reprojection is wrong?

flynntflynnt Global Mapper UserPosts: 3
edited April 2009 in Technical Support
Hi there,

I have a MrSID dataset that is in UTM 31, Datum ED50. I'm looking at a point that is at a certain latitude/longitude (let's call it A). When I reproject the .sid file into Geographic, WGS84, I get a different latitude/longtitude (lat/long B) for the same point. I'm still somewhat of a newb to GIS, but is this correct?

When I reproject the same file with gdalwarp, the latitude and longitude stay the same (lat/long A). This is what I would expect. However, for the same point, Google Earth shows the same latitude/longitude as GlobalMapper after reprojection (lat/long B). I'm confused.

Who is wrong and why? any ideas?

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2009
    The difference is correct. The latitude and longitude location of a place is actually only valid when referenced to a particular earth model, or datum. Different models of the earth (datums) may differ by up to several hundred meters for the same latitude/longitude coordinate pair. Global Mapper automatically converts coordinates between datums to make sure they always refer to the same location on the earth.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • flynntflynnt Global Mapper User Posts: 3
    edited April 2009
    Huh, I wouldn't have figured that. I guess I'll need to read up on this some more. Are there any good online resources that explain this in detail?

    thanks in advance
  • flynntflynnt Global Mapper User Posts: 3
    edited April 2009
    Hi, i just wanted to make sure I was clear...

    If someone told me an airport was at a particular lat/long, and in the image i was looking at, it was indeed there. Then I reproject the image, I would expect the x,y location in the image to change, but not the lat/long of the airport. If that is indeed the case, I really need to read up and understand this stuff better.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited April 2009
    Yes, if you change the datum the lat/lon value for a particular location can change (typically less than 100 meters, but can be 10 times that). Most people don't know of this stuff outside of the mapping/GIS community and for things like an airport getting within 100 meters or so is fine. See Geodetic Datum Overview for more details.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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