Outdoor Navigator, leading the edge
I have been giving Outdoor Navigator a workout going up the outside of
Vancouver Island and running the inside passage from Ketchikan out to Cape
Spencer, then over to Kayak Island thence to Cape Clear at the end of
Montague Island and on to Seward.
It takes about 400 megs to store all the charts in SE Alaska and all the
way to the edge of Kodiak Island. When I was leaving for this trip, I did
not know we would go beyond Puget Sound, but I loaded what I could into a
128 meg memory stick to use with my NX80 PDA and took all the chart
booklets I have for the area, Port Hardy to Skagway, the sailing directions
for Northern BC and a few other odds and ends. In order to get enough
charts loaded on the 128 meg memory stick I loaded all the charts with
scales over 80,000 to 1, some of the 40,000 to one stuff, the Wrangell
Narrows and all the detail stuff near Cordova, Whittier and Prince William
Sound.
In the end we had to buy a paper chart for Keku Strait and Rocky Pass with
The Summit in order to take the short cut from Ketchikan to Kake. I should
have loaded the electronic versions of 17372 but I failed to notice the
lack and it's implications until we were underway and it was too late. I
had all the charts on an 80 meg USB disk drive, but I could not get the
hotsynch system to work on any of the machines I encountered along the way
and so could not do any loading. If you don't have a dedicated laptop that
you can take with set up to do hotsynchs you should expect to encounter
trouble trying to do this in the field, unless you have plenty of time,
which I did not.
It turns out that the chart 17372 that I had on the portable drive was a
1997 edition, whereas the present version on Maptech's site is the latest
printed version of Sept. 2003 with corrections to Dec. 2003. This would not
seem to have been a serious problem but it is always nice to have the
latest versions.
I don't like to use paper charts to negotiate tricky narrow channels
without setting the chart up with pencil drawn range lines which I can pick
up visually. I made up about a dozen of these ranges to mark our channel
edges so that we could keep positioned with precision even with up to 6
knots of current alternating pushing from behind and ahead. We had no
trouble, except for the initial entry coming from the south just past red
mark #10, about 56 degrees, 36 minutes, 45 seconds north. The time was just
after low slack, tide about 2 feet over datum and the depth was shoaling to
6 feet, we backed up immediately and anchored near 20 seconds north, just
out of the direct current. After waiting 2 hours and making dinner, the
tide having risen to 8 feet over datum, we went up the channel without
incident., this was just about sunset, but twilight lasts for several hours
and we were well north of The Summit by then.
The Douglass's book on Alaska describes this spot near 45 seconds north as
shallow, but the warning is inadequate in my estimation. In any event we
never saw less than 8 feet under the transducer, anywhere, and I think our
choice of waiting 2 hours and having 8 feet above datum, according to the
tide tables and our increased depth as measured by the sounder, were
prudent measures. This channel is basically a saltwater river with the tide
coming up it from both directions. The amount of current is such that the
bottom being soft sticky mud and small to large boulders, all of which is
constantly being rearranged, that you can anticipate things not being
exactly as the chart shows.
The Maptech system for Outdoor Navigator is clumsy for maintaining many
hundreds of charts. Jeffrey says he is working very hard on the solution. I
hope he is sweating appropriately in the semi-tropics where this is being
done. I picked up a 512 meg memory stick at Costco for $60 and loaded about
223 charts, which Outdoor Navigator will not load completely. How it
decides which ones it will load is not clear. I had to unload about 23 to
get back to the limit it will load of 200. There are about 148 charts in SE
Alaska, so I have about 52 more from South Central Alaska loaded as well.
If you have the Palm file utility that will allow renaming directories you
can create and name another one OutdoorNav2 and load charts in it. You can
place another 200 in that directory. Then when you want to use the second
directory, rename it with OutdoorNav1 and then rename OutdoorNav2 to
OutdoorNav and you can use the second directory set. This is a really
clumsy but effective way to get the job done. If you like this trick but
think it is REALLY clumsy, tell Jeffrey down in tropical land. Perhaps he
will then take pity on those of us north of 35 north and provide a better fix.
