What is a Simple Mercator?

[feverzsj]

according to georef.cpp, toSM()/fromSM(), which is used as enc projection. It looks like a common mercator, but takes into account the latitude of natural origin which seems to be the enc cell center.
So, what the major difference and benfit between the Simple Mercator and common mercator. Will it distorts features cross cells?

[seandepagnier]

I think taking the latitude of origin is only to render on the right offset. Regular mercator charts can be overlayed at any time and align correctly, so it is common mercator. There is not distortion.

I could be wrong, but I believe simple mercator means it is using spherical coodinates rather than elliptical.

[bdbcat]

feverzsj...

boat-alexander is correct. SM uses spherical calculations with no eccentricity or elliptical correction terms. Read the code and you will notice the performance penalty incurred when calculating the correction terms.

We do a lot of this calculation in OpenCPN.....

Dave

[feverzsj]

Thanks for the replying!
So, this projection is Spherical Mercator which references lat0/lon0?
But, why a fixed scale factor 0.9996 is applied?
Furthermore, each cell now reference different point, won't that be a problem?