Difference maps are used to highlight the differences between two variables on one image. Suppose you make two contour maps, one of Normal Temperature, the other of La Niña temperature. It can be difficult to make precise observations by toggling between the maps. Creating a difference map can give you a more precise understanding of the differences between them. This technique can be used on different variables and map types.
How to make a Difference Map
Click on Edit and select Create Formula.
A box will appear asking for a Name and Formula.
Give your map an appropriate name.
In Formula, type the name of one variable minus another variable. Then click on Add Formula.
Example: To find the difference between La Niña and Normal Temperatures, type La Nina Anomaly (anomaly is a generic term for difference from normal) in Name and l-n in Formula. Then click on Add Formula.
In the Visualization State box, under Formulas, double click on the name
of your difference map. Select the type of map that you want to make.
On the left side of the new pop-up box:
Click on the second magnifying glass: 3D Grid or 2D Grid. Do not double click or the box will disappear and you will have to repeat step 3.
Single click on the variable.
Repeat this step for the right hand side.
Click OK.
In this example, the left box would have La Nina Temperature (3D Grid)
highlighted and the right side would have Normal Temperature (3D Grid)
highlighted.
You can now manipulate the appearance of your map.
Note: To get an anomaly map, in this case, it is important to subtract the Normal from La Niña because we want to look at how La Niña differs from the Normal. The cooler temperatures will appear as negative numbers, the warmer temperatures as positive. If our formula were n-l, negative numbers would denote the warmer temperatures during la Nina.
