Didger can instill a degree of confidence about the accuracy of your digitizing project because it reports root-mean-square (RMS) error values. Digitizing, by nature, is never exact but you can use the calibration RMS error values to make statements such as, "I am 95 percent sure that this digitized point is within two meters of the true location on the ground." This lends a certain credibility to your digitizing project.
The final Didger project accuracy is determined by the care you put into the calibration process. If you demand a high degree of precision and accuracy for your project, take care when working through the calibration from selecting the calibration points to digitizing them. If accuracy is not highly important to your results, calibration requires very little effort. However, it is always a good idea to verify that RMS values are reasonable before proceeding with a project.
The number of calibration points needed to determine RMS depends on which georeference method is selected. See Spatial Transformation Methods for information on the number of calibration points necessary for each method. A larger number of points are better because you can then use the RMS error values with more confidence when interpreting your final digitized data. However, more calibration points do not necessarily provide a more accurate project, only more confidence in the reported RMS values.
There are three points to consider that can affect the error values and ultimately the RMS values.
1. How well did you define the X and Y coordinates for your calibration points?
§ When you choose your calibration points, determining the exact XY coordinates for the points is important. If you are determining XY coordinates from an axis, you must rely on using a straight edge and ruler to determine exact coordinates. In these cases, the points must be measured and recorded as accurately as possible.
§ The calibration point coordinates must also be typed into the calibration grid during the calibration process. You should be careful to enter the correct coordinates during this stage. If you find a problem with your XY coordinates, correct any mistakes in the calibration grid coordinate values. Refer to the tutorial for more information on the calibration grid and calibrating. If you change a value in the grid, you do not have to redigitize the point (or points) if you are confident that you clicked in the precise location when digitizing the calibration points.
2. How precise were you when you clicked the calibration points?
§ Even with exact calibration point coordinates, you should use care when clicking the points. Make sure the pointer is directly over the points when you click them onscreen or on the digitizing tablet.
§ If the RMS value is not acceptable and your XY coordinates are correct, you can redigitize the points. Refer to the tutorial for more information on calibrating.
3. Finally, there are four conditions under which Didger does not calculate RMS values, or the RMS values are meaningless.
§ when you are using one or two log axes
§ when you only have three calibration points with an affine polynomial georeferencing method
§ when you select the thin plate spline, natural cubic spline, Marcov spline, exponential spline, rational quadratic spline, or inverse distance squared georeferencing methods
§ when your project uses different X and Y scaling (not the same number of units per inch in both dimensions)
RMS Error Value
Calculating Allowable or Acceptable RMS Error
An Example of Allowable Error Based on Map Scale
An Example of Allowable Error Based on a Percentage Value