Open CPN and google earth

AntarcticPilot

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The only caveat is to check the date of the image you are using. Google's are often quite old, and in areas where the sea-bed changes rapidly or where there has been recent construction, it could be misleading. But in an area where the chart survey is old, it's likely to be more recent than the chart. However, man-made changes are likely to have made it onto a chart through corrections if not through new editions.

Satellite images may have position errors of up to around 100m; don't assume (as the article seems to) that the positions given by images will match your GPS. The more recent the image, the smaller the likely offset, but unless an image has been tied to ground control points, it may have position errors.

Probably a bit geeky, but also you should be aware that different satellites have sensors with different spectral responses. That means that you can't assume that things that look the same on adjacent images actually are the same. Further, the penetration through water also depends on the spectral response, and different satellites will penetrate to different maximum depths.

In general a useful technique for places off the beaten track.
 

GHA

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The only caveat is to check the date of the image you are using. Google's are often quite old, and in areas where the sea-bed changes rapidly or where there has been recent construction, it could be misleading. But in an area where the chart survey is old, it's likely to be more recent than the chart. However, man-made changes are likely to have made it onto a chart through corrections if not through new editions.

Satellite images may have position errors of up to around 100m; don't assume (as the article seems to) that the positions given by images will match your GPS. The more recent the image, the smaller the likely offset, but unless an image has been tied to ground control points, it may have position errors.

Probably a bit geeky, but also you should be aware that different satellites have sensors with different spectral responses. That means that you can't assume that things that look the same on adjacent images actually are the same. Further, the penetration through water also depends on the spectral response, and different satellites will penetrate to different maximum depths.

In general a useful technique for places off the beaten track.

All very true though in my experience google images are likely to be very accurate and more likely to be newer than charts. Easy to check datum accuracy in opencpn with a few bearings if you want (routes are an easy way to do this, a route with 2 waypoints for each bearing) or just sail/walk to a landmark. Also sasplanet allows you download from different sources to check for differences.
Can be extrememly useful as an addition source of info, not just off the beaten track..

Might be imagining this but sat images seem to mostly be taken at low water, which is really handy! Anything to do with the moon position which would make this happen? Or is it just coincidence... :)

OCHEkZp.jpg
 

Roberto

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Might be imagining this but sat images seem to mostly be taken at low water, which is really handy! Anything to do with the moon position which would make this happen? Or is it just coincidence... :)


In France they (authorities) have done a campaign of detailed (military) aerial photographs at very low altitude for all the coast, actually they made two campaigns, the second one was to photograph the shore limit closer to LAT, or more realistically the springiest low water springs: sometimes differences with Google or Bing sat images are striking. The problem is georeferencing as they are not accessible through Sasplanet so one has to use either MapCal or OpenCpn wefax plugin.

Do such images exist for UK waters?
 

AntarcticPilot

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Might be imagining this but sat images seem to mostly be taken at low water, which is really handy! Anything to do with the moon position which would make this happen? Or is it just coincidence... :)

Many remote sensing satellites are in "sun synchronous" orbits, which means that they pass over at the same time each day. The older satellites - the Landsat Series and SPOT - took images continuously so there was an image taken under similar conditions every 16 days; the purpose was to provide long-term monitoring, so the regular acquiisition of comparable images was (and remains) important. These have resolutions up to about 15 m (ETM+). Higher resolutions satelites take images as requested; the availability depends on a request having been made, and as the sensors are steerable, there's no hard and fast repeat period between images. I guess that someone requested that the high-res images you show be taken at low water! Publicly available images tend to be ones that are past their sell by date for commercial exploitation.
 
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