American polyconic - latitude of origin?

[Piotr]

I see a parameter. It's latitude of origin. I can't see the difference between 0 and 45 degrees with this parameter. What it's for?

[daan]

There will be no visible effect on a map using the projection directly. Latitude of origin is provided so that calculations will have the desired effect when the projection is only part of a more extended chain of calculations. How to apply an ellipsoidal datum might depend on the latitude of origin. Positioning two projections with respect to each other for blending might (but not necessarily) require a latitude of origin. The vertical offset of coordinates when output to vector format is determined by latitude of origin. And so on.

— daan