Problem with misaligned spatial projection (.prj) - British National Grid
I’m new to HEC-RAS and having problems with projected coordinate systems of my HEC-RAS (5.0.3) geometry and the WMS maps in RAS Mapper. I imported georeferenced cross sections via a .CSV file (i.e. not using GIS), however, they are offset by ~120m from the river seen on WMS maps (Google, Open Streetmap etc).
I found a workaround by manually editing the Easting & Northing in the .prj file which aligned my geometry and the WMS correctly - however, I'm wondering if this may affect 2D modelling e.g. generating misprojected flood maps. I'd be grateful for any suggestions please?
.prj file PROJCS["OSGB 1936 / British National Grid",GEOGCS["OSGB 1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northing",-100000],UNIT["Meter",1]]
Re: Problem with misaligned spatial projection (.prj) - British National Grid
Ok, so it seems that the WMS maps are projected to different coordinate systems e.g. Google Maps Global Mercator (Ras Mapper>Map Layers>Image Info).
I've gone back to my original .prj (British National Grid, 27700) to avoid potential issues with misaligned 2D mapping (e.g. my terrain layer is offset from my 1D geometry due to different projection systems), so have opted for using terrain and/or OS maps as my backdrop.