T&T: Chart &, Mistakes

[Jim Healy]

Randy wrote:

"When we were on the Erie Canal this summer we frequently noticed that our
boat was running on dry land, according to our Navionics powered Raymarine
C-120."



There are several factors that affect the placement of the boat icon on a
chartplotter.  I have C-MAP chips in a Raymarine System, and it behaves as
described by Randy and other posters.



The lat and lon reported on the GPS device is right, probably to 2 -5 meters
accuracy.  But the placement of the boat icon on the screen of the
Chartplotter by the chartplotter's ["expensive"] is not accurate.  One
possible cause was discussed earlier, that being changes in the routing of
the waterway after the digitized chart raster was created.  The more likely
cause, though, lies in the math used to actually place the boat icon on the
screen.



The reason is (not trying to be a wiseguy here) the earth is not flat.  But,
of course, paper charts of the earth's surface are flat.  To deal with the
problem of correctly representing the earth's land features on a flat piece
of paper, there are several different ways of transferring a survey to
paper.  Mercator projections treat the earth's geometry in one way,
polyconic in another, etc, etc, all in an attempt to manage this problem for
a specific region of the earth's surface.  Chapman's will give you a writeup
of celestial surveying if you care.



In recreational GPS technology, the ["expensive"] software that places the
boat icon on the map segment shown on the chartplotter translates the lat
and lon determined by the GPS chip into an X and Y offset into the chart
raster.  The digital raster of the chart is a representation of a flat paper
chart, not a curved section of the earth's surface.  The  index point of the
digital raster is not in the center of the raster, either; it's truly at one
of the corners (0, 0) of the raster, so there's more room for computational
error at the far ends of the raster than near the index point.  There is
more error in wide area charts, and there is more error in charts that have
high positive and negative latitudes.  My chips for the Chesapeake Bay south
through Florida are highly accurate.  That's because the surveying
technology more closely approximates those curved pieces of earth to a flat
piece of paper, so when that flat piece of paper is later digitized, there
is less error in placing the icon on the digitized raster.



This is all made more complicated by the way Chartplotter quilt charts
together.  Some overlap like real paper charts do, others are sharply
cutoff.  Mine is the latter.  So on the Erie Canal, for example, I see
actual discontinuities in shore lines, recommended route lines, etc, at
*some* quilt boundaries.



My Chartplotter does have a way to adjust the tracking offset, but I find
its usefulness to be limited, because as soon as the heading changes, the
computational error changes in relationship to the new heading.  I find it
more confusing in practice to use the offset than to leave it off.



One last thing about this.  Any of you that use software products like
Street Atlas or Microsoft Streets will also observe this phenomenon on land,
as well, although there, it's much more likely that the placement of the
road is wrong on the map.



I know this is long winded, but I hope it's helpful.



Jim Healy, aboard Sanctuary