Manual – Distortion Grid
The distortion grid is a comprehensible method to visualize the accuracy of an old map. All settings concerning the configuration of the distortion grid are made in the Distortion Grid tab.
Show Distortion Grid
Select the Show button to display the distortion grid. Select the Show Undistorted Grid button to show a grid that is rotated and scaled, but not distorted.
Enter the Mesh Size in meters for the grid's mesh size in the according text field. The mesh size always refers to the scale of the new map, i.e. enter the mesh size in meters relative to the new map. The minimum number of grid lines is 4, the maximum number of lines is 200. Enter a mesh size that results in a grid with a number of lines within these limits.
Mesh Size Units
Select between meters and degrees in the menu following the Mesh Size text field. If degrees are selected, a graticule of longitude / latitude lines is generated in both the old map and the new reference map. Degrees are only available if OpenStreetMap is used as reference map, and if visualizations are generated for the old map (see Direction of Transformation for more information about this feature).
The Extension menu allows for the selection of the shape of the distortion grid. Selecting Rectangle will result in a rectangular grid containing all points. Choosing Around Points the grid will only be drawn around the points inside their convex hull. To customize the extension of the grid, first draw a limiting polygon using the pen tool, then choose Custom Polygon. The polygon will disappear from the map, but will be used to limit the extension of the grid when you hit the Compute button.
Set the smoothness of the grid lines with the according slider. Drag the slider the left for a angular grid lines and to the right for smooth grid lines.
Choose the size of the labeling coordinates by changing Label Size. Select None if you don't want any labels.
The Frequency menu allows for selecting the number of labeled lines, e.g. a value of 4 creates a label for every fourth grid line.
Adapt the color, width and type of the lines for optimum readability.
More / Exaggeration Factor / Offset
Enter an exaggeration factor in the dialog that opens when the More button is pressed. Distortions in the grid are enforced when the exaggeration factor is larger than 1 and diminished when smaller than 1. The exaggeration factor should be set to 1 in most cases. A factor different to 1 is only useful when maps with very low distortion are analyzed. In such cases, an exaggeration factor larger than 1 can show otherwise unseen distortions.
If the exaggeration factor is not 1, a red warning text is displayed to remind you of this important fact.
Also accessible via the More button is an option to horizontally and vertically offset the position where the lines of the distortion grid are generated. This feature is useful to align distortion grids with a specific point. Grid lines are usually placed at multiples of the mesh size. If the horizontal or vertical offset differs from 0, grid lines are computed at intermediate positions.
The maximum offset is equal to the mesh size. If the values entered are large than the mesh size, the remainder of the division "entered offset / mesh size" is used.
For a grid along lines of equal longitude and latitude, the offsets must be in degrees.