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Old 01-06-2021, 14:27   #1
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Location: Bush Alaska
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Help: B&G/Lowrance/Simrad Custom Charts

I bought a Simrad Go and want a chart for a large lake in Alaska for it.

I purchased the Alaska Lakes chart and it less detail for a 60 mile long lake than a typical desk top globe does. I got my money back.

I did a bunch of research and didn't find much info, but was able to find some guides on creating your own charts downloading google satellite images with sas.planet and insight map creator.

The chart I am trying to make is roughly 100km(~65 miles) by 100km, of which my boat is roughly in the center sitting on a trailer in my yard.

After a few failed attempts I was able to get the chart to display on my Simrad Go screen, I had to change the Atlas version to 10 in Insight Map Maker.

So at this point I can make different charts(google, bing, arcgis, satellite, road, etc....) with different names all on the same card and choose which one I want to use in the plotter. However, they are all roughly off by 200m(~700ft) which I feel is a pretty significant error.

I did a test where I made a very small chart of just my village, and it seemed much more accurate, probably within a few feet.

I don't know if my problem is in the charts that sas.planet downloads (I doubt it since they all have the same error), something wrong with the way I download them, something wrong with the way I save them from sas.planet with the .kml file, something wrong with the way insight map creator translates the .kml file into a .jgw file, something wrong with my settings in map creator, something wrong with the gps in my plotter (I dont think so since it did display close on a much smaller chart)....... I'm at a loss.

I suspect my problem is some sort of rounding error, but I don't know.... I am kind of a loss at what to try next.....

Has anyone run into this issue before or have any ideas on how I should go about troubleshooting my process?

Thanks!
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Old 02-06-2021, 10:23   #2
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Re: Help: B&G/Lowrance/Simrad Custom Charts

too unique a problem then?
back to googling
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Old 15-06-2021, 14:17   #3
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Re: Help: B&G/Lowrance/Simrad Custom Charts

Well, if anyone runs across this in the future, I figured it out.
I am still not sure the exact issue, but still suspect it to be a rounding problem.

The Problem......
I was stitching tiles of satellite images downloaded into sas.planet into a single file and creating a world file for it then using those two files in insight map creator to create another world file and finally converting to the charts that can be read by the plotter. I was working with a rather large image, zoom 16 google satellite tiles 131 wide and 103 tall..... When loaded on the plotter my position(roughly center of chart) would be off by somewhere around 750ft.

The fix.....
I found that I could 'stitch' in any grid size from sas.planet I wanted, it doesn't need to be a single huge jpeg file. So I selected the same grid size/pattern as the tiles downloaded and made it 131 wide and 103 tall, this created a huge number of files 131*103*2 and then insight map creator created another world file for each of them, so really working off of 131*103*3 files in the working folder, but it went smoothly and made a chart that shows my position accurately within a few feet and shows all my previous paths plotted on the charter as beginning and ending at locations I know they were instead of inland or cutting across peninsulas etc....

Hopefully that helps someone down the road someday.

next when I have time, is figuring out how to download different versions/dates of google satellite to piece together the clearest picture without clouds or shadows, etc.....
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