Update README.md

README.md

@@ -57,6 +57,10 @@ Per-class pixel distribution:
 
 To convert depth maps to their corresponding meter values:
 depth value (meter) = pixel value*100.0/255.0
+
+The reference frames convention used in the data collection of TopAir:
+<img src="./assets/ref_frames.jpeg" width="100%"/>
+
 ### Dataset Description
 
 <!-- Provide a longer summary of what this dataset is. -->