total length measurement of an ellipse

classic Classic list List threaded Threaded
3 messages Options
Reply | Threaded
Open this post in threaded view
|

total length measurement of an ellipse

austin_millbarge
I am trying to get the total length measured of an ellipse (or partial of one), and it doesn't appear to be reporting the correct length.  I create an ellipse in the following way:

1) select Ellipse Arc (Axis)

2) Click on origin (0,0) as center of ellipse.

3) Go up to 300mils to (0,300) to set endpoint of major axis.

4) Go to the right 100mils to (100,0) to set endpoint of minor axis.

5) Starting at 0 degree, end at 180 degrees to create a half ellipse.


When I select the half ellipse, and measure Total Length of Selected Entities (and this ellipse is the only thing selected), I get a length of 711.0605mils.  Using some of the online calculators to calculate the circumference of an ellipse, using A=100mils (x-axis radius) and B=300mils (y-axis radius), the total circumference of the ellipse should be 1336.49mils.  Divided by 2 (since I have half an ellipse), I should get 668.25mils...NOT 711.061mils which LibreCAD is reporting.  Am I doing something wrong here?  I get the same wrong calculation for a full ellipse.  Changing dimensions doesn't seem to work, nor changing decimal settings or grid spacings (was thinking it was a rounding error).  If I use the same ellipse command, but create a circle (300mils major and minor axis), and measure the length, I get the correct response of 1884.96mils.

Any suggestions?
Reply | Threaded
Open this post in threaded view
|

Re: total length measurement of an ellipse

austin_millbarge
Any thoughts or responses on this?  Really would like to know if this is a bug or not.
Reply | Threaded
Open this post in threaded view
|

Re: total length measurement of an ellipse

dellus
I have tested it and have to confirm your finding. LibreCAD calculates the length wrong. Other CAD systems do it right, for instance QCAD, of which LibreCAD is an offspring.
As there are hardly any developers engaged at the moment and there are more important things to fix, we will have to live with this. But it's good to know.
I'm not a maths crack at all, but googling has shown that calculating the length of an ellipse is not a trivial thing, complicated formulas. Maybe we should check if the length of splines is done right.