Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  Electronic Charts for Isle Royale

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Electronic Charts for Isle Royale
jimh posted 09-05-2013 10:33 AM ET (US)   Profile for jimh   Send Email to jimh  
I recently spent eight days navigating the waters around Isle Royale in northwestern Lake Superior. I have five sets of charts for the area:

--a NOAA paper chart, 14976;

--a NOAA electronic chart in raster format, essentially an electronic copy of the 14976 paper chart;

--a NOAA electronic chart in vector format;

--a Lowrance Insight electronic chart;

--a Navionics electronic chart, Great Lakes West Platinum Plus (900P+)

The NOAA electronic charts are available on my laptop computer, which I was not carrying on this trip, so they were not available to me as I was navigating up there. I am including them for reference in this discussion. The Lowrance and Navionics electronic charts were available on my HDS-8 chart plotter. And, of course, the paper chart was available, if I wanted to unroll it. Most of the time while underway the paper chart was not available at the helm. For much of the trip, I used the Navionics electronic chart on the HDS-8 as the primary source of chart data.

When approaching the entrance to McCargoe Cove from the North, I could easily see two large rocks offshore about a half mile, known as Ollies Dogs Rocks. However, these rocks were not shown on the Navionics chart. Only a shoal area was indicated in the region of the rocks. Below are four views of this area on four different charts:

Ollies Dogs Rocks


Lowrance INSIGHT

NOAA Raster

NOAA Vector

The Navionics chart is quite different than the other three. It fails to show the rocks above the surface of the water. These are large rocks, easily seen, and extend at least six feet above the surface of Lake Superior. It was quite disturbing to discover they were not very clearly indicated on the Navionics chart.

The Lowrance chart does a better job of presenting the hazard. The chart clearly shows two rocks above the water surface. The NOAA charts show them, too.

prj posted 09-05-2013 01:51 PM ET (US)     Profile for prj  Send Email to prj     
I find that, graphically, the Navionics version of the shoal would serve as a much better warning than the undifferentiated asterisks on the Lowrance INSIGHT version. Those asterisks could simply go unnoticed, as another numerical depth indication while the large blue mass of shallow water would immediately grab my attention.

I most often use the NOAA Booklet Charts, as they are far less cumbersome to use on an open boat than the full chart version. While orienting yourself to the proper page can be a challenge, once there, you seldom need to turn pages due to the scale of the documents. Prior to departing on a trip, I'll print and spiral bind the Booklets into a single 8 1/2 x 11 document with an acetate cover and drop it in the console.

Here is NOAA Chart 14976 in Booklet format:

6992WHALER posted 09-05-2013 03:11 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
The detail of the Garmin Blue Sea G2 charts falls in between the NOAA Vector and Raster charts.

The Garmin chart shows the rocks surrounded by the dark blue.

jimh posted 09-05-2013 05:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is another comparison, this time in the area of Crystal Cove. As we were heading out of Crystal Cove, one of our cruising companions called our attention to a rock awash not too far away. This rock awash is a very serious hazard. Here is how it was shown on the four electronic charts:

Crystal Cove

Navionics. Note that again the hazard (the rock) is not shown, only a shoal area.

Lowrance Insight. The rocks are shown, but they are drawn as if on land.

NOAA Raster. Shoal and rocks are indicated.

NOAA Vector. I don't think our actual track line was as close to the hazard as this plot shows--at least I hope not!

jimh posted 09-05-2013 05:43 PM ET (US)     Profile for jimh  Send Email to jimh     
Finally, here is a very interesting situation, occurring on the SW end of the island, as you approach the backdoor entrance to Washington Harbor. Offshore there is a very prominent rock, at least ten feet above the water. Compare how it is presented on the four charts:

Southwest Isle Royale, Washington Harbor

Navionics. The rock is shown as a small islet.

Lowrance Insight. The rock and shoal are completely omitted. It would be very dangerous to navigate with this chart in this area at any time when visibility was reduced or at night.

NOAA Raster.

NOAA Vector. Note our track line goes over the island.

saumon posted 09-05-2013 09:07 PM ET (US)     Profile for saumon  Send Email to saumon     
This is exactly the same as I found in this previous post: .

