-Measurements...

http://en.wikipedia.org/wiki/Feret_diameter

Cheers,
Ben



> Many Thanks,
>
> Kate
>
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> ImageJ mailing list: http://imagej.nih.gov/ij/list.html

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