Re: o-charts for OpenCPN


I've attached a screenshot of the North Portland Channel in Navionics with Sonar charts turned on and set to the most detail, also a shot of the NOAA raster chart. Lots more detail in Navionics, but where does it come from?

What was the water level when you were there, Mark? NOAA gives depths at "mean low low water", but at low tide and a drought stricken river...

“'MLLW' means the 0.0 tidal elevation. It is determined by averaging each day's lowest tide at a particular location over a period of 19 years. It is the tidal datum for vertical tidal references in the salt water area."

On 8/27/2021 4:25 PM, Mark N wrote:
Just use your depth sounder when in shallow water and don't trust
Navionics' depth contour lines.
Just happened a week ago:
Attached are two images I spit out of sas.planet showing my track on a satellite image and on the navionics image. We are in the North Portland Channel on the south side of Hayden Island and the current is heading to the northwest.
If you zoom in on the navionics, you will see we were in 14-16 feet of water and we were well aware of the 2-4 foot area ahead.  We were about to head to the north to view the nice floating homes and boats along there.  I've done this many times before.
Luckily, we were in the no wake zone and my buddy, who was piloting, was watching the depth gauge because he didn't want to hurt my boat.  He commented on the depth just before the depth sounder squealed its head off at three foot deep and scared the sh#t outa me.  What's it talking about?  We finally nursed her around and got the hell out of there.
After getting home, I dropped the gps track on my sas.planet imagery and, lo and behold, we have new bottom features!  I doubt we will see NOAA or navionics update this area.
John <>
My toast would be, may our country always be successful, but whether successful or otherwise, always right. (John Quincy Adams)
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