Most raster charts for marine use are created in a convenient and universally accepted Mercator projection. Mercator projection allows charts to be “quilted” (or connected together on a world-wide view). However, some raster charts still use other projections for reasons of convenience, tradition, data availability etc.
These charts can’t be directly “quilted” with the rest of Mercator-based “world”.
In US, NOAA uses non-Mercator, primarily Polyconic projection raster charts, for coverage of Great Lakes area. Polyconic was a predominant projection for US charts from mid-1800s to early 20th century.
To fit non-Mercator chart into Mercator world, it needs to be re-projected, that is – chart image needs to be transformed to match Mercator projection requirements.
While differences may not be as clearly visible on a large scale (small area) chart, small scale charts are very telling. Here is an example of small scale chart (NOAA 14500, Great Lakes overview) in original Polyconic and reprojected Mercator.
Note how parallels and meridians are not perpendicular on the Polyconic projection chart above and intersect under 90 degree angle on the Mercator chart below. Mercator projection is conformal, i.e. – able to preserve angles and courses, including those between latitude and longitude lines.
Chart reprojection is available in current release of PolarView NS.