I mentioned that working with hundreds of charts on Outdoor Navigator is
generally clumsy. There are problems attempting to have a complete set. It
is real easy to miss downloading a chart, since you can't verify except by
hand whether you have loaded all the current charts. The hand verification
is itself subject to mistakes. It would help if Maptech's system would
provide a count of charts in a group of chart numbers with which you could
at least verify the totals to be expected.
A technical note: I like to set up range marks on obvious points of land
like the end of a point or a very visible buoy or channel mark. You can use
these range lines to keep center channel or use to mark the edge to keep
from going over the line. The range line is constructed by drawing a line
from any 2 marks which you can see visually or maybe even on the radar and
then staying on or to one side of the line.
Regards,
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
Mike Maurice mikem@yachtsdelivered.com
At 05:04 PM 5/11/05 -0700, you wrote:
It turns out that the chart 17372 that I had on the portable drive was a
1997 edition, whereas the present version on Maptech's site is the latest
printed version of Sept. 2003 with corrections to Dec. 2003. This would not
I failed to mention a minor technical problem with the inset charts used on
17372, for The Summit and Devils Elbow, both of which are 10,000 to one
scale. They are in NAD 1927 datum, which I assume is not the same as CONUS
1927, but NAD 1927 Alaska. In any event since accuracy of 10 yards or less
is needed to traverse The Summit and the Elbow, if you are using a GPS to
drive a chart plotter or even if you are doing your plotting by hand, you
need to set the GPS into NAD1927 Alaska. Otherwise your locations will be
off by some amount in latitude and longitude. We set up one of the 2 Garmin
76's that we had with the Alaska datum and the other with the normal WGS84
datum. Then we just swapped plugs between the correct unit depending on
which chart datum we were using at any given moment. The large scale
insets were using NAD 1927 and the main chart sections were in WGS84.
Outdoor Navigator does not switch the datum that the GPS is using in it's
data stream that it is sending, so you have to change the datum by hand.
This is true of many chart plotting systems. Since most US charts are in
WGS84 datum this is not generally a problem. But as you can see it can be
with some of the insets. As far as I can tell, charts for the lower 48 have
all been corrected, but you can't be sure unless you inspect by eye. In
Alaska the insets produced from Army Corp maps are apparently still in NAD
1927.
In practice this is not a problem unless you are working at close quarters
in a narrow channel.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
WHOA!!!! Wait a minute here!!
NEVER EVER change the datum of your GPS receiver!!!
The datum of the GPS constellation is WGS84 so the
positions generated by the GPS should always be
relative to the WGS84 datum.
However - if you have a chart which uses a different
datum, change the presentation datum in your chart-plotter
to the datum to the chart (assuming it doesn't switch
automagically).
What is the issue here?? because if the GPS position is
being transmitted to other places, like via AIS or
a DCS position query, the receivers will (must!)
assume the GPS position is relative to the GPS datum!
If you change the datum in your GPS receiver and it
gets transmitted to someone else, you position will
appear offset according to the alternate datum.
For example of how this can go very wrong, a month
or so ago, a large container ship in an Asian port
was sending AIS data, but the shipboard personnel
had adjusted the datum in the GPS to match their
chart. at the harbor VTS fixed AIS base station,
the container ship was show to be THREE MILES
inland while tied-up at the wharf.
if this had been a collision avoidance situation,
the outcome could have been VERY BAD indeed.
moral of the story - the lat/lon generated by
the GPS must be referenced to the GPS datum: WGS84.
if you need to display a chart using a different datum,
do that transformation within the chart plotter ONLY.
do not let non-WGS84 coordinates lose running around
your navigation equipment unless you are very, very
sure you know what is going on.
yes - i understand that if you have an integrated
GPS/Chartplotter it is not always clear what gets
adjusted if you have to change the chart datum.
what SHOULD happen is now understood, but there is
a lot of equipment out there programmed by people
who didn't understand this critical distinction.
so it pays to ask the manufacturer exactly what
is going on.
