Hi,
we used to have a process where we would open up some Irish national grid in MapInfo create points and then re-save the file as British National Grid. I'm trying to replicate this in FME, however when using either the reprojector or setting within the read/write i'm seeing a few metres difference. are you able to advise?
Best answer by mark2atsafe
View original
+21
Which coordinate system are you going from/to in EPSG terms?
Which coordinate system are you going from/to in EPSG terms?
I'm going from EPSG:29902 to EPSG:27700
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I'm going from EPSG:29902 to EPSG:27700
Hi Mark_1spatial
An example IG value is: 334316|374675
when converted to BNG using mapinfo: 146779|530100
when converted to BNG using FME: 146778|530103
as you can see only slightly different....
[moderator note: edited with info from other answer]
+2
@alicedevereux I'm seeing slightly different to you:
And when I convert that BNG coordinate to Lat Long and check in Google Maps it is the same place as the IG value in Belfast (SE corner of Royal Mail building)
+25
Just looking into this now. Taking quite a while!
Anyway, how are you carrying out the reprojection? Have you tried the GridInQuestReprojector? I don't know much about it, but it says that the purpose is to convert "the national coordinate systems of Great Britain, Northern Ireland and the Republic of Ireland" so that would be something to look at for sure. The problem there is that I can't get it to work! At all. I will continue trying.
The other issue is that there must be a datum transformation of some sort going on, and I just can't find which it is. The CsmapReprojector lets me choose, and I can get multiple results from different transformations, I just can't find the ones that your MapInfo version is using!
So, will check some more and get back to you shortly.
+21
Just looking into this now. Taking quite a while!
Anyway, how are you carrying out the reprojection? Have you tried the GridInQuestReprojector? I don't know much about it, but it says that the purpose is to convert "the national coordinate systems of Great Britain, Northern Ireland and the Republic of Ireland" so that would be something to look at for sure. The problem there is that I can't get it to work! At all. I will continue trying.
The other issue is that there must be a datum transformation of some sort going on, and I just can't find which it is. The CsmapReprojector lets me choose, and I can get multiple results from different transformations, I just can't find the ones that your MapInfo version is using!
So, will check some more and get back to you shortly.
+25
Just looking into this now. Taking quite a while!
Anyway, how are you carrying out the reprojection? Have you tried the GridInQuestReprojector? I don't know much about it, but it says that the purpose is to convert "the national coordinate systems of Great Britain, Northern Ireland and the Republic of Ireland" so that would be something to look at for sure. The problem there is that I can't get it to work! At all. I will continue trying.
The other issue is that there must be a datum transformation of some sort going on, and I just can't find which it is. The CsmapReprojector lets me choose, and I can get multiple results from different transformations, I just can't find the ones that your MapInfo version is using!
So, will check some more and get back to you shortly.
However... I think that's all redundant. What you should be using is the GridInquest transformer. That is the official Irish tool for converting coordinate systems. I finally got it to work (the settings are tricky and depend on where your data is). This is what I used:
Obviously you might want to tweak this a little (maybe your source is 2015, not 2002) but I think this provides the definitive answer: 146776.21347994561,530102.7005781912
If you want your data to be the same as MapInfo provides, then I think we'll need to know what transformations they are actually doing.
You could investigate by converting Irish Grid to ETRS89 Geodetic using both MapInfo and the GridInquest transformer, and see what the difference is. If there's no difference, then it must be in the conversion to OSGB that the difference occurs.
Hope this helps!
+25
+2
Yep I found that but got dragged into a FME Server performance issue ;)
+2
However... I think that's all redundant. What you should be using is the GridInquest transformer. That is the official Irish tool for converting coordinate systems. I finally got it to work (the settings are tricky and depend on where your data is). This is what I used:
Obviously you might want to tweak this a little (maybe your source is 2015, not 2002) but I think this provides the definitive answer: 146776.21347994561,530102.7005781912
If you want your data to be the same as MapInfo provides, then I think we'll need to know what transformations they are actually doing.
You could investigate by converting Irish Grid to ETRS89 Geodetic using both MapInfo and the GridInquest transformer, and see what the difference is. If there's no difference, then it must be in the conversion to OSGB that the difference occurs.
