Learning to Measure Image Features Using ImageJ

The principle is that if you know the size of one feature in an image then you can use this as a guide to measure other features.

In our example we’ll use the image below. As we know the size of the Sun and we can use this as a ‘baseline’ to measure other features in the image. To download, left-click the image to enlarge it and then right-click and choose ‘Save Picture As…’.

First open registax, it’ll appear as a small bar like below.

Choose ‘File’, ‘Open’ and find the image that you want to work with (if you’re practicing with this image make sure you’ve saved it first!), click open.

Another window will open containing the image like below.

Before we can measure anything on the image (such as the prominence) we need to tell ImageJ a ‘baseline’ measurement. We know the diameter of the Sun so we can use this. If you don’t know or can’t remember this click here.

Click on the line tool as shown below.

Now draw a line across the centre of the Sun (like marking on its diameter) like below. Try to be as accurate as possible and as you do it keep an eye on the ‘length’ value in the ImageJ window. Don’t move your mouse after you finish the line as this will make the ‘length’ value disappear.

Now, remembering the length value, click ‘Analyse’, ‘Set Scale’ like below left to get the menu like below right.

Type into the ‘Known Distance’ box the size of the Sun and into the ‘Unit of Length’ box whatever units you’ve used, I’ve chosen to use kilometres like below. Then click ok.

Now we can measure other features of the image by drawing more lines.

To measure the prominence draw a line from the edge of the Sun to the furthest point of the prominence like below

The size of the prominence is shown in the ImageJ window as the length value, in this case it is 363,648 km.

This value will only be as accurate as your measurements so you need to bear this in mind when working with the figures. In this particular case it would be sensible to say that the prominence is about 364,000 km big (28 times the Earth’s diameter!!!).

It is possible to take measurements using other tools in ImageJ, such as the circle drawer etc, but we’ll leave it to you to investigate these.