# Proj4js: Is this correct implementation of Azimuthal Equidistant relative to an arbitrary center?

If I wanted to interpolate points along a Great Circle Arc, it makes sense that either endpoint of the Great Circle is a satisfactory starting place. Next, I suppose I'll need a projection to preserve distances along the Great Circle. Unless someone corrects me, I think an Azimuthal Equidistant projection centered on either endpoint of the Great Circle would work.

From Wikipedia, with respect to an Azimuthal Equidistant projection "all points on the map are at proportionately correct distances from the center point", and additionally, it's "useful for showing airline distances from [the] center point of projection".

I suspect I can implement the Azimuthal Equidistant in Proj4js, but I've never really customized a projection before, so basically I'm just curious whether I'm doing it properly..

Now if I go to SpatialReference.org and search for Azimuthal Equidistant, I see both north and south pole-centered customizations of the projection. Respectively, they're implemented in Proj4js like so:

`Proj4js.defs["ESRI:102016"] = "+proj=aeqd +lat_0=90 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";`

`Proj4js.defs["ESRI:102019"] = "+proj=aeqd +lat_0=-90 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";`

It follows that the only value changing in either projection is the latitude, itself, `+lat_0=`, which changes from `90` to `-90`. So I'm pretty sure I just need to set these `+lat_0` and `+lon_0` appropriately, and bingo--the Azimuthal Equidistant is properly defined.

So with all of this said..

Is this a valid way to define an Azimuthal Equidistant projection relative to a specific place, like Columbia, South Carolina? (Lon: -81.020742, Lat: 34.008654)

`Proj4js.defs["CUSTOM:10001"] = "+proj=aeqd +lat_0=34.008654 +lon_0=-81.020742 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";`

I hate to ask such a Yes or No question, so please note the additional opportunity to remark on whether this is a wise choice of projection considering the scenario (interpolating points at fixed distances along a Great Circle Arc).

• Interesting, @AndreJ. Regarding your edit in that post (problem with reprojecting ..on an ellipsoid), that makes me wonder how I can test the `aeqd` implementation to make sure it's working as expected before relying on any values it emits. :/ – elrobis Oct 13 '14 at 19:22
• Based on your understanding of that bug/flaw, would you be comfortable using the projection definition above for "hemisphere-sized" regions of interest? For instance, if I were interested in interpolating points along an arc with endpoints in Boston MA, and Honolulu HI, with the `aeqd` centered at either endpoint, would it be acceptable? Your edit in that answer seems to imply (to me at least) that any problems projecting to `aedq` from an ellipsoid are only apparent at the extremities. Am I wrong? Any idea what the safety threshold might be--that is, maximum reliable distance? – elrobis Oct 13 '14 at 19:37