I used OCPN last year with CM93
charts and was reasonably happy with the zoom capability. Most of the time I could zoom in enough to distinguish my
anchor marks and follow tracks I had made.
This year I installed and used the new S63 chart capability (first the beta and now the official release) using official
charts from the Australian Hydrographic Office. However the zoom capability is now much reduced. I now find that I cannot distinguish my
anchor marks, they appear to be all on top of each other, and trying to visually follow tracks made before is almost impossible.
There is an issue in
Australia in that many of the official charts are not properly georeferenced and also some have not been updated since 1770, they are
Captain Cooks charts! So, whilst the
shipping lanes and
commercial traffic areas have good charts with lots of detail and you can zoom in a long way before overzoom kicks in, in other areas there is little detail and the zoom level is very poor.
With the
current overzoom capabilities for S63 charts in some areas the
boat length, as displayed, is about 0.5nm long. However, in these areas you really need to be within 10-20m of your anchor point to know you are safe, particularly when in the
Great Barrier Reef. The
GPS signals generally are good enough to get you within 3-10m, i.e. within a
boat length of where you really need to be.
Now there are at least two opinions on zooming in more than the embedded scale of the chart suggests. This is what overzoom is.
1. Charts used for
navigation should not be zoomed in more than the official scale will allow as there is no more detail to be found and the overzoom suggests a greater level of accuracy than is supported by the data.
2. Charts should be zoomable to the level of accuracy of the supplied location information, i.e. the
GPS signal. Whilst the information displayed by the official chart will/may not be accurate, the location, i.e. lat/lon, will be accurate to any other information on the chart which is also derived by the location information. This would allow a
skipper to approach a GPS location visually on OCPN and arrive at a known location.
In 1. the chart is being used for
navigation based on marked depths, dangers, land, lights, bouys, etc. In 2. the chart is not being used for navigation, rather the plotting in 2D of the location, tracks and waypoints.
In case 1. there really should be no overzoom. In case 2. overzoom should be allowed to at least the GPS accuracy at the time. There is a potential problem though. This is where someone is in the second case, overzoomed, but attempting navigation using the chart information being displayed. In
Australia this would be highly dangerous as I suspect it would in the rest of the world.
The questions are:
a. Should overzoom be allowed at all?
b. If overzoom is allowed, how much should be allowed?
c. How should it be indicated that the chart information being displayed at this level of overzoom is not accurate and MUST NOT be used for navigation purposes?
I have attached a PDF with 3
screen shots taken of Lady Musgrave Island
lagoon at the south end of the GBR showing the issue. This
lagoon is safe, but does have some isolated
coral heads that come straight up from the bottom, 7-8m, to within <1m of the surface. Between 10:00 & 15:00 it is easy to see them, outside of this, or with overcast or strong
wind it is almost impossible. I have marked the
safe anchorages, many of the bommies (good diving!) so know where I can go. I just need to be able to get there. So I would like to be able to zoom in quite a bit further than is shown. Note: the
current beta, 2302, does allow greater overzoom for CM93 charts.
Any ideas?