FYI, here are the Garmin BlueChart G2 maps (HCA015R) for the sames areas:

Ollies Dogs Rocks:

Crystal Cove:

Washington Harbor:

It seems to me that the Garmin maps are much closer to the NOAA charts than their competitors.

jimh posted 09-05-2013 09:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Saumon--Many thanks for taking the time to gather those screen shots of the GARMIN charts in the three areas of Isle Royale. I agree with your assessment: the GARMIN charts are better than the Lowrance Insight or Navionics Platinum+ charts.

I could email you my track line in .GPX format, and you could plot it on the GARMIN charts. It would be interesting to see if my track line runs over land and rocks on the GARMIN charts, as it does on some of the others.

saumon posted 09-06-2013 07:22 AM ET (US)     Profile for saumon  Send Email to saumon     
Yes, no problem. Is there other places than the Crystal Cove area that you "run aground" on the chart?
jimh posted 09-06-2013 08:15 AM ET (US)     Profile for jimh  Send Email to jimh     

Stop the presses!

I just discovered that the Navionics chart presentation was being affected by my user settings. I had customized the presentation to turn off a lot of features, like "show international boundaries", or "show light sectors," and many others. It turns out that in the menu structure for customizing presentation, representation of hazards like rocks is part of the "Other elements" group. I had this turned off.

One would not think that presenting serious hazards like rocks awash would be a feature that could be turned off, and, even stranger, that this feature would be listed under "Other elements" in the menu structure. However, that appears to be the case. With the Navionics charts set to "Show All", the rock hazards are clearly shown. Here are two new screenshots, showing the Navionics presentation for Crystal Cove and Ollies Dogs Rocks:

Navionics Chart; All Features ON; Crystal Cove

Navionics Chart; All Features ON; Ollies Dogs Rocks

This is a much better presentation. I always say, "I learn something every time I go boating." I certainly learned something in this thread--don't turn off "Other elements" in your Navionics chart presentation options.

saumon posted 09-06-2013 10:03 AM ET (US)     Profile for saumon  Send Email to saumon     
Another user setting that can greatly affect tracks presentation is the recording interval.

On a Humminbird chart plotter that I had, the "live" or active track was fine, recording a point every second, but the default setting for the saved track, probably to save memory, was 10 seconds, meaning it record only one out of every 10 actual points.

As a result, the circumvention of a tip could appear fine live but, when reviewing it on the saved track, it would seem like you ran over ground.

6992WHALER posted 09-06-2013 10:27 AM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Jim did the setting change effect the Lowrance presentation?

I am glad you found that error in the settings. I have to say that I have been surprised at the number of times I have found myself uncomfortable with your choice of course lines because of rocks that appeared on my Garmin Charts but not on yours charts.

jimh posted 09-06-2013 01:54 PM ET (US)     Profile for jimh  Send Email to jimh     
John--I don't think the setting affected the Lowrance presentation, as it probably has its own set of options. I will check the Lowrance chart options to see if something might be awry with the settings.
prj posted 09-06-2013 05:32 PM ET (US)     Profile for prj  Send Email to prj     
Boy, those Garmin charts are beautifully presented.

What does the little one-legged seabird grid represent on the Navionics chart? What does the small subtext adjacent to the depth number represent on the Navionics chart?

K Albus posted 09-06-2013 05:49 PM ET (US)     Profile for K Albus  Send Email to K Albus     
The little birds represent "National Park" or the equivalent, and the subscript numbers are inches of depth. I believe the inches result from the conversion from metric to English units.
prj posted 09-07-2013 03:50 PM ET (US)     Profile for prj  Send Email to prj     
Thanks Kevin. If I recall, you solved that bird mystery in the past here.
6992WHALER posted 09-07-2013 05:03 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
saumon, Is that the Garmin G2 Vision or the G2 maps?
jimh posted 09-07-2013 08:50 PM ET (US)     Profile for jimh  Send Email to jimh     
I checked the settings on the Lowrance Insight charts; they were set to "full" feature display. I can't imaging there is a setting that would cause the chart display to suppress the display of small islands, which is what is happening in the Lowrance chart for the SW end of IRNP. That is a rather significant omission of an important detail.
saumon posted 09-07-2013 09:40 PM ET (US)     Profile for saumon  Send Email to saumon     

saumon, Is that the Garmin G2 Vision or the G2 maps?

Those screenshots are from the regular G2 map (non-Vision). Having both, I find out that the only added feature of the Vision maps are the 3D view but, on the other hand, the regular G2 maps have a lot more depth soundings and contour lines.