Hope this helps!
Hi @Mark2AtSafe I was going to suggest the GridInQuestIIReprojector but the test workspace failed when I didn't choose a Vertical Datum :
Source coordinate system 'Irish Grid' cannot reproject to destination coordinate system 'OSGB36 / British National Grid', either 'Out of Area' coordinates or invalid coordinate system parametersI thought it was getting cute because outside of Ireland! It doesn't like some parameter combos
+25
Hi @Mark2AtSafe I was going to suggest the GridInQuestIIReprojector but the test workspace failed when I didn't choose a Vertical Datum :
Source coordinate system 'Irish Grid' cannot reproject to destination coordinate system 'OSGB36 / British National Grid', either 'Out of Area' coordinates or invalid coordinate system parametersI thought it was getting cute because outside of Ireland! It doesn't like some parameter combos
I'm also struggling to get gridinQuestIIReprojector to work. i also get the error of
"Source coordinate system 'Irish Grid' cannot reproject to destination coordinate system 'OSGB36 / British National Grid', either 'Out of Area' coordinates or invalid coordinate system parameters
Reprojector: Reprojection failed"
Any ideas, i wonder if there is a problem because i'm taking the csv and creating a geometry using the vertexCreator that this is then causing the problem that it doesn't recognise it as Irish Grid?
For extra info i've got lots of points that are all in Northern Ireland, the one its currently failing on is: 333548|374449
+2
I'm also struggling to get gridinQuestIIReprojector to work. i also get the error of
"Source coordinate system 'Irish Grid' cannot reproject to destination coordinate system 'OSGB36 / British National Grid', either 'Out of Area' coordinates or invalid coordinate system parameters
Reprojector: Reprojection failed"
Any ideas, i wonder if there is a problem because i'm taking the csv and creating a geometry using the vertexCreator that this is then causing the problem that it doesn't recognise it as Irish Grid?
For extra info i've got lots of points that are all in Northern Ireland, the one its currently failing on is: 333548|374449
Here's mine working with FME 2017.0
and I'm using a VertexCreator as well
and here are the results:
I've also used the same parameters, but its still fails, what I have noticed is if you have one record in the csv it works, but if there are more than one record in the file it errors...very odd!
+2
I've also used the same parameters, but its still fails, what I have noticed is if you have one record in the csv it works, but if there are more than one record in the file it errors...very odd!
Sounds like a bug to me
Back to your original question, I'm not sure you will match the MapInfo conversion exactly. Depends which is considered to be more correct.
Thanks Mark. I will revert back to using the re-projector. As you say it depends on which is deemed more correct. Thanks to all trying to help.
+25
I've also used the same parameters, but its still fails, what I have noticed is if you have one record in the csv it works, but if there are more than one record in the file it errors...very odd!
+2
I've also used the same parameters, but its still fails, what I have noticed is if you have one record in the csv it works, but if there are more than one record in the file it errors...very odd!
@Mark2AtSafe I'm using a Creator transformer, OK with one feature, more than one fails:
Also if the input feature is EPSG 29902 it throws a warning, but even setting to Irish Grid it still fails the second feature through.ig2bng-error.fmw
+25
@Mark2AtSafe I'm using a Creator transformer, OK with one feature, more than one fails:
Also if the input feature is EPSG 29902 it throws a warning, but even setting to Irish Grid it still fails the second feature through.ig2bng-error.fmw
+25
+25
OK, here's the definitive answer, with info straight from our developers.
Normally GridInquest is not meant to be used for Irish to British grid conversions, but I mentioned that it's a fairly common practice and so our developers will fix it so that an Irish -> British conversion doesn't cause an out of range error in the GridInquest transformer. We're hoping for this to make it into FME2017.1
So those points we transformed. That will work and not cause an error.
Under the covers I think it's a two-step process where we convert Irish Grid to ETRS89, using "the official polynomial transformation parameters", then ETRS89 to OSTN2015 using the NTv2 grid. But you wouldn't really need to know that.
So - in short - try the GridInquest transformers in 2017.1 when it is released.
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