# Interpretation of RMS Errors on Digitized Maps: Decrease RMS Error Using Control Points (Especially for GATE-Geospatial 2022)

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If an RMS error is within acceptable range, assumption of accurate transformation.

If errors are made in digitizing control points or inputting latitude and longitude of control points, transformation can still be wrong.

- Latitude/Longitude on paper maps erroneous
- Inaccurate placement of control points will lead to errors in transformation

As an example, we can shift the location of control point 2 and 3 on a third quadrangle by increasing their x values by a constant. The RMS error would remain about the same because the object formed by the four control points retains the shape of a parallelogram. But the soil lines would deviate from their locations on the source map.

The same problem occurs if we increase the x values of control points 1 and 2 by a constant, and decrease the x values of control point 3 and 4 by a constant. In fact, the RMS error would be well within the tolerance value if the object formed by the shifted control points remains a parallelogram.

We typically use the four corner points of the source map as control points. This practice makes sense because the exact readings of longitude and latitude are usually shown at those points.

Therefore, the use of more control points means a better coverage of the entire map in transformation. In other situation, control points that are closer to the map features of interest should be used instead of the corner points.