### Questions on projection in GEOGRID "module_map_utils.F"

Posted:

**Tue Jul 24, 2018 12:21 pm**Howdy folks:

I have been looking at the source "module_map_utils.F" in GEOGRID and there are a few things I don't quite understand.

Particularly, I am looking into all related functions on the Lambert conical comformal projection. I extracted the coordinates (in degrees) from a WRF .NC file, I need to convert this into spherical scale for another CFD simulation code so that means I will have rewrite the code to convert them. I have access to the original namelist.wps, so the Lambert projection settings are known.

Browsing through module_map_utils.F, in particular subroutines set_lc, lc_cone, ijll_lc, llij_lc, while I can recognize the functions being used for the projection, there are few things I don't quite get.

1) Why are the Earth radius and delta x used in the calculation of the cone position in the calculation of pole point in subroutine set_lc? My hunch is to figure out how many "i"s there should be at the first true latitude, but if the conversion is from angles (spherical) to angles (projected), I don't know what the use of this could be. Does GEOGRID really map this to "i,j" directly?

2) I understand that the reference long / lat position will be the center of the domain. However, do I expect, reference long be the vertical axis, as opposed to the standard longitude? In other words, will the projected domain be adjusted such that the reference longitude will be orthogonal to the true latitudes? (I hope not, but I needed a confirmation).

3) A very naive question ... ultimately I want to convert this into UTM distances (in m) for my other simulation. Can I just take the projected long lats and simply convert them without the intermediate step of reverting the Lambert projection (in degrees) in linear coordinates (also in degrees)?

Thank you very much in advance, Gerry.

I have been looking at the source "module_map_utils.F" in GEOGRID and there are a few things I don't quite understand.

Particularly, I am looking into all related functions on the Lambert conical comformal projection. I extracted the coordinates (in degrees) from a WRF .NC file, I need to convert this into spherical scale for another CFD simulation code so that means I will have rewrite the code to convert them. I have access to the original namelist.wps, so the Lambert projection settings are known.

Browsing through module_map_utils.F, in particular subroutines set_lc, lc_cone, ijll_lc, llij_lc, while I can recognize the functions being used for the projection, there are few things I don't quite get.

1) Why are the Earth radius and delta x used in the calculation of the cone position in the calculation of pole point in subroutine set_lc? My hunch is to figure out how many "i"s there should be at the first true latitude, but if the conversion is from angles (spherical) to angles (projected), I don't know what the use of this could be. Does GEOGRID really map this to "i,j" directly?

2) I understand that the reference long / lat position will be the center of the domain. However, do I expect, reference long be the vertical axis, as opposed to the standard longitude? In other words, will the projected domain be adjusted such that the reference longitude will be orthogonal to the true latitudes? (I hope not, but I needed a confirmation).

3) A very naive question ... ultimately I want to convert this into UTM distances (in m) for my other simulation. Can I just take the projected long lats and simply convert them without the intermediate step of reverting the Lambert projection (in degrees) in linear coordinates (also in degrees)?

Thank you very much in advance, Gerry.