The latest ASPRS Positional Accuracy Standards for Digital Geospatial Data shows the correlation between vertical RMSE and contour interval. The updated ASPRS standard uses accuracy based upon the RMSE (Root Mean Square Error) at the ground scale. The ASPRS Accuracy Standards for Large Scale Maps from 1990 used RMSE, however the difference comes in how the standards represent the accuracy classes. “The legacy ASPRS map standard of 1990 uses Class 1 for higher accuracy and Classes 2 and 3 for data with lower accuracy while the new 2014 standard refers to the map accuracy by the value of RMSE without limiting it to any class.”

Due to the differences in how accuracy classes are represented, users needed a way to relate the two standards to one another. The following equation was created to determine how RMSE and the minimum Contour Interval are related.

** RMSEz = 1/3 x contour interval (CI)**

The LP360 Control Report states the vertical accuracy class and the minimum contour interval based upon their relationship to the RMSEz.

The vertical accuracy class is based off the RMSE value. In the example above the vertical accuracy class is 9 cm. Knowing this information one can then use the above equation to determine what is the minimum contour interval equivalent that should be used with that data.

9=1/3 x **CI**

**CI** = 27 cm