I'm not familiar with open source drawing options outside OpenOffice.org, but it is a great drawing tool. Unfortunately, there isn't an easy way to measure the radius of an ellipse or circle, or between two centers, since the center is displayed only when you're in rotation mode. You could draw two intersecting lines over the shape to show the center, aligning them with the circle, but that's a little bit of a hack. (To do so, draw the shape, draw a line roughly vertical through the center, and use the alignment feature to align that line in the center of the shape--select both, right-click, and choose Align > Center. Repeat those steps with the horizontal line. The intersection point will be the center.)
However, if you just need to measure the diameter, you can use Draw to do what you need.
To draw a circle or ellipse on a tiff/jpg:
- In OpenOffice.org, choose File > New > Drawing.
- Choose Insert > Picture > From File and bring in the graphic you want.
- Click the circle/ellipse tool and draw the shape you want. Hold down Shift to keep the circle proportionate.
- To see the measurements of the entire ellipse, right-click on it and choose Position and Size. From here you can rotate precisely, see and change dimensions and size, and change the pivot point of the object.
- If you want to measure the circle or ellipse to get information not displayed in the Position and Size window, you can use a dimension line. Use the Lines and Arrows icon in the Drawing toolbar at the bottom. Use the Dimension Line icon to draw the line over the part of the circle or ellipse you want to measure. Choose View > Zoom to zoom in for more precision. (See note at beginning of this reply for a workaround of how to display the center consistently.)
This was first published in October 2005