MAPC2MAPC Help pages : Errors
How to ....
Grids and Datums
Map Images
System Issues

Calibration Inaccuracy Calculations

Internally, the program needs to create an equation that gives the relationship between pixels on the screen and coodinates on the map.

Sorry for the algebra, but these are :
  Latitude = a + b*xpixel + c*ypixel and Longitude = d + e*xpixel + f*ypixel

The six coefficients a,b,c,d,e,f are the values for an Affine Transformation (Google it!)

The program calculates these values to give the best fit. For example if pixel  413 410 is a calibration point at 39.361124 9.289135 but the calculation puts that coordinate at 416 414 then the 'error' is 5 pixels (Pythagoras!) ; the program finds values that gives the lowest value for the total error across all calibration points. (Three points will always fit exactly but using four or more means any error becomes apparent.)

Ideally the figure should be zero but it is usually impossible to be that accurate. You should be able to get to single digits : 1,2,3 or 4 though.  The program tells you the point with the largest calculated error but that may not be the point that is wrong. You can move a calibration point a few pixels by right-clicking on it - or it may be easier to right-click and then delete and then middle-click on the map to add a calibration point.

If the map is calibrated with a grid (e.g. Ordnance Survey, UTM, etc)  the program will maintain two sets of calculations : One linking Grid Coordinates to pixels and one for Latitude and Longitude as above. These are reported separately :

Average projection inaccuracy in original grid=904,0511
Largest inaccuracy at point 4 at 413 4407 -1,5 904,3
Inaccuracy will be reduced by moving this point right 1 up 904

These figures are so high because this particular calibration is faulty.

Some actions on the map will change these figure 'Warp map to Lat/Long' will lose the original grid and the  figure will become zero.