如图，在同一平面内，AB垂直于地面， B、C、D在地面上的同一水平线上，AD = 2m，AC = 1.3m，CD = 2.1m，求AB的高度。---**Chart/Diagram Description:**
* **Type:** Geometric diagram.
* **Main Elements:**
* **Points:** Point A (appears to be the top of a tree), Point C, Point D. Points C and D are located on the surface of a body of water (labeled implicitly as a pond).
* **Lines:**
* A vertical line segment representing the tree, with its top labeled A and its base implicitly below A, resting on the ground near the pond.
* A line segment connecting points C and D, representing the surface of the pond. The area below this line segment is shaded or textured to indicate water.
* A straight line segment connecting point A to point C.
* A straight line segment connecting point A to point D.
* **Labels:** Points are labeled as A, C, and D. The object at A is depicted as a tree. The area between C and D is depicted as a pond or body of water.
* **Relative Position and Direction:** Point A is positioned above points C and D. Points C and D are on a horizontal line segment, representing the pond's surface. Lines AC and AD extend from A to C and D, respectively.
**Textual Information:**
* There is no textual information such as question stems, options, or other explanations present in the image.