# Polyconic Projection for China?

I have a set of locations/points and I am trying to compute the nearest distance of these points from a Polyline. The points and the polyline lie in China and the Geographic Coordinate System is WGS 1984. I have some questions regarding this.

Since my task involves computing distances, I am trying to use an equidistant projection. The following pdf suggests that the best equidistant projection for China is the Polyconic projection.

http://rcg.gvc.gu.se/data/ChinaPrecip/_notes/projection.pdf

The ESRI Polyconic projection page indicates that the parameters I will have to set include False Easting, False Northing, Central Meridian and Latitude of Origin.

http://resources.arcgis.com/en/help/main/10.1/index.html#//003r0000003t000000

What would be the best value for these parameters over China?

Also, I previously thought that the selection of central meridian and latitude of origin does not impact distortions. The ESRI link above, however, under the "Properties" and "Distance" section says "The scale along each parallel and along the central meridian of the projection is accurate. Distortion increases along the meridians as the distance from the central meridian increases. " This seems to imply that the selection of central meridian impacts distortions or am I understanding this incorrectly?

Yes, the location central meridian will change the pattern of distortions in most map projections. In the polyconic projection the length of the central meridian is correct, and also along latitude lines (which are not straight). The latitude of origin doesn't affect the pattern of distortions, but only sets the Y (Northing) = 0 location. For China, if your area of interest is the entire country, somewhere around 105 E is the central longitude.

• Thanks. Is it correct to assume that to measure distances polyconic or in general equidistant projections are best? Note in my case I have a bunch of points (cities) and a polyline (river) and I want to measure how close each point is to the river (i.e. the nearest distance of the points to the river). Jul 24 '15 at 15:54