Help Understanding Coordinate Systems


I recently received this from Mapit support regarding a difference in coordinates I had observed.

The employee was out using Mapit and forgot to use orthometric height to OSGM15 geoid and so I converted the lat/lon manually after and noticed a shift between the conversion using Ordnance Survey software and the Mapit British National Grid output coordinates. Here is the response from Mapit:

“Hi Dave, this is related to the fact that Mapit is not using the grid to convert coordinates.
It uses standard 7 parameters Helmert Transformation between WGS84 and OSGB36.
For maximum accuracy, I would suggest logging all the data in WGS84 in form of lat, lon and get the data converted to 27700 using professional software which is utilising OSGM15 grid.
The support for OSGM15 conversion of the coordinates possibly will be supported directly in Mapit in the future, however now all transformations are being done using PRJ4 library and not grid conversions, which is always a less accurate method.”

My question is: Does Reachview3 have the same issue when converting to BNG and OSGM15?


Hi Dave,

ReachView 3 supports EPSG:7405 (OSGB 1936 / British National Grid + ODN height). However, we don’t have OSGM15 vertical datum there. Also, for now, there is no possibility to apply local grids in ReachView 3. This feature is in our roadmap. However, I can’t provide you with an ETA.

So I’m struggling to know which app to use here - there is a standard error between Reachview and Mapit XY outputs - which is more accurate?

Hi Dave,

Sorry for the misleading answer. ReachView3 supports EPSG:7405 that provides British National Grid coordinates and ODN heights. Heights in OSGM15 and ODN are the same, so EPSG:7405 should suit your needs. The conversion with the grid and the geoid works fine in our app, so ReachView should be more accurate.

If you have coordinates in ETRS, you can check the results using the official Coordinate transformation tool.

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