Fun with ENC charts

Looks like NOAA may be using automated conversion software to create ENC charts from satellite images. It works great most of the time, but sometimes things don’t go as planned. In this chart (US5NY43) Mid Hudson Suspension Bridge seems to be charted based on a shade of the bridge rather than bridge itself, moving it somewhat North. Stated accuracy of 1:40000 scale chart is about 140′ so this one is probably just about within the limit.

Good navigators always remember that chart is not a substitute for a proper lookout.

P.S. A bit more on this – the following is my guess, correct me if I am wrong.

So, NOAA may be trying to do the right thing. Note that the satellite image is taken from position slightly due North of the bridge, rather than vertically. From that point of view bridge as tall as this one (134 ft) would be visually shifted South relative to anything at “sea”/water level. On the other hand, if the sun was at high noon and directly overhead, shade from the bridge would be precisely under it at exactly the sea level, and would provide a perfect outline of the bridge on a chart. I think this method works “most of the time”, in particular for images taken during summer months in Southerly latitudes.

Here we have a relatively high Northern latitude, and photo likely not taken at noon, so the sun is low in the southern sky. Because of that, the shade is located North of the bridge. The “real” bridge position should probably be charted in the middle between the shade and the bridge on the satellite photo.

9 Responses to Fun with ENC charts

  1. R Fairman says:

    Thanks Seems you monitor the yahoo forum site and respond with support when needed. Interesting error on the compilation of the chart. Are they using Google earth as the source image

  2. polarnavy says:

    Which Yahoo forum do you mean? I was contacted by a user directly.

    I am guessing that both NOAA and Google use the same satellite data – there are only so many sources available at a given resolution.

  3. R Fairman says:

    The problem was originally posted on

    http://groups.yahoo.com/group/seaclear_mapping/message/1552

    I suggested he contact Polar Navy direct

  4. P. Porteus says:

    I am the user that discovered this. I am also a Sea Clear II user, and thus use the Yahoo group. Might I suggest a user group for Polar View as well. If the questions and problems go out to an entire group, there is a better chance that another user might have seen the problem, or could also benefit from the fix.

  5. WillieC says:

    Well with regard to the photo, the center of the bridge at SEA LEVEL will be in line with the CENTER of the piers at SEA LEVEL so the NOAA projection is way north. In other words the center of the bridge at the sea level is just slightly north of the deck.

      • P.Porteus says:

        I reported this to NOAA and apparently they do care, here is the response I received.

        OCS Inquiry: 15292

        Original message: The Mid Hudson fixed bridge is charted approx 140 feet of where it actually is.

        OCS response: Thank you for your interest in NOAA’s products. In response to your inquiry, NDB will be issuing a source document to have the symbology relocated to the centerline position of the GP for the ENC and Raster Chart Products. Best regards.

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