Defining the axes

After you have started a new document , you will need to tell ScanIt how pixel coordinates translate to axes coordinates. You do this by picking points in your image that have known graph-coordinates. To start defining the axes, make sure you are in Axes definition mode. You can either select this mode from the Edit menu, or from the mode list in the top left of the window.

the general case

Generally, you need three points with known coordinates to define your axes. You click with the left mouse button in the graph to add a marker at known coordinates. You will be prompted for the coordinates. It is advised that you pick a marker on the point where the two axes cross, and one point each on the other far end of each axis, like so:

example graph

3 suitable markers at known coordinates for axes definition. Picture from: Jasper van Baten, "CFD: A Design and Scale Up Tool for Multiphase Reactors", PhD Thesis, University of Amsterdam, 2000.

The hint in the top of the window will tell you how many points are set and how many more are required. You will need three sets of known coordinates in total.

If you are not happy with a marker, you can click on the marker to modify its graph coordinates, or to delete it. You cannot move the marker; if it needs to be moved, delete it and re-insert it.

Once you have set your third point, you will automatically be prompted to accept the axes. You can then select whether you want you X or Y axis (or none/both) to scale in a logarithmic manner. The bottom of the window will then tell you how much the graph coordinates will modify if you move one pixel horizontally or vertically in the window. It will also show you - in degrees - the angle between the X and Y axes, based on the known coordinates you have provided. Usually this needs to be close to 90 degrees. If this number is not correct, check your axis specifications. Sometimes your axes cannot be accepted; for example if you have picked your three points on a straight line.

If you have accepted the axes, you can start gathering data. You can also cancel the accepted axes, so that you can modify them. To do this, click anywhere in the image while the axes have been accepted and you are in Define axes mode. To remove all defined axis information, choose Clear axis information from the Data menu.

Once your axes have been accepted, the graph coordinates under your mouse pointer will be displayed in the right of the status bar at the bottom of the screen.

the special case

Sometimes, you do not know the X- and Y-coordinate at the same location. This happens when the X- and Y-axis do not cross, or the coordinate values at the crossing are not known. In this case, you can get away by selecting two points with known X-coordinate at unknown but equal Y-coordinate (i.e. on the X-axis), and after that, selecting two points with known Y-coordinate at unknown but equal X-coordinate (i.e. on the Y-axis). So, 4 points in total:

example graph

2 suitable X-markers and 2 suitable Y-markers for axes definition. Picture from: Jasper van Baten, "CFD: A Design and Scale Up Tool for Multiphase Reactors", PhD Thesis, University of Amsterdam, 2000.

You will need to start by picking a point at the X-axis. Make sure that the Y-coordinate is blank, i.e. clear the initial value for Y. The hint in the top of the screen will tell you what marker is expected in what order.