The Ruler tool helps you position images or elements precisely. The Ruler tool calculates the distance between any two points in the workspace. When you measure from one point to another, a nonprinting line is drawn, and the options bar and Info panel show the following information:
Position with the Ruler tool
The starting location (X and Y)
The horizontal (W) and vertical (H) distances traveled from the x and y axes
The angle measured relative to the axis (A)
The total length traveled (D1)
The two lengths traveled (D1 and D2), when you use a protractor
All measurements except the angle are calculated in the unit of measure currently set in the Units & Rulers preference dialog box.
If your document has an existing measuring line, selecting the Ruler tool causes it to be displayed.
To resize the line, drag one end of an existing measuring line.
To move the line, place the pointer on the line away from either endpoint, and drag the line.
To remove the line, place the pointer on the line away from either endpoint, and drag the line out of the image, or click Clear in the tool options bar.
You can drag out a measure line on an image feature that should be horizontal or vertical, and then choose Image > Image Rotation > Arbitrary. The correct angle of rotation required to straighten the image is automatically entered into the Rotate Canvas dialog box.