Problem with PLY and REF Points when Georeferencing BSB Charts

[GPS-Marco]

Originally Posted by HamishB

Mercator is like the standard map of the world you see with Greenland distorted to be bigger than Australia. Only areas along the equator (the central parallel) remain undistorted. More local charts using the Mercator projection will reset the standard parallel to be a nearby latitude, but the distortion remains as you move away from it.
...
Hamish

Ciao Hamish, The sentence above in not precise and may imply some misunderstanding. Mercator projection is not distorted when you go far from the equator (shapes of "small objects" are always kept). The only "distorsion" is the scale that changes while latitude change (so the Greenland....). This implies that you do not need to change the parallel of projection that always remains the equator (at least for our nautical charts). You just shall take into consideration that the scale value of a small chart far from the equator shall be corrected by the mercator scale formulas.

Moreover: both merc and tmerc are conformal and then x and y scales are identical. If a map does not have an identical x and y scale, then it is neither merc nor tmerc (and nor UTM). I noticed that in this case OCPN complains about this x/y scale inconsistency (just give a look to the log).

Ciao, Marco.

[Totobeloeil]

Marco,

Thanks for your explanation.
There is one thing I don't understand.

Originally Posted by GPS-Marco

Moreover: both merc and tmerc are conformal and then x and y scales are identical.

I believed this was true only locally. When going far away from Equator, the scale goes down, no ?

So a large circle (60 degrees wide, for example) should not appear as a circle ?

Or is there another projection used by OCPN ? (a 'world' projection with a scale based on Equator)

Sorry for all these questions ... I promise I won't do it again ..

Jean-Pascal

[Totobeloeil]

This is a 60 degreees circle. Left bottom corner is 0°00'00"N 0°00'00"E.
The file is calibrated with 121 ref points.

In my opinion, it should extend itself North to fill all the red box.
Or did I miss something ?

Jean-Pascal

N.B. I did not install any patch.

[GPS-Marco]

Originally Posted by Totobeloeil

This is a 60 degreees circle. Left bottom corner is 0°00'00"N 0°00'00"E.
The file is calibrated with 121 ref points.

In my opinion, it should extend itself North to fill all the red box.
Or did I miss something ?

Jean-Pascal

N.B. I did not install any patch.

Jean, how are you "georeferencing" this immaginary circle? How do you calculate the lat/lon of each x/y pixel? Are you using the mercator formulas?

A "circle" 60 degrees wide and 60 degrees high is not a circle on earth surface (think to meridians on earth that gets closer while you go north...). A real circle 60 degrees high (i.e. spanning 60 degrees in latitude) drown on the earth surface from the equator up, would be wider than 60 degrees in longitude at its maximum width (at 30° north, meridians are closer than at equator).

And no: a circle on earth is not a circle on conformal projection. shapes are kept only on limited areas. (i.e on conformal maps infinitesimal circles are projected as circles). And this is true always: close or far from the equator.

Tissot's Indicatrix - Wikipedia, the free encyclopedia

Ciao, Marco.

[Totobeloeil]

Originally Posted by GPS-Marco

Jean, how are you "georeferencing" this immaginary circle? How do you calculate the lat/lon of each x/y pixel? Are you using the mercator formulas?

It is calibrated, not really georeferenced. I built a hdr file and a bmp file and then made the attached kap / bsb with mc2bsbh, nconvert and tif2bsb. So, no formula.
I think it is a valid test however.

at 30° north, meridians are closer than at equator

Yes : I think this wide circle should look like an egg.

N.B. : the red box calculated by OCPN (is it a bounding box ?) looks better, as it seems higher. I don't understand why it looks different.

Well, perhaps it was simply not a good idea ...

Tissot's Indicatrix - Wikipedia, the free encyclopedia

The applet makes sens :
http://www.btinternet.com/~se16/js/tissot.htm

Thanks for your concern.

A plus,
Jean-Pascal

[Totobeloeil]

Marco,

I understand now is that OCPN does not really project the chart (after calculating coordinates from each of the 121 ref points).

OCPN simply displays the chart : a position it calculated from only 2 ref points.

So the chart has to be projected outside OCPN. I did it for the 60 degrees wide circle and it looks OK now.

Thanks for your help.

Jean-Pascal

[GPS-Marco]

Originally Posted by Totobeloeil

Marco,

I understand now is that OCPN does not really project the chart (after calculating coordinates from each of the 121 ref points).

OCPN simply displays the chart : a position it calculated from only 2 ref points.

So the chart has to be projected outside OCPN. I did it for the 60 degrees wide circle and it looks OK now.

Thanks for your help.

Jean-Pascal

Exact! Just note that when you say "a position it calculated from only 2 ref points", this is true only for small scale charts since OCPN switches to a pure mercator calculation instead of trying to use the georeferentiation by reference points approximation polinomial (that is always bad for very small scale charts).

Ciao, Marco.