Projection Errors

[kjames]

Hi all, I think this is a fairly well discussed issue. Previous threads are massive in some cases – difficult to find answers without heaps to time to read up.

I’m fairly new to chart navigation and the use of Mercator charts. I’ve got an Ipad, with Inav and navionics charts. I’m at latitude 35 deg south (approx). Found that the disparity between track line and chart was about 200m / 0.1nm. Is this about what I should expect due to projection error? Just seems so much. We even managed to get in a bit of strife in shallows on the weekend – missed the channel and bottomed out – was able to back out though . Note, track file is accurate as a kmz file.

What I was mainly interested in was whether there is a way to correct this through chart software – from what I can ascertain opencpn can perform corrections. Have I chosen the wrong chart system ? Any advice appreciated. Cheers

[GordMay]

You might find Jeremy Hood’s tutorial informative.
➥ Navigational charts

[barnakiel]

It is not a projection error. It is a hardware / software or chart accuracy error.

On a cheapo netbook with a cheapo puck gps we get accuracy of about 10 meters from the chart feature. I think the same applies to chart plotters.

b.

[kjames]

Originally Posted by barnakiel

It is not a projection error. It is a hardware / software or chart accuracy error.

b.

Barnakiel, given track was totally accurate when exported to google earth then I think hardware is ok. I contacted chart supplier x-traverse who are totally confident that chart is ok, and said it was projection. I've looked at settings and can't find an issue there. But I just cannot accept result - not useless but not far off that. Cheers

[barnakiel]

Originally Posted by kjames

Barnakiel, given track was totally accurate when exported to google earth then I think hardware is ok. I contacted chart supplier x-traverse who are totally confident that chart is ok, and said it was projection. I've looked at settings and can't find an issue there. But I just cannot accept result - not useless but not far off that. Cheers

In this context, there is no such thing as projection error.

Maybe you mean the software using different geodetic/reference datum than the one the chart has been made to? Check out the parameters, most of the time the datum is supposed to be set to WGS-84, unless stated otherwise in chart specifications.

Projection has nothing to do with it.

If any seller tells you their chart/software is OK and you get 1 cable of error then probably change the software and the chart to something that is reliable.

b.

[kjames]

Originally Posted by barnakiel

In this context, there is no such thing as projection error.

Maybe you mean the software using different geodetic/reference datum than the one the chart has been made to? Check out the parameters, most of the time the datum is supposed to be set to WGS-84, unless stated otherwise in chart specifications.

Projection has nothing to do with it.

If any seller tells you their chart/software is OK and you get 1 cable of error then probably change the software and the chart to something that is reliable.

b.

Barnakiel, I was hoping it would be something simple like the datum. With the inavx system you can't tell, well at least I can't tell what datum etc is. Frustrating to say the least. Cheers

[barnakiel]

Originally Posted by kjames

Barnakiel, I was hoping it would be something simple like the datum. With the inavx system you can't tell, well at least I can't tell what datum etc is. Frustrating to say the least. Cheers

?

I have just recently used an iPhone with Navionics and the positions were spot on. I cannot tell what software it was though. If there are more than one make of software that uses the same chart, then perhaps you can test the other software with the same chart?

Are you sure the GPS had the fix? Asking because I know that some tablets (perhaps iPad too) use sorts of inertial navigation and mobile towers triangulation while/if their GPS unit cannot get the fix (e.g in a long tunnel).

b.

[kjames]

Barnakiel, think the Navionics charts I have, are arranged for use only by INavx so not sure if I can try chart on other s/ware. Conversely, I don't have other charts I can use on the Ipad. Cannot be GPS / satellite issue as there is just no doubt tracking is spot on when exported to Google. thanks for your suggestions, I'll keep working on it. Cheers

[kjames]

I'm still trying to understand this issue, and basically determine what is a reasonable expectation for accuracy. I thought it might help to post an image of the 'track line' on the InavX output and one over a google photo.

On the Inavx image you can see the displacement of the track path to the west or southwest of the channel, whereas on the aerial photo it is pretty much central to the channel. The scale on the Inavx image gives an indication of scale. You can also see the channel markers - obviously with the error involved one cannot find the channel other than with the markers. Further north the markers are widely spaced and that's where we got in a bit of strife on the weekend. Having thought about it a bit I guess the main concern is accuracy in coastal locations. Here you can't get in too much trouble.

[Sailbad]

Gps is an AID to navigation and should be used together with other devices.

[kjames]

Originally Posted by Sailbad

Gps is an AID to navigation and should be used together with other devices.

SB, I accept that. Not the issue here. I want to find out how good / mediocre this equipment is and particularly to be sure to get it set up right. I’m also interested in what other chart plotter alternatives can do in comparison.

I'd hope not to be approaching a coast or a hazard in the water at night but I’m sure that will happen at times. At this stage it appears this equipment won’t save me and given "Selective Availability” probably we shouldn’t ever expect that.

Cheers

[barnakiel]

Doh. From the pictures attached it looks like the chart datum is not the charting software uses!

PITA there is no other software / charts that you can cross-reference and find out which party fails to deliver.

Is the Navionics chart file transferable to say a PC? Perhaps you can test it under a PC charting software?

b.

[barnakiel]

Originally Posted by kjames

(...) think the Navionics charts I have, are arranged for use only by INavx so not sure if I can try chart on other s/ware (...)

Re-read this. PITA. There is definitely an error in how the software plots against Navionics chart.

The seller talking of a 'projection error' is, well, just a seller, talking.

b.

[kjames]

Originally Posted by barnakiel

Doh. From the pictures attached it looks like the chart datum is not the charting software uses!

PITA there is no other software / charts that you can cross-reference and find out which party fails to deliver.

Is the Navionics chart file transferable to say a PC? Perhaps you can test it under a PC charting software?

b.

Howdy B, everything Apple is locked up pretty much - dont think you can transfer it. Whilst I'm very happy with the Ipad, I reckon I'd have this sorted on a PC by now. I think with Apple everything has to be perfect cos you can't adjust it.

It's occurred to me that mercator vs lat/long can only produce a north-south distortion, so I am now feeling sure there is a chart problem. Cheers

[barnakiel]

Yep. Run a quick test with the most archaic to the most recent on this machine: netbook/puck/raster/vector/google for our current location. Below you have the results.

1 cable off is not acceptable in the area you are.

Cheers,
b.