Reprojection of Raster Charts

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.
14500 Polyconic

14500 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.

4 Responses to Reprojection of Raster Charts

    • polarnavy says:

      Honestly, quite a number of users asked about support for non-Mercator charts. Pretty much everyone on Great Lakes that uses raster charts is affected and a number of other areas too.

      So, I finally had to move on it 🙂

  1. wayne; thomas says:

    Couldn’t figure out why some St Lawrence R charts weren’t showing up. But found this to explain and downloaded 1.6

    I am currently transitioning from Nobeltec VNS 8 and can not see the charts there either. I was wondering if the reprojected chart is available in BSB format that I could load into VNS.

    thanks — wayne

    • polarnavy says:

      PolarView reprojects charts for internal use only. You can not use these reprojected charts with other software.
      They need to do their own homework 🙂

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