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.