If you compare any of the screenshots I posted with those posted by jimh for the same areas, you'll note there's a lot more bathymetric contour lines in the deeper areas, much like a topo map. The G2-Vision maps, that I didn't posted, ressemble much more to the Navionics charts with a little bit more details...

6992WHALER posted 09-07-2013 11:32 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Maybe my unit 3206 is just to old but my G2 charts do not have as nice of detail as yours.
saumon posted 09-07-2013 11:58 PM ET (US)     Profile for saumon  Send Email to saumon     
It could be. Did you have the latest software update for your unit?

As an example, that same map (G2 non-Vision HCA015R) didn't show that much details when viewed with the older MapSource software on my computer while the newer HomePort software show all the detailed contour lines...

Tomorrow, I'll try to show, side by side, three views of the same area using the Navionics Gold, the Garmin BlueChart G2 and the Garmin BlueChart G2 Vision, for the sake of comparision.

saumon posted 09-08-2013 01:15 AM ET (US)     Profile for saumon  Send Email to saumon     

by looking at the change history log ( ), it seems they added the support for the BlueChart G2 version 11.5 and later when releasing the 3.80 software version. If your software is up-to-date and still not showing much details, it could simply be that you have an older version of the chart.

You can usually see the chart revision number on the screen when they load at start-up. Let me know which one you have...

jimh posted 09-08-2013 07:27 AM ET (US)     Profile for jimh  Send Email to jimh     
In addition to some variation among the several charts regarding how certain hazards were presented, I also noticed that all of the electronic charts I had seem to show the land features as being in a slightly offset position from the deduced position of my GPS receiver. Just about all the time during the cruise the GPS receiver (internal to my Lowrance HDS-8) was obtaining a 3D position fix with augmentation by using the WAAS. There typically were ten satellites in view, the HDOP index was in the range of 1.0 to 1.5, and the estimate of fix accuracy was about 30-feet. I am comfortable with the boat position fix from the GPS receiver being accurate.

The track recording rate was high enough to correctly show the course. (The setting for track recording rate was "AUTO", which records a track point whenever the boat heading changes.) I am comfortable with the recorded track position as being accurate.

In the area of Southwest Isle Royale, as we entered into Washington Harbor, the track of my position plotted on the various electronic charts shows the boat crossing over land. I sent the track to Saumon in GPX format, he imported the track into his computer charting application, Garmin Homeport, and he emailed me a screen shot showing the GPS receiver position track on the Garmin charts for that area. The result is seen below:

Garmin Chart with vessel track

As you can see, the plot of my track on the Garmin chart also takes us over land. This seems to agree with all the other charts, leading to two possible explanations:

--the general position of land features in the Isle Royale area is drawn incorrectly on all the charts, or

--the position fix from my GPS receiver is in error

Based on my experience with the GPS receiver position fix and other charts in other locations, the deduced position has been very accurate. For areas with high resolution charts, for example, a marina with individual boat slips shown, the GPS receiver position fix usually is so accurate the boat is shown precisely located, even to the position relative to finger docks and slips.

I suspect that all the electronic charts have been developed from the same base, the NOAA paper chart cartography. If that data has a slight offset in position of the land masses, it would be reasonable that this error might show up in all the electronic charts. This would explain why all the electronic charts seem to show the same error with regard to the vessel track in the SW Isle Royale area shown in the above screen captures.

silentpardner posted 09-08-2013 06:39 PM ET (US)     Profile for silentpardner  Send Email to silentpardner     
[A NOAA Web article] [explains] the accuracy of NOAA charts. The accuracy of what you see on a [chart plotter] is a product of the entire system, not just the charts used to compose the chart you see on your machine.
jimh posted 09-08-2013 06:54 PM ET (US)     Profile for jimh  Send Email to jimh     
The salient question in this discussion seems to be very simple, so let me just lay it out for the second time, and we can have an direct answer on the question, rather than a suggestion to go off to read something tangentially related. Thus, the question:

Is the error seen in the GPS track as plotted on six different electronic charts due to:

--an error in the position deduced by my GPS receiver at that particular time and place, or,

--and error in the charted position of the land on all six charts which are likely all taken from the same survey.

I have already expressed my opinion, but let me repeat it for clarity:

The error is due to an error in the charted position of the land which is reflected in all six charts because all of them are based on the same (old) survey.

