Global Mapper v24.0

All my exported maps come into autocad in same place no matter what projection I use

I'm trying to place a model for use in VuCity, the site is Belfast and I've been told by the VuCity support guys I must use Irish Transverse Mercator projection. Now, Northern Irish OS tiles use Irish grid projection so when I bring in a tile on Irish grid settings it's co-ordinates line up with where it opens in Autocad as you'd expect. When I bring the same OS tile in on ITM projection it opens up in Global Mapper in a very different place, again as you'd expect.


Here's the problem, when I export the ITM tile on an ITM projection it open up in Autocad in the exact same place as the Irish Grid OS tile. I've gone into tools>configure and set projection to ITM before exporting, I've shut down, re-opened and set to ITM before bringing it in and checked again before export but it always opens up in the same place. Anyone have any ideas on what I'm doing wrong?

Answers

  • Blooper
    Blooper Global Mapper User
  • Blooper
    Blooper Global Mapper User

    Ignore the post above, Couldn't get these to load as separate pics.

    As you can see the projection types are different but the co-ordinates are the same however the latitude and longitude are completely different, maybe this is how it's supposed to work but I'm not a mapping expert and it's got me baffled.

  • Hi Blooper,

    What about ye! Greetings from Perth in Western Australia! As a County Antrim man myself I'm happy to help...

    It appears that you are caught in the middle of a projection transition between ITM (new) and IG (old). This Wikipedia article will maybe help explain, particularly the bit about "as a result ITM coordinates had to be obviously different so users would not confuse the two. This was done by shifting the ITM false origin further into the Atlantic and thereby creating substantially different co-ordinate numbers for any given location" :

    https://en.wikipedia.org/wiki/Irish_Transverse_Mercator

    So Donegall Street has definitely not moved and is still about 54.6 deg LAT and -5.9 deg LON and always will be while Earth remains (roughly) spherical. However, under IG (old) its roughly at 334,000 mE & 374,500 mN and under ITM (new) its at about 734,000 mE & 874,500 mN i.e. a approximately 400,000 m shift in Easting and 500,000 m shift in Northing (I say approximately because there is a scale factor difference in there, but let's not worry about that!). So the powers that be (OSi and OSNI) have made the new ITM co-ords to be obviously bigger that the old IG co-ords to help minimise confusion i.e. you should be able to "look" at co-ords and more or less tell which projection they are based on. So if they start with a "3" they are old IG and if they start with a "7" or "8" they are new ITM.

    This projection shift has been built into Global Mapper as it takes everything back to Lat/Lon (as far as I know) and then re-projects depending on the datum/projection that the user selects. However when you export to AutoCAD it simply goes off the plane Cartesian coordinates (N, E) = (x, y) and it couldn't care less about datum and projection. That is why your X, Y coordinates are the same above (and are correct for old IG), but those coordinates are in a different location based on new ITM that is why you are getting different Lat/Lon.

    I hope that helps. I find its helpful in GIS in general to use EPSG codes for datum/projection info and they are easy to search up on the net. Here are the two in question:

    TM65 / Irish Grid - Ireland - EPSG:29902

    IRENET95 / Irish Transverse Mercator - EPSG:2157

    If you use EPSG codes you can't go wrong! See attached screen grabs.

    Best of luck to you and get back if still not clear.

    Cheers,

    Alistair.



  • Hi Blooper,

    I've just re-read your question and realised that I've answered a totally different question than the one you were asking - sorry. I have no idea why this is happening to you! ?

    I exported the red "Belfast CBD" box shown in my screen grabs above as DXF in both IG (old) and ITM (new) and they come into AutoCAD as per their respective coordinates i.e. in different places. See screen grabs and attached dxf's in zip folder. I've also attached a GM package file in both IG (old) and ITM (New) in case that is helpful for you.

    May be worth contacting GM helpdesk...?

    Cheers,

    /al








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