RenderTexture depth

I'm able to take a "screenshot" when Im rendering a normal color rendertexture, and save it into a texture2d
When I change to depth mode in the rendertexture, it doesnt input anything.

Does somebody know how to convert a depth rendertexture to a normal texture2d?
I have a high "map camera" and I need depth info from above, so I can use that in a shader, but I don't want to render twice.

I'm using this from the Unity docs:

Code (CSharp):

1.     Texture2D RTImage(Camera camera)
2.     {
3.         // The Render Texture in RenderTexture.active is the one
4.         // that will be read by ReadPixels.
5.         var currentRT = RenderTexture.active;
6.         RenderTexture.active = camera.targetTexture;
7.  
8.         // Render the camera's view.
9.         camera.Render();
10.  
11.         // Make a new texture and read the active Render Texture into it.
12.         Texture2D image = new Texture2D(camera.targetTexture.width, camera.targetTexture.height);
13.         image.ReadPixels(new Rect(0, 0, camera.targetTexture.width, camera.targetTexture.height), 0, 0);
14.         image.Apply();
15.  
16.         // Replace the original active Render Texture.
17.         RenderTexture.active = currentRT;
18.         return image;
19.     }

Also I've tried using this:

Code (CSharp):

1.         Shader.SetGlobalTexture("_LevelDepth", DepthCamera.targetTexture, UnityEngine.Rendering.RenderTextureSubElement.Depth);