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Latitude/Longitude lines must run parallel to the edges of the screen for the calibration to be

acceptable.

The

3 point method

uses an affine transformation to calculate the calibration coefficients. This is a

linear transformation and will work with maps which are rotated or skewed but the lat/Lon or

Easting/Northing grid must be linear (i.e. be straight lines) after the

map projection calculations

have been applied. As an example, a map with curved lat/Lon lines may in fact be linear when

projected and therefore calibrated in the Transverse Mercator projection.

The

4,5,6 point method

use a least squares method fit to the linear Affine method to do the

transformation. This method will work with maps which are rotated or skewed. It is the same as the 3

point method but gives an overdetermined calibration taking into account all the points. It uses the

same method as the 3 point but gives a calibration which is an average of all the points.

The

7 or more point method

has the option of using a 2nd order polynomial to do the

transformation. This method will work with maps which are rotated or skewed and have a large

curve in the Lat/Lon or Grid lines. The polynomial option is located on the Options button on the

map calibration window.

While the polynomial method can calibrate maps with curved grids the curve must be constant for all

parts of the map. Example

if a Latitude line towards the bottom of the map is curved a line near the

top of the map must have the same amount of curve, the method cannot give an accurate calibration

if the map is warped by different amounts over the extent of the map.

If after calibrating you find the waypoints etc do not stay where originally placed on the map this

indicates the calibration is not accurate enough, the forward calibration is different to the inverse

calibration, the map is most likely warped or needs to be calibrated using a different map projection.

When dealing with maps with curved Lat/Lon lines and you are not sure of the projection it is

advisable to try calibrating using the Transverse Mercator projection as this is the most common for

general purpose topographic maps.

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