# WPS Lambert Geogrid LonLat mismatch

#### MarcTree

##### New member
Hi,

I'm getting an offset converting between long/lat mass grid coordinates and x/y synthesized geo coordinates in the geogrid file. In my example this is around 40-50m for a file with dx/dy of 1000m. Using geogrid version 4.2

Namelist parameters:
map_proj = 'lambert',
ref_lat = 35.085,
ref_lon = 130.95,
truelat1 = 35.085,
stand_lon = 130.95,

Resulting lambert variable in both the geogrid and output netcdf file, which is effectively the same as namelist inputs:
char Lambert;
:grid_mapping_name = "lambert_conformal_conic";
:latitude_of_projection_origin = 35.084991455078125; // double
:longitude_of_central_meridian = 130.9499969482422; // double
:standard_parallel = 35.084999084472656; // double
:_CoordinateTransformType = "Projection";
:_CoordinateAxisTypes = "GeoX GeoY";

I get the same conversion to Lon/Lat using MIKE Zero python and MATLAB toolboxes and the following website:

Using these parameters error is greater at the grid corners, indicating the earth radius is wrong. If I change the ellipsoid to a sphere of 6370000m the error is consistent at all points. For example, before and after:
+proj=lcc +lat_0=35.084991455078125 +lon_0=130.9499969482422 +lat_1=35.084999084472656 +lat_2=35.084999084472656 +a=6371229 +b=6371229 +no_defs
+proj=lcc +lat_0=35.084991455078125 +lon_0=130.9499969482422 +lat_1=35.084999084472656 +lat_2=35.084999084472656 +a=6370000 +b=6370000 +no_defs

After some iteration I get a nice conversion with the following projection string:
+proj=lcc +lat_0=35.08505847 +lon_0=130.95043037 +lat_1=35.08505847 +lat_2=35.08505847 +a=6370000 +b=6370000 +no_defs

So the real question is which coordinates do I trust? XLONG_M, XLAT_M and synthesized x/y using the iterated projection string above? Or x/y using the namelist parameters?
+proj=lcc +lat_0=35.085 +lon_0=130.95 +lat_1=35.085 +lat_2=35.085 +a=6370000 +b=6370000 +no_defs

My worry is XLONG/XLAT are synthesized incorrectly in the first place. Which coordinates are forcings interpolated onto for the WRF simulation?

WPS assumes a spherical earth with a radius of 6370km.

I would suggest that you use the coordinate created by WPS for WRF simulation.