If you can't get an answer any other way, you can
connect your laptop serial port to the NMEA0183 output
and watch the data stream with a terminal program.
if the GPS coordinates jump back and forth when you
switch the datum back and forth between WGS84
and something else, you know there is a problem.
your manufacturer may or may not understand
the problem, but at least you will know
what's going on, which is better than not knowing.
this is all coming to light because of commercial
AIS deployment. when the GPS data never left
the bridge, arguably it didn't matter too much.
but if it gets transmitted to someone else,
they must know the datum to interpret the
coordinates. and yes, a surprising amount
of "commercial grade" "IMO Approved" equipment
has this problem. in fact, it hadn't occurred
to the IMO this was an issue until the
deployment started and surfaced the problem.
and no, NMEA2000 will not make this magically
disappear.
sigh
-mo
Mike Maurice wrote:
Mike Maurice mikem@yachtsdelivered.com
At 05:04 PM 5/11/05 -0700, you wrote:
Outdoor Navigator does not switch the datum that the GPS is using in it's
data stream that it is sending, so you have to change the datum by hand.
This is true of many chart plotting systems. Since most US charts are in
WGS84 datum this is not generally a problem. But as you can see it can be
with some of the insets. As far as I can tell, charts for the lower 48 have
all been corrected, but you can't be sure unless you inspect by eye. In
Alaska the insets produced from Army Corp maps are apparently still in NAD
1927.
In practice this is not a problem unless you are working at close quarters
in a narrow channel.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
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Maybe I misunderstand, but it seems clear to me that positions on Garmin's Blue
Charts are already adjusted to match the WGS84 datum. I have checked this on
several of the northern BC charts which were originally produced on NAD 27, and
in great detail on the intricate route through Venn Passage on BC chart 3955.
If I use the paper chart, I have to adjust. If I use the electronic chart data,
it's already done for me. My friend's Cap'n navigation system with charting
from NDI seems to be exactly the same way.
It seems to me there's no need to change anything.
Richard Cook
New Moon - Bounty 257
Mike O'Dell wrote:
WHOA!!!! Wait a minute here!!
NEVER EVER change the datum of your GPS receiver!!!
The datum of the GPS constellation is WGS84 so the
positions generated by the GPS should always be
relative to the WGS84 datum.
However - if you have a chart which uses a different
datum, change the presentation datum in your chart-plotter
to the datum to the chart (assuming it doesn't switch
automagically).
What is the issue here?? because if the GPS position is
being transmitted to other places, like via AIS or
a DCS position query, the receivers will (must!)
assume the GPS position is relative to the GPS datum!
If you change the datum in your GPS receiver and it
gets transmitted to someone else, you position will
appear offset according to the alternate datum.
For example of how this can go very wrong, a month
or so ago, a large container ship in an Asian port
was sending AIS data, but the shipboard personnel
had adjusted the datum in the GPS to match their
chart. at the harbor VTS fixed AIS base station,
the container ship was show to be THREE MILES
inland while tied-up at the wharf.
if this had been a collision avoidance situation,
the outcome could have been VERY BAD indeed.
moral of the story - the lat/lon generated by
the GPS must be referenced to the GPS datum: WGS84.
if you need to display a chart using a different datum,
do that transformation within the chart plotter ONLY.
do not let non-WGS84 coordinates lose running around
your navigation equipment unless you are very, very
sure you know what is going on.
yes - i understand that if you have an integrated
GPS/Chartplotter it is not always clear what gets
adjusted if you have to change the chart datum.
what SHOULD happen is now understood, but there is
a lot of equipment out there programmed by people
who didn't understand this critical distinction.
so it pays to ask the manufacturer exactly what
is going on.
