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Old 20-11-2010, 05:39   #1
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The CM93 Offset Feature

I have been playing around a bit with this new feature, that was introduced in beta 2.2.1111. I had the impression from the announcement that this feature was supposed to be corrections for each cm93 cell separately, but have found that this is not the case.

I have tested charts around Penrhyn, also called Tongareva in the NE Cook Islands.
Pic 1. CM93 and the NZ raster-charts agree pretty well at this scale.
Click image for larger version

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Pic 2. At the next smaller scale CM93 charts, things are not so good.

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Pic 3. Using the new tool I correct this (approximately).
Click image for larger version

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Pic 4. Going back to the original larger scale chart, shows that the corrections has applied to this as well.
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Obviously, a useful correction tool must apply to individual (faulty) charts.
Hopefully we can establish a list with reasonable corrections to apply to individual charts.

This new feature needs to be discussed. See this post as a start.....

To get started, tick the box att the bottom of ToolBox-> Vectorcharts, then"OK". Now tick the box att the bottom of ToolBox->Settings. Start to play with the right-click menu.

Thomas
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Old 20-11-2010, 07:15   #2
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I haven't used the offset tool yet, but I can offer one bit of advice--If you enter an offset, the screen should have a blinking red "MANUAL OFFSET" display (like the Overzoom). Otherwise you have created a monster.

I was visiting on an Ocean Alexander this year, and the high end chart plotter showed the boat icon 30 miles off position. I RTFM, and the plotter had a feature which would let you put in an offset by positioning the cursor where you knew the boat was and pressing a button. However, there was nothing obvious that told the next user that you had put in an offset, rendering this chart plotter extremely dangerous.

PS-- Penrhyn is one of my favorite places in the whole world, but I wouldn't trust anything but my eyeballs there.
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Old 20-11-2010, 09:38   #3
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Quote:
Originally Posted by donradcliffe View Post
I haven't used the offset tool yet, but I can offer one bit of advice--If you enter an offset, the screen should have a blinking red "MANUAL OFFSET" display (like the Overzoom). Otherwise you have created a monster.

I was visiting on an Ocean Alexander this year, and the high end chart plotter showed the boat icon 30 miles off position. I RTFM, and the plotter had a feature which would let you put in an offset by positioning the cursor where you knew the boat was and pressing a button. However, there was nothing obvious that told the next user that you had put in an offset, rendering this chart plotter extremely dangerous.

PS-- Penrhyn is one of my favorite places in the whole world, but I wouldn't trust anything but my eyeballs there.
The new CM93 does give you a warning when the feature is on, I just did not include i in the screen shots. Here it is.Click image for larger version

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The tool to do the adjustments looks pretty much like the measuring tool. Activate the tool, click on the location you you know the boat is in, move the cursor to the own ship icon, click again and you will see the icon jump to the correct position. The value of the correction will appear in Toolbox->Settings-> CM93 offset display, where numerical values also can be entered manually.

You are spot on in your "PS" !

Thomas
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Old 20-11-2010, 09:39   #4
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Quote:
Originally Posted by donradcliffe View Post
I haven't used the offset tool yet, but I can offer one bit of advice--If you enter an offset, the screen should have a blinking red "MANUAL OFFSET" display (like the Overzoom). Otherwise you have created a monster.

I was visiting on an Ocean Alexander this year, and the high end chart plotter showed the boat icon 30 miles off position. I RTFM, and the plotter had a feature which would let you put in an offset by positioning the cursor where you knew the boat was and pressing a button. However, there was nothing obvious that told the next user that you had put in an offset, rendering this chart plotter extremely dangerous.

PS-- Penrhyn is one of my favorite places in the whole world, but I wouldn't trust anything but my eyeballs there.
The new CM93 does give you a warning when the feature is on, I just did not include i in the screen shots. Here it is.
Click image for larger version

Name:	Screenshot-OpenCPN 2.2.1111.png
Views:	92
Size:	85.4 KB
ID:	21321

The tool to do the adjustments looks pretty much like the measuring tool. Activate the tool, click on the location you you know the boat is in, move the cursor to the own ship icon, click again and you will see the icon jump to the correct position. The value of the correction will appear in Toolbox->Settings-> CM93 offset display, where numerical values also can be entered manually.

You are spot on in your "PS" !

Thomas
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Old 23-11-2010, 15:04   #5
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Offset for other type of chart.

Hello,

I saw the new tool on the offset of the CM93. Suppose there is no malfunction. This is not my purpose here.

If I load the BSB raster Brazil, I see that most of these cards are like WGS84. But some are not. For example, chart 9001.kp: Datum COA.

Likewise with Raster charts of the USA. A number of these maps are standard WGS84 but the others are Datum : NAD83.

Is it useful to have the same tool to offset the charts for these ?

Best regards

Gilletarom.
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Old 26-11-2010, 15:31   #6
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Originally Posted by Gilletarom View Post
Hello,

I saw the new tool on the offset of the CM93. Suppose there is no malfunction. This is not my purpose here.

If I load the BSB raster Brazil, I see that most of these cards are like WGS84. But some are not. For example, chart 9001.kp: Datum COA.

Likewise with Raster charts of the USA. A number of these maps are standard WGS84 but the others are Datum : NAD83.

Is it useful to have the same tool to offset the charts for these ?

Best regards

Gilletarom.
OpenCPN compensates automatically for quite a few different chart datums. For the US, WGS84 and NAD83 are for practical purposes considered the same, unless you require better than 1 m accuracy.
But... there are a lot of raster charts out there, that nevertheless could do with some manual corrections. Once again, isolated islands in the Pacific are good candidates.
A user can already do this now, by adjusting the "DTM" values in the header of the kap file. A graphic tool on top of the existing code could let users adjust individual raster charts. These DTM values would then have to be saved in a file.

To try DTM corrections: These are lat & long diffs in seconds of arc.
Apply with normal signs, + for N & E, - for S & W. Think of corrections needed to move the incorrect position to the true position.
Remember to use "bsbfix" after editing a kapfile. Also use "Force full database rebuild" in the chart tab.

Thomas
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