Different results from Global Mapper and Blue Marble

jnguyenjnguyen Global Mapper UserPosts: 3
edited March 2013 in Projection Questions
Hi,

Anyone please help me.

I tried to convert these numbers (x: 379328, y: 9923571 - UTM18S, WGS84) to PSAD56 (CSEG code: 24878) using GlobalMapper and Blue Marble. I hace 2 different results. In GlobalMapper, the result is 379560.57, 9923946.8. In Blue Marble, the result is 379551.83, 9923937.73. I noticed that the Blue Marble use 7 parameters, and the GlobalMapper 3 parameters and the numbers are quite different between the two. Anyway, I edited the GlobalMapper projection to be exactly the same as Blue Marble and the numbers now are 379094.665, 9923142.24.

Now, I am quite confused. Which one is right?

Thanks
Joe

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    Joe,

    I assume that you added a new custom datum with the 7-parameter transformation? The 7-parameter transforms are tricky as there are 2 standards for specifying the transformation. You may need to negate the rotation X, Y, and Z parameters when adding the transform to Global Mapper depending on which standard is used in the database where you got it. I suspect that will make it agree with Blue Marble.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • jnguyenjnguyen Global Mapper User Posts: 3
    edited March 2013
    Hi Mike,

    Thanks for your quick response.
    Now if I modify the Global Mapper with only 3 parameters to be the same as Blue Marble (X-axis Shift -60.31, Y: 245.935, Z: 31.008) as a new custome datum, then the new numbers are 379322.65, 9923540.03 which do not agree with those of Blue Marble.

    Joe
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited March 2013
    Joe,

    You should keep all 7 parameters as specific for the transform, just try making the 3 rotation parameters the negative of what you originally used and keep the dx, dy, dz, and scale what you had originally. You can't use the dx, dy, and dz of a 7-parameter transformation for the 3-parameter transform. That will not generate a good transformation.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • jnguyenjnguyen Global Mapper User Posts: 3
    edited March 2013
    Thank you MIke!

    I have solved my problem.

    Joe
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