Non-Cylindrical Grid Mapping
Limit of Capability

This illustration shows the typical limitation of the four-point map-referencing geometry:-

The points circled in red are those used to reference the map.

The red grid lines were produced by creating a text overlay for each of the lat/long intersection points. RealMaps overlayed this file on the map in accordance with its normal four-point arithmetic but I coloured them in a paint package to make them stand out.

I think this illustrates the strengths and weaknesses of this geometric conversion quite well:-

• It can cope with skew as well as distortion from non-rectangular lat/long projections.
• It is versatile and easy to apply since it is not necessary to know the original projection type.
• Accuracy improves as the area covered decreases in size - this means that the system works best for high detail maps.
• For low-detail maps, such as that illustrated, careful selection of the referencing points can reduce errors.
• The system never accurately reflects curved lat/long lines.
• On typical maps the longitude lines are straight and therefore likely to be more accurate in the conversion.

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