Problem with misaligned spatial projection (.prj) - British National Grid

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Problem with misaligned spatial projection (.prj) - British National Grid

Igloo_79
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've used .prj files (27700: OSGB 1936/British National Grid) I downloaded (http://spatialreference.org/ref/epsg/27700/) as well as those generated by QGIS, leading to the same results.

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]]
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Re: Problem with misaligned spatial projection (.prj) - British National Grid

Gabriel
I had the exact same issue, never found a solution...
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Re: Problem with misaligned spatial projection (.prj) - British National Grid

Gabriel
In reply to this post by Igloo_79
I ended up editing http://spatialreference.org/ref/epsg/27700/ as follows:

PARAMETER["false_easting",400107],PARAMETER["false_northing",-100056]

Are these the same values you got?
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Re: Problem with misaligned spatial projection (.prj) - British National Grid

Igloo_79
Mine were very similar. Have you experienced offset issues when loading terrain (e.g. DTM with the standard 27700 UCS), or when performing 2D floodplain modelling?

I am concerned that whilst the 1D network may appear to line up neatly with the WMS imagery, the offset could affect 2D flood mapping.
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Re: Problem with misaligned spatial projection (.prj) - British National Grid

lkhoury
I had the same issue. It turned out that the survey data was adjusted by a correction factor (eg: 1.006). Are you sure your survey data point do not have any issues?
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Re: Problem with misaligned spatial projection (.prj) - British National Grid

Igloo_79
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.