[Proj] Conic projection, but centric (gnomonic)

[Lukasz Komsta]

Dear All,

In late 1970s, Institute of Botany of Jagiellonian University in Krakow (Poland) developed a geobotanical grid. This grid has interesting history, because the details of projection were never officially published. After my reverse engineering followed by further numerical investigation by my colleague it was revealed that this grid is based on something which is called in mapproj package "central conic projection" with one tangent (with quite strange Earth radius, but it is only a detail). The projection is from Earth center, not parallel nor stereographic.

I implemented this grid in various languages and the detailed info is here: https://atpol.sourceforge.io/.

The creators of original grid officially confirmed that this projection is same with their original idea and this caused rising of interest in usage.

However, there is no possibility to set this projection in PROJ4 (as far as i know), which results in incompatibility of many GIS systems with this botanical grid.

The closest solution is

+proj=eqdc +lat_1=52 +lat_2=52 +lat_0=52 +lon_0=19 +axis=esu +a=6390000
+b=6390000 +ellps=sphere +x_0=330000 +y_0=-350000

...but this results in quite significant inaccuracy, as EQDC is not gnomonic (central) projection.

So, my questions are:

1. Is there a possibility to achieve desired projection using PROJ4 ?
2. If not, are there any possibility to implement such a functionality in the future to allow GIS users using this grid?

Thank you very much in advance

Lukasz Komsta