CLLocationCoordinate2D to MKMapPoint using PROJ4 - mkmapview

Question

I am attempting to use the Proj4 library to transform different map projections for use with Apple Map Kit. For testing purposes I thought I would start with duplicating the MKMapPointForCoordinate function using Proj4.
These are the definitions I am using:
+proj=longlat +ellps=WGS84 +datum=WGS84 +nadgrids=#null +no_defs
+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=#null +no_defs
When I use the coordinate 47.5, -97.3 the Proj4 transformation returns:
6028687.013553943, -10834169.44145535
But MKMapPointForCoordinate returns:
61646948.12444445, 93835627.5941129 which is an order of magnitude off.
The world size for Map Kit is 268435456 x 268435456

Answer 1

With the following definitions:
+proj=longlat +ellps=WGS84 +datum=WGS84 +nadgrids=#null +no_defs
+proj=merc +lon_0=0 +lat_ts=0 +x_0=134217728.0 +y_0=134217728.0 +a=6378137 +b=6378137 + k=1.0 +units=m +nadgrids=#null +no_defs
there is one more step to close the gap between Mapkit and Proj4:
a linear regression in x and
a linear regression in y.
let const_x = 114180219.657211
let a1_x = -0.850708929130513
private func mk_x_forProjX(projX: Double) -> Double {
return (projX - self.const_x) / (self.a1_x + 1)
}
private func proj_x_forMKX(mkX: Double) -> Double {
return mkX * (self.a1_x + 1) + self.const_x
}
let const_y = 154255236.342789
let a1_y = -1.14929107086948
private func mk_y_forProjY(projY: Double) -> Double {
return (projY - self.const_y) / (self.a1_y + 1)
}
private func proj_y_forMKY(mkY: Double) -> Double {
return mkY * (self.a1_y + 1) + self.const_y
}
The fit is not perfect, but the precision is satisfying enough.
One last point,
be sure to use Double when converting degrees <-> radians,
i.e :
static func DegreesToRadians (value:Double) -> Double {
return value * M_PI / 180.0
}
static func RadiansToDegrees (value:Double) -> Double {
return value * 180.0 / M_PI
}