If someone has the opposing position, that is, that the error is due to my GPS receiver failing to deduce the correct position, please let me know, and please make it clear. I don't understand how I am supposed to make an inference from reading some article on another website about this question.

Let's have some clear and unequivocal opinions on this, if you please.

jimh posted 09-09-2013 04:51 PM ET (US)     Profile for jimh  Send Email to jimh     
Saumon weighed in on the question of the error in my track being due to the charts being wrong. He took the track data I sent him and plotted it over a map from YahooMaps. Here is a screen capture showing my track at the dock in Washington Harbor. The track and the map agree precisely. They show the boat right at the small dock just north of the main pier.

LINK to large image
Track of my boat Washington Harbor at the small boat dock.

Next, here is the track plotted on the map for the passage through Thompson Island and near the wreck of the AMERICA. The plot over YahooMaps shows the boat track correctly. The boat did not travel over land.

LINK to large image
Track of my boat passing close to Thompson Island

In plotting the track on a map that is independent of the NOAA maps, Steve has shown:

--the GPS receiver was accurately tracking the position

--the maps developed from an independent source, not from NOAA, show the track precisely as it was actually run.

Based on this evidence from Steve, it is clear that the error seen with the NOAA charts, all five electronic versions, is a result of an error in the location of the land on those charts, and not from an error in the GPS receiver. When the GPS derived track is plotted against a satellite derived map, the accuracy of the track is excellent.

This presentation completely contradicts the theory put forth by SilentPartner that the cause of the error was a problem in my GPS receiver (which for some reason SilentPartner made in a completely different thread, in which he says that my "HDS 8/GPS antenna configuration is incapable of recording accurate GPS readings in the area you are boating." I have never accepted this theory, as I don't see any reason why a GPS receiver would not work well in that area. The "G" in GPS is, after all, for "global" and the system works everywhere on the globe.

jimh posted 09-09-2013 05:02 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is another verification of the accuracy of the GPS receiver track my HDS produced, with the track plotted over GoogleEarth.

Near the wreck America and passing Thomson Island, the track fits perfectly with the details shown in this satellite view.

At the Washington Harbor docks in this satellite view, both the fuel dock to the north and the small boat dock in the center correspond exactly with my recorded track.

6992WHALER posted 09-09-2013 07:16 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
My Garmin also has the North gap track going over land.
My radar image overlaid on the chart plotter moved to show the chart and radar not agreeing were the land was. My GPS receiver agreed with the real time Radar image. So that would tell me that the GPS position is correct while the chart data on the screen is incorrect.
Garmin has improved the charts. In 2006 using the Garmin Blues Sea charts the display showed that I was 1/8 of a mile on land the entire length of Washington harbor.

6992WHALER posted 09-09-2013 07:45 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     

My Garmin 78 is using US ALL & Canadian West Coast g2 HXUS039R
FEB 2013 010-c1018-20

The 3206 uses its internal g2 charts and I also have the G2 RUS040R US East G2 Feb 2012 010-C1014-10 map on a Data Card.

All three of these maps cover Isle Royal, but none of them give me the same detail on home port or on either unit as the detail you are showing in your screen shots. The 3206 and 78 are updated.

I have every unit set to show the most detail.

jimh posted 09-11-2013 01:54 PM ET (US)     Profile for jimh  Send Email to jimh     
I carefully looked at the paper chart, NOAA 14976, for an indication of the date of the survey. I only found a note that says the hydrologic survey was done by the Army Corps of Engineers sometime prior to 1974. Since the Air Force NAVSTAR GPS was not in operation in 1974, it seems reasonable to conclude the survey did not use any GPS receiver positions.

It should be realized that Isle Royale is a rather remote island. Its closest approach to Michigan is about 56-miles. Its closest approach to Minnesota is about 14-miles. These long distances over water may have limited the precision of any land survey done prior to the now common use of GPS-receiver measurements. An error of 100-feet in the charted position of the island is reasonable, and seems to be apparent in the electronic chart cartography developed from the NOAA sources.

It would be an interesting sidebar topic to explore the details of the land survey of Isle Royale. I think the United States Geologic Survey may have some more details to tell us about when the position of Isle Royale was fixed.

saumon posted 09-11-2013 03:58 PM ET (US)     Profile for saumon  Send Email to saumon     
Found this site:

When you enter "Isle Royale" in the search box, it return 416 results and you can preview the maps. Those from the 70's, 60's and 50's didn't include their land contour sources but the same identical maps fron the 40's indicate that the land contours were taken from a 1908 Michigan Geological survey.

