# Formula for forward-inverse RMSE?

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05-08-2021 04:02 AM
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New Contributor II

Hello! I am struggling with understanding the three types of RMS error reported while georeferencing in ArcGIS Pro. All of the documentation seems to include this paragraph (emphasis added):

The forward residual shows you the error in the same units as the data frame spatial reference. The inverse residual shows you the error in the pixel units. The forward-inverse residual is a measure of how close your accuracy is, measured in pixels. All residuals closer to zero are considered more accurate.

OK, so I understand how forward and inverse RMSE are calculated, but what the heck is forward-inverse?! The description is a non-explanation; it sounds exactly the same as inverse RMSE. Obviously it isn't, though, because it is given as a fractional decimal less than 1. Can anyone explain or provide the mathematical formula for forward-inverse RMSE so I can understand what it actually is and why it might be useful to interpret along with the other two?

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