chart differences

[martinworswick]

My onboard plotter is a raymarine rc435 and i knew my position was quite a way off-it shows me in openwater when i'm tied in my slip,i thought i'd compare it to bing maps in ovitalmap on an ipad and a ge2kap chart in opencpn on a rpi-the coloured dots i've made are my estimates of the position of the gps receiver

[Steadman Uhlich]

Verrrry interesting!

Thanks for posting the comparison.

I recently sailed up Penobscot Bay Maine, and compared what I saw on a Raymarine Chartplotter (recent model, large hybrid touch screen) with what I saw on an iPad next to it, that has SEAiq on it.

It seemed to me that the SEAiq chart and IPad GPS position of our boat was more accurate to what I observed with my eyes on the passing landmarks on the many small islands and ATONS, though I did not do have any screengrabs and did not take photos to show, but I now wish I had. What I recall was it appeared to be a striking difference.

I am sorry I cannot provide more details, as I do not own the gear and I am off the boat after the voyage ended a few days ago.

[CaptTom]

Older chartplotters sometimes defaulted to a different datum. It's also possible to inadvertently change the datum. This will cause everything to be in the wrong position.

There should be a page which shows HDOP (horizontal dilution of precision) or just DOP. Nowadays expect that number to be not muich more than a couple of meters (half-dozen feet.) If that number is small, and you're using the right datum, just about any chartplotter ever made should put you very close to exactly the correct location, unless you're in an area that's not well charted, like some inland canals in the US and Canada, or some remote islands.

[martinworswick]

Originally Posted by CaptTom

Older chartplotters sometimes defaulted to a different datum. It's also possible to inadvertently change the datum. This will cause everything to be in the wrong position.

There should be a page which shows HDOP (horizontal dilution of precision) or just DOP. Nowadays expect that number to be not muich more than a couple of meters (half-dozen feet.) If that number is small, and you're using the right datum, just about any chartplotter ever made should put you very close to exactly the correct location, unless you're in an area that's not well charted, like some inland canals in the US and Canada, or some remote islands.

i'll check that next weekend,its been that way ever since i bought the boat-assuming the previous owner had it all set correctly could be a mistake i suppose....