The map published in 1909 list the persons who made the survey, from 1867-1871, and resurveyed certains parts (like the Western portion) in 1905-1906.

I guess NOAA only do the bathymetric part and take their land contours from geological surveys, so more info on that can probably be obtained via the Michigan Geological survey ( )

silentpardner posted 09-14-2013 01:36 AM ET (US)     Profile for silentpardner  Send Email to silentpardner     
Any of y'all ever overlay a radar picture over your chartplotter's maps in that area? That might be a way to determine if your charts are accurate.
jimh posted 09-14-2013 08:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Please see the post earlier in the thread where John describes how his RADAR overlay agreed with the GPS receiver and indicated the electronic charts were in error.
K Albus posted 10-03-2013 09:29 PM ET (US)     Profile for K Albus  Send Email to K Albus     
I realize I'm a little late to the party, but here's a link to a screen capture of my track approaching Washington Harbor, taken from my Simrad NSS8 with a Navionics Gold chart card:
jimh posted 10-03-2013 10:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the contribution to the thread. Since the plot of your track on the NAVIONICS GOLD chart appears to correctly show the position of the two islands, that is, your track does not go over land like all the others, it seems reasonable to conclude the NAVIONICS GOLD chart more accurately shows the position of the islands than the others.
Chuck Tribolet posted 02-11-2014 10:16 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
"The "G" in GPS is, after all, for "global" and the system works everywhere on the globe."

Actually, GPS doesn't work all that well in the far northern
and southern latitudes because the satellites don't get that
far north and south. The Russian GLONASS satellites are in
a more steeply inclined orbit that does cover those areas
better. That's understandable given that the Russians have
a great deal of land mass up there.


jimh posted 02-13-2014 09:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck has introduced a sidebar topic: does the Air Force NAVSTAR Global Positioning System actually cover the globe? Chuck says NAVSTAR GPS "does not work all that well" at extremely high latitude regions because of the orbits of the satellites.

The satellites in the NAVSTAR GPS are in orbits with a 55-degree inclination. This suggests that the highest latitude which a NAVSTAR GPS satellite will reach in terms of its ground position on the Earth is going to be 55-degrees latitude. The satellites in GLONASS are in orbits with 64.8-degree inclination, which suggest the GLONASS satellites will pass overhead at some points on the earth at latitude 64.8-degree.

I can't find any reasonably authoritative resource that confirms Chuck's statement that "GPS doesn't work all that well in the far northern
and southern latitudes." It certainly works. I suppose the question is one of a comparative, that is, how well does it work?

What I can find is some mention that a position fix solution obtained with GLONASS could have a better (lower) dilution of precision compared to the NAVSTAR GPS, due to the orbit inclination of the GLONASS satellites.

Users in higher latitude areas, such as Canada, obtain better GLONASS derived dilution of precision (DOP) than users of GPS. This is due to the high inclination angle of GLONASS: 64.8 degrees compared to 55 degrees for GPS.


However, I think the distinction is moot for us as boaters because in the higher latitudes--very high, say above 70-degrees--there is not much recreational boating occurring. For recreational boating use, the NAVSTAR GPS does not suffer any particularly inferior performance relative to GLONASS. at least not at any latitude in which any of us are likely to be boating.

Dilution of precision is just one influence on the accuracy of the the position fix solution. Ionospheric effects are another influence. In polar regions there are often severe ionospheric disturbances. I don't know if the orbital limitations are the greatest impediment to position fix accuracy at high latitudes.

jimh posted 02-13-2014 09:46 AM ET (US)     Profile for jimh  Send Email to jimh     
About marine chart plotters in general and the GNSS receivers typically provided with them, I will say that the present offerings are really not very impressive. As boaters we are at least one generation behind users of mobile devices. Many smartphones and other mobile devices now provide GNSS receivers that can use the NAVSTAR GPS and GLONASS. And there are professional grade GNSS receivers that can use positioning signals other than the Course Acquisition L1 signal of NAVSTAR GPS. Most marine chart plotters seem to be offering commodity-grade GNSS receivers that use only GPS and only the L1 signal. We should expect to see better GNSS receivers becoming part of marine chart plotter products soon.

