Problem with linear measure

[Roberta Rocha]

Hi everybody

I work with reptiles. Now we're developing a biometry research with Boa snakes, comparing traditional measurements (with a string or a metric tape) to computer measurements (with ImageJ). I think I'm not processing image in a correct way, maybe some problem with resolution. My ImageJ measures have a big error in comparison to analogic measurements, about eight to fourteen centimeters. I tried to attach the picture but the size is surpassing the maximum permitted in mailist. The known distance used to set the scale is a 20 cm white ruler, placed next to the snake.

My steps are these: 

1) With a segmented line, I get the measure of the rule in pixels.
2) I Set the scale.
3) I draw a segmented line over the snake's back.
4) I analyze measure (ctrl+m).

The snake in the picture I tried to attach had 174 cm length in traditional way, while in ImageJ it measured 160 cm. Even if we can't get the same measure, we should reach a more similar value in this scaling.

I guess there's something more to do, maybe convert the bits or resolution. Also thought about straighten the segmented line to get the measures, but I didn't get how to do this.

I'll appreciate any help. If it is necessary, I can send the picture to anyone's email to try to measure.

Best regards.

Roberta Braga - UFC - Ceará, Brazil
 
 
ROBERTA ROCHA
--------------------------------------------------------------------------
Médica Veterinária - CRMV/CE n°2099
Núcleo Regional de Ofiologia - NUROF
Universidade Federal do Ceará - UFC
(85) 3366.9801 / 3366.9810
[hidden email] / [hidden email]

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ImageJ mailing list: http://imagej.nih.gov/ij/list.html

[Michael Schmid]

Hi Roberta,

two possible causes for inaccurate line measurements:

- Make sure you have the last version of ImageJ (Help>Update ImageJ). There were problems with line measurements in some old versions.

- Make sure the image is not distorted. The camera axis must be exactly perpendicular to the plane where the snake is placed. Also, many consumer cameras have bad barrel distortion at the wide-angle side of the zoom lens. You can put a few rulers (or other objects with well-defined size like sheets of paper) in different parts of the image (center, near corner) and with different orientations to see whether all show the same length. If they don't, the image is distorted.

By the way, I'd propose to use the straight line, not the segmented line for calibration with the ruler. When using the segmented line, a small extra segment created when trying to finish the line by double-clicking might go unnoticed.

Michael
________________________________________________________________
On Sep 19, 2012, at 19:50, Roberta Rocha wrote:

> Hi everybody
>
> I work with reptiles. Now we're developing a biometry research with Boa snakes, comparing traditional measurements (with a string or a metric tape) to computer measurements (with ImageJ). I think I'm not processing image in a correct way, maybe some problem with resolution. My ImageJ measures have a big error in comparison to analogic measurements, about eight to fourteen centimeters. I tried to attach the picture but the size is surpassing the maximum permitted in mailist. The known distance used to set the scale is a 20 cm white ruler, placed next to the snake.
>
> My steps are these:
>
> 1) With a segmented line, I get the measure of the rule in pixels.
> 2) I Set the scale.
> 3) I draw a segmented line over the snake's back.
> 4) I analyze measure (ctrl+m).
>
> The snake in the picture I tried to attach had 174 cm length in traditional way, while in ImageJ it measured 160 cm. Even if we can't get the same measure, we should reach a more similar value in this scaling.
>
> I guess there's something more to do, maybe convert the bits or resolution. Also thought about straighten the segmented line to get the measures, but I didn't get how to do this.
>
> I'll appreciate any help. If it is necessary, I can send the picture to anyone's email to try to measure.
>
> Best regards.
>
> Roberta Braga - UFC - Ceará, Brazil
>  
>  
> ROBERTA ROCHA
> --------------------------------------------------------------------------
> Médica Veterinária - CRMV/CE n°2099
> Núcleo Regional de Ofiologia - NUROF
> Universidade Federal do Ceará - UFC
> (85) 3366.9801 / 3366.9810
> [hidden email] / [hidden email]
>
> --
> ImageJ mailing list: http://imagej.nih.gov/ij/list.html

--
ImageJ mailing list: http://imagej.nih.gov/ij/list.html