If you can't get an answer any other way, you can
connect your laptop serial port to the NMEA0183 output
and watch the data stream with a terminal program.
if the GPS coordinates jump back and forth when you
switch the datum back and forth between WGS84
and something else, you know there is a problem.
your manufacturer may or may not understand
the problem, but at least you will know
what's going on, which is better than not knowing.
this is all coming to light because of commercial
AIS deployment. when the GPS data never left
the bridge, arguably it didn't matter too much.
but if it gets transmitted to someone else,
they must know the datum to interpret the
coordinates. and yes, a surprising amount
of "commercial grade" "IMO Approved" equipment
has this problem. in fact, it hadn't occurred
to the IMO this was an issue until the
deployment started and surfaced the problem.
and no, NMEA2000 will not make this magically
disappear.
sigh
-mo
Mike Maurice wrote:
Mike Maurice mikem@yachtsdelivered.com
At 05:04 PM 5/11/05 -0700, you wrote:
Outdoor Navigator does not switch the datum that the GPS is using in it's
data stream that it is sending, so you have to change the datum by hand.
This is true of many chart plotting systems. Since most US charts are in
WGS84 datum this is not generally a problem. But as you can see it can be
with some of the insets. As far as I can tell, charts for the lower 48 have
all been corrected, but you can't be sure unless you inspect by eye. In
Alaska the insets produced from Army Corp maps are apparently still in NAD
1927.
In practice this is not a problem unless you are working at close quarters
in a narrow channel.
Mike
Correct. the Blue Charts are relative to WGS84.
(Blue Chart data is supplied by Transas which traces
its roots to hydrographic group of the Russian Navy!)
and if you must take the coordinates from the GPS and
use them with a paper chart with a different datum,
yes indeed, you may have no other choice but to have the
GPS do the datum conversion.
but do understand
that if the GPS is squawking something other than WGS84,
you can have an "issue" if you send the coordinates
to someone else (like in a DSC distress message)
and they don't know the datum upon which they are based.
since DSC and AIS messages assume GPS-derived coordinates,
that also implies the WGS84 datum.
i've been thinking seriously about developing a Lat/Lon
repeater display that does datum correction. with one
of those at the nav station, this issue of paper charts
won't require using the GPS Rx to do the conversion.
-mo
Richard G. Cook wrote:
Maybe I misunderstand, but it seems clear to me that positions on Garmin's Blue
Charts are already adjusted to match the WGS84 datum. I have checked this on
several of the northern BC charts which were originally produced on NAD 27, and
in great detail on the intricate route through Venn Passage on BC chart 3955.
If I use the paper chart, I have to adjust. If I use the electronic chart data,
it's already done for me. My friend's Cap'n navigation system with charting
from NDI seems to be exactly the same way.
It seems to me there's no need to change anything.
Richard Cook
"Mike O'Dell" mo@ccr.org
At 08:27 PM 5/13/05 -0400, you wrote:
NEVER EVER change the datum of your GPS receiver!!!
The datum of the GPS constellation is WGS84 so the
positions generated by the GPS should always be
re
Never is a really long time.
Go back and read what I wrote. Outdoor Navigator does not have the
provision for changing the datum.
In which case there is no choice, but to change the datum in the GPS.
Since, you are making absolute statements, get this.
My explanation of the situation was clear, yours has most likely confused
at least a few people.
If your position is being broadcast to others, then changing the datum may
confuse them.
In the meantime not using the best datum for the plotting problem at hand
may run you aground.
Argh!!
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
i believe we are in violent agreement.
and yes, your summary is succinct and clearly captures the
difficulty of the situation.
maybe what i should have said is,
"if there is any way to avoid it,
do not change the datum in the GPS receiver."
alas, given paper charts with an alternate datum, you have no choice.
Argh indeed!
cheers,
-mo
Mike Maurice wrote:
If your position is being broadcast to others, then changing the datum may
confuse them.
In the meantime not using the best datum for the plotting problem at hand
may run you aground.
Argh!!