To get back on our topic, the electronic charts for Isle Royale, I am not changing my position regarding the source of the error seen in my GPS-derived position fix and tracks showing my boat as having traveled over land. The source of the error was not from my GPS-derived position fix, but rather from an error in the chart data. In particular, the error was not due to a problem with GPS accuracy at the latitude of my boat, which was in latitude 48-degrees-North.

6992WHALER posted 02-13-2014 12:42 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
It might be interesting to get a NOAA paper chart of the area and measure the latitude and longitude for the middle of the north gap and compare it to the position according to the GPS,
to see if the original NOAA chart data is incorrect or if the error occurred in the transfer from paper to electronic charts.

I think my experience with my real time radar and the GPS position not to mention my seaman's eye all agreeing on my position in the North Gap while at the same time the map on the chart plotter showed me in a different location. Makes me very confident to say the electronic map has faulty data entered in to it for this area. And the GPS recorded location is correct.

Tom W Clark posted 02-13-2014 01:04 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
[I deleted this comment because it was completely off topic and I thought it was rather mean spirited. Maybe Tom was having a bad day. Will give him the benefit of a doubt.--jimh]
jimh posted 02-13-2014 08:13 PM ET (US)     Profile for jimh  Send Email to jimh     
I think the last major ship to go down in the vicinity of Isle Royale was the EMPEROR in 1947. We visited that wreck site in August on our cruise (which is the topic of this discussion).

It is a bit ironic that the present day situation at that shoal is there is a dive buoy marking the wreck, but there still is not an aid to navigation to mark the danger. However, it is charted.

patmcqueen posted 02-15-2014 06:35 PM ET (US)     Profile for patmcqueen  Send Email to patmcqueen     
with EarthNC you can overlay the charts and the google earth imagery. The significant differences certainly demonstrate Jim's point. I wish I could change the shading so it was easier to see the photo over the chart ...,-88.69195342590332&z=15


Chuck Tribolet posted 02-15-2014 07:19 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Could the electronic chart error be a use of the wrong datum?
I don't know what the difference is between NAD27 and WGS84 in
that area.

MMM, when I get a chance I'll have a look at my electronics
topo sheets and see what they say.


jimh posted 02-16-2014 12:00 PM ET (US)     Profile for jimh  Send Email to jimh     
The EarthNC chart overlay very nicely shows the difference in satellite and charted view around the point of interest near Thomson Island.
Chuck Tribolet posted 02-16-2014 07:57 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
It's not datum. The different between NAD27 CONUS and WGS84
(the mostly likely datum screw-up) is very small in that area.
The latitudes are the same, NAD27 CONUS longitude is 0.005 minutes
less than WGS84.

By way of comparison, in Monterey, CA, NAD27 CONUS lattitude
is 0.004 minutes greater than WGS84, and latitude is 0.061
minutes less than WGS84.

Still not bad given they surveyed NAD27 CONUS starting
somewhere in the midwest using a transit, tape, and thermometer.

JimH: Just for grins can you e-mail me your Washington Harbor
track? [Sent--jimh]


Chuck Tribolet posted 02-17-2014 08:35 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
JimH sent me his Washington Harbor track line. I used Garmin
Mapsource to overlay it on the Garmin topo sheet product from
about 1999. It looks just like JimH's Google Earth overlay.

I guess the USGS (does topo sheets) got it righter than NOAA
(does nautical charts).


jimh posted 02-19-2014 09:28 PM ET (US)     Profile for jimh  Send Email to jimh     
I posted the chart and track graphic Chuck sent me.

To follow up on my question--go back many posts in the thread to find it--of what is in error, the position deduced by my GPS receiver with a very low HDOP and with WAAS augmentation of precision, or the several electronic charts, all derived from NOAA surveys, it seems crystal clear to me that the chart data is off and the GPS receiver fix is fine.

My basis for this is the several highly correlated tracks and land mass positions found in charts not based on the NOAA survey. When the track is plotted over satellite images or U.S. Geologic Survey maps, it shows a much truer presentation of where the boat went in relation to the land. Case closed for me.

porthole2 posted 05-16-2015 09:34 AM ET (US)     Profile for porthole2  Send Email to porthole2     
I see this is an old discussion [yes, and since your new article was on a different topic, I moved it to its own thread.--jimh]

Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.