"Richard G. Cook" newmoon1@prodigy.net
At 07:49 PM 5/13/05 -0600, you wrote:
Maybe I misunderstand, but it seems clear to me that positions on Garmin's
Blue
Charts are already adjusted to match the WGS84 datum. I have checked this on
I think you may find that The Summit and the Devils Elbow inserts on chart
17372 are in NAD 1927 format, even on Blue Charts. That the Blue Chart
stuff has NOT been compensated for.
Since I don't have the Blue Charts for those areas, I can't know for sure.
But, since the latest present paper charts from NOAA (17372 inserts) are in
NAD 1927 you can not assume that these Blue Charts are on WGS84. Find out.
Once again, it appears that you read only what you want. Note my original
post, which stated that the lower 48 charts are most likely? compensated
and that Alaskan stuff and some foreign charts are may/still be on other
datums. And this includes electronic stuff. If you want to write about
generalities, instead of using specific charts and circumstances, then you
may be wrong and possibly confuse others who know less about this.
The facts that I stated in my post concerning char 17372 are accurate and
may or may not reflect the state of any electronic chart datum made from
them. But, to assume that any paper chart which is current and is using a
datum other than WGS84 has been corrected where it is being used in a
electronic system is not a safe assumption. In the case of chart 17372 my
facts were exactly correct, from that it is a fair conclusion that there
may be other such charts.
Novice navigators get themselves into trouble making assumptions. Datums
are one of the hidden gotchas which can bite those who are relying on the
vague notions of others who have not checked their charts with a magnifying
glass.
In case you never read or have forgotten that I reported that the Blue
Chart location for the Pt. Arena rock located just north of Pt. Arena
Calif, near 38-45 North is off by about 3/4 of a mile on the Blue Charts,
whether this mistake has been corrected I do not know. But the Blue Charts
are not to be used for Navigation. So much for assumptions. If you run into
it Garmin will not be liable for your damage. I have no idea how many other
errors there are in the Blue Charts, but I do not use them for these
reasons and have no plans to do so. In the case of the Pt. Arena Rock, it
is my estimate that since the charts are in vector format, someone who does
not know the area, picked the wrong rock. But of course they completely
missed the real rock about 3/4 mile further north and very near the traffic
line from Pt. Arena to Ft. Bragg. This particular rock is very dangerous as
it is just awash at high tide.
Look guys and gals, I know you are in this Pleasure Boating for the fun of
it. But these kinds of mistakes kill novices just as quickly as pros. I
don't tell you these things to hurt your feelings. They are meant to keep
you out of trouble and afloat without resorting to the emergency pumps.
Besides, if I lost a boat from one of these nearly invisible mistakes, you
can be sure that I would hear about for a long time, maybe forever. Now
that, is a long time!!!
Regards,
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
Outdoor Navigator does not
have the provision for changing the datum.
In which case there is no choice, but to change the datum in the GPS.
Since, you are making absolute statements, get this.
My explanation of the situation was clear, yours has most
likely confused at least a few people.
Mike, while your explanation was clear, it was also clearly incorrect. As
you know, I wrote Outdoor Navigator. Every line. Every statement. Every
handling of map transformations. It just doesn't work the way you describe.
The software expects the lat/lon to be in WGS84. The transformation of that
to the physical location on the map image is done by a high order polynomial
equation, completely independent of the map datum. Maptech is the one that
creates the polynomial but I'd be very surprised if there was more than 1
pixel of error throughout the inset that you reference.
You made this exact same argument about a year ago. I gave a long detailed
explanation of the .KAP file handling showing how the vessel position is
determined by this polynomial fit. I challenged you to give a specific
example with a specific lat/lon on WGS84. You never replied, on list or
off.
Outdoor Navigator specifically does not allow changing the datum - you are
correct in that. The reason is that it would only create erroneous position
displays. Again, the challenge is up to you. Give me a lat/lon and a chart
number and I'll gladly verify that it works perfectly. If it doesn't, I'll
get Maptech to make the correction.
...Jeff
================
Jeffrey Siegel
M/V aCappella
DeFever 53PH
W1ACA/WDB4350